                   FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

CENTER FOR BIOLOGICAL DIVERSITY;         
PACIFIC ENVIRONMENT,
                Plaintiffs-Appellants,
                  v.                            No. 10-35123
KENNETH LEE SALAZAR, Secretary
of the Interior; U.S. FISH AND
                                                 D.C. No.
                                             3:08-cv-00159-RRB
WILDLIFE SERVICE,                                 OPINION
              Defendants-Appellees,
ALASKA OIL AND GAS ASSOCIATION,
     Defendant-intervenor-Appellee.
                                         
       Appeal from the United States District Court
                for the District of Alaska
     Ralph R. Beistline, Chief District Judge, Presiding

                   Argued and Submitted
              June 28, 2012—Fairbanks, Alaska

                    Filed August 21, 2012

    Before: Alfred T. Goodwin, William A. Fletcher, and
            Milan D. Smith, Jr., Circuit Judges.

           Opinion by Judge William A. Fletcher




                              9521
9524    CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR




                       COUNSEL

Rebecca Noblin, CENTER FOR BIOLOGICAL DIVER-
SITY, Anchorage, Alaska, Kassia Rhoades Siegel, CENTER
         CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR      9525
FOR BIOLOGICAL DIVERSITY, Joshua Tree, California,
for the appellants.

Dean Keith Dunsmore, ENVIRONMENT & NATURAL
RESOURCES, Anchorage, Alaska, Kristen L. Gustafson,
David C. Shilton, UNITED STATES DEPARTMENT OF
JUSTICE, Washington, D.C., for the appellees.

Jeffrey Wayne Leppo, Ryan P. Steen, Jason T. Morgan,
STOEL RIVES, LLP, Seattle, Washington, for the intervenor-
appellee.


                           OPINION

W. FLETCHER, Circuit Judge:

   This case involves U.S. Fish and Wildlife Service
(“Service”) regulations under Section 101(a)(5)(A) of the
Marine Mammal Protection Act (“MMPA”) that authorize
incidental take of polar bears and Pacific walruses resulting
from oil and gas exploration activities in the Chukchi Sea and
on the adjacent coast of Alaska. The Center for Biological
Diversity and Pacific Environment (collectively “Plaintiffs”)
brought suit challenging the regulations and accompanying
environmental review documents under the MMPA, Endan-
gered Species Act (“ESA”), and National Environmental Pol-
icy Act (“NEPA”). The district court granted summary
judgment to the Service. We affirm.

                      I.   Background

   The Chukchi Sea off the North Slope of Alaska is a promis-
ing location for oil and gas exploration and development. It
also is home to polar bears and Pacific walruses, both of
which are marine mammals protected under the MMPA.
There are two polar bear stocks in Alaska, with a total esti-
9526      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
mated population of about 3,500 animals. Surveys taken
between 1975 and 1990 estimated the total population of
Pacific walruses in the area to be between 200,000 and
250,000 animals. Both polar bears and Pacific walruses
migrate seasonally with the advance and retreat of the sea ice
habitat on which they rely for survival. In May 2008, the Ser-
vice listed the polar bear as a threatened species under the
ESA because of projected reductions in sea ice caused by cli-
mate change. 73 Fed. Reg. 28,212 (May 15, 2008). The
Pacific walrus is not presently listed as threatened or endan-
gered under the ESA.

           A.   Incidental Take Under the MMPA

   The MMPA generally prohibits the “take” of marine mam-
mals. 16 U.S.C. § 1371(a). “Take” is defined broadly under
the MMPA to encompass “harassment,” including any act of
“torment” or “annoyance” that “has the potential to injure . . .
or . . . disturb a marine mammal or marine mammal stock in
the wild by causing disruption of behavioral patterns, includ-
ing, but not limited to, migration, breathing, nursing, breed-
ing, feeding, or sheltering.” Id. § 1362(13), (18)(A)(i)-(ii).
Unlawful take is subject to a civil penalty of up to $10,000 for
each violation. Id. § 1375(a)(1). A knowing violation is sub-
ject to a criminal penalty of up to a year in prison and a
$20,000 fine. Id. § 1375(b).

   The MMPA allows several exceptions to the general take
prohibition, including take for scientific research and for pub-
lic display, as well as incidental take in the course of commer-
cial fishing. Id. § 1371(a)(1)-(2). At issue in this appeal is an
exception under Section 101(a)(5)(A) for incidental, but not
intentional, take of “small numbers” of marine mammals from
“a specified activity (other than commercial fishing) within a
specified geographical region.” Id. § 1371(a)(5)(A)(i). The
Service will authorize such take of “small numbers” of mam-
mals for up to five years if it determines that the total inciden-
tal take would have a “negligible impact” on the relevant
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR       9527
species or stock and would not have an “unmitigable adverse
impact” on availability for specified subsistence uses. Id.
§ 1371(a)(5)(A)(i)(I). If the Service makes the required find-
ings, it may issue regulations — such as those challenged in
this appeal — specifying permissible methods of take pursu-
ant to the activity, specifying other means of effecting the
least practicable adverse impact on the species, and specifying
monitoring and reporting requirements for the authorized
take. Id. § 1371(a)(5)(A)(i)(II).

   In 1983, the Service promulgated regulations implementing
Section 101(a)(5) of the MMPA. 48 Fed. Reg. 31,220 (July 7,
1983) (codified at 50 C.F.R. § 18.27). The implementing reg-
ulations establish a two-step process: first, the Service issues
incidental take regulations that govern a specified activity in
a specified geographic region for up to five years; second, the
Service issues letters of authorization (“LOAs”) to individual
applicants authorizing their incidental take under the regula-
tions. 50 C.F.R. § 18.27(e)-(f). Before issuing an LOA, the
Service must determine that the level of anticipated incidental
take is consistent with the five-year regulations. Id.
§ 18.27(f)(2). The implementing regulations define “small
numbers” as “a portion of a marine mammal species or stock
whose taking would have a negligible impact on that species
or stock.” Id. § 18.27(c). They define “negligible impact” as
an impact that is not reasonably likely or expected to “ad-
versely affect the species or stock through effects on annual
rates of recruitment or survival.” Id.

   The oil and gas industry for more than two decades has
requested and received incidental take authorization for its
exploration, development, and production activities off the
coast of northwestern Alaska. Between 1993 and 2006, the
Service issued a series of regulations authorizing incidental
take of polar bears and Pacific walruses in the Beaufort Sea.
58 Fed. Reg. 60,402 (Nov. 16, 1993); 60 Fed. Reg. 42,805
(Aug. 17, 1995); 64 Fed. Reg. 4,328 (Jan. 28, 1999); 65 Fed.
Reg. 5,275 (Feb. 3, 2000); 65 Fed. Reg. 16,828 (Mar. 30,
9528      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
2000); 68 Fed. Reg. 66,744 (Nov. 28, 2003); 71 Fed. Reg.
43,926 (Aug. 2, 2006). In 1991, the Service issued regulations
authorizing incidental take in the adjacent Chukchi Sea. 56
Fed. Reg. 27,443 (June 14, 1991). Little to no oil and gas
exploration occurred in the Chukchi Sea over the next fifteen
years. However, new opportunities for exploration and devel-
opment in the Chukchi Sea prompted the Alaska Oil and Gas
Association (“Association”) to request another set of five-year
incidental take regulations in 2005.

             B.   2008 Chukchi Sea Regulations

   In response to the Association’s request, the Service in June
2007 published proposed regulations authorizing incidental,
nonlethal take of polar bears and Pacific walruses resulting
from oil and gas exploration activities in the Chukchi Sea. 72
Fed. Reg. 30,670 (June 1, 2007). Previous incidental take reg-
ulations in the Beaufort and Chukchi Seas covered oil and gas
exploration, development, and production. The new regula-
tions cover only exploration activities — such as onshore and
offshore seismic surveys, exploratory drilling, and associated
support operations.

   In July 2007, Plaintiffs filed comments with the Service
criticizing the proposed incidental take regulations. The
Marine Mammal Commission, an independent federal agency
created under the MMPA to advise the Service, submitted
comments recommending that the Service defer issuing final
regulations until it developed more effective monitoring and
mitigation strategies and gathered more information about the
effects of exploration activities on the mammals.

  In March 2008, the Service issued an Environmental
Assessment (“EA”) for the proposed regulations pursuant to
NEPA. 40 C.F.R. § 1508.9. The EA concludes that the inci-
dental take regulations, along with accompanying mitigation
measures, “would result in no measurable impacts o[n] the
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR       9529
physical environment,” and “the overall impact would be neg-
ligible on polar bear and Pacific walrus populations.”

   Because promulgation of the regulations would constitute
“agency action” under Section 7 of the ESA, the Service’s
Marine Mammal Office consulted internally with the Fair-
banks Fish and Wildlife Field Office regarding the regula-
tions’ effects on the threatened polar bear. 16 U.S.C.
§ 1536(a)(2); 50 C.F.R. § 402.02. In May 2008, the Fairbanks
office issued a Biological Opinion (“BiOp”) concluding that
the incidental take regulations were not likely to jeopardize
the continued existence of the polar bear. The BiOp notes that
“(1) the Regulations do not authorize[ ] lethal take, (2) the
Chuckchi Sea Regulations will be implemented in a similar
manner to the Beaufort Sea Regulations, which have been in
place almost continuously since 1993, and (3) few bears are
likely to be encountered, and those that are encountered are
likely to alter their behavior only temporarily if at all.” The
BiOp does not consider effects on the Pacific walrus because
the species is not listed as threatened or endangered under the
ESA.

   In June 2008, the Service issued a final rule for the Chukchi
Sea incidental take regulations. 73 Fed. Reg. 33,212 (June 11,
2008) (codified at 50 C.F.R. §§ 18.111-18.119). The regula-
tions encompass a geographic area of about 90,000 square
miles, including the waters and seabed of the Chukchi Sea, as
well as adjacent coastal land extending about 25 miles inland.
The regulations anticipate up to four offshore seismic survey
vessels operating in a given year, each accompanied by three
support vessels, and up to three drill ships, each drilling as
many as four wells and accompanied by icebreakers, barges,
helicopters, and supply ships. Id. at 33,215-16. Onshore, the
regulations anticipate the drilling of as many as six wells and
the construction of up to 100 miles of roads and four airfield
runways. Id. at 33,216. The final rule acknowledges that non-
lethal, incidental harassment of polar bears and Pacific wal-
ruses is reasonably likely or expected to occur as a result of
9530      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
the proposed activities. Id. at 33,223-32. However, it notes
that onshore activities are not expected to occur near known
polar bear denning areas or walrus haulouts, and that offshore
activities will occur during the open water season (July
through November) to avoid disturbing pack ice on which the
mammals rely. Id. at 33,214. The rule incorporates into its
analysis mitigation measures that would be imposed on the
activities, such as restrictions on the location and spacing of
offshore seismic surveys. See, e.g., id. at 33,216-18.

   The final rule concludes, with “a high level of confidence,”
that “any incidental take reasonably likely to result from the
effects of the proposed activities, as mitigated through this
regulatory process, will be limited to small numbers of wal-
ruses and polar bears.” Id. at 33,234-36. The Service explains
that

    the number of animals likely to be affected is small,
    because: (1) A small portion of the Pacific walrus
    population or the Chukchi Sea and Southern Beau-
    fort Sea polar bear populations will be present in the
    area of Industry activities, (2) of that portion, a small
    percentage will come in contact with Industry activi-
    ties, and (3) the response by those animals will likely
    be minimal changes in behavior.

Id. at 33,236.

   The final rule also concludes that the incidental take autho-
rized under the regulations would have only a “negligible
impact” on the polar bears and Pacific walruses. It concludes
that “any incidental take reasonably likely to result from the
effects of oil and gas related exploration activities during the
period of the rule, in the Chukchi Sea and adjacent western
coast of Alaska[,] will have no more than a negligible effect
on the rates of recruitment and survival of polar bears and
Pacific walruses in the Chukchi Sea Region.” Id.
         CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR       9531
   The regulations require a separate LOA for each proposed
exploration activity. Applicants for an LOA must submit an
operations plan, a polar bear interaction plan, and a site-
specific mitigation and monitoring plan. 50 C.F.R. §§ 18.114,
18.118. The Service will tailor its mitigation and monitoring
requirements based on the location, timing, and nature of the
proposed activity. Id. § 18.116(b). The regulations do not
authorize lethal or intentional take. Id. § 18.117(a). In July
2008, the Service began issuing LOAs for exploration activi-
ties in the Chukchi Sea under the incidental take regulations.
The regulations are valid through June 11, 2013. Id. § 18.113.

                 C.     Procedural Background

   Plaintiffs filed suit against the Service, alleging that the
five-year incidental take regulations, the accompanying BiOp,
and the EA fail to comply with the MMPA, ESA, and NEPA.
The Association intervened as co-defendants.

   Plaintiffs had previously challenged the Service’s 2006 reg-
ulations authorizing incidental take of polar bears and Pacific
walruses from oil and gas activities in and along the Beaufort
Sea. In December 2009, we upheld the Service’s 2006 Beau-
fort Sea regulations under the MMPA and NEPA. Ctr. for
Biological Diversity v. Kempthorne, 588 F.3d 701 (9th Cir.
2009). Among other things, we let stand the Service’s deter-
minations that the authorized incidental take would have only
a “negligible impact” on the marine mammal populations, and
that the regulations would not have a significant impact on the
environment. Id. at 710-12.

  A month after our decision in Kempthorne, the district
court in this case granted summary judgment to the Service
and the Association. Plaintiffs timely appealed.

                  II.    Standard of Review

  We review de novo a district court’s grant or denial of sum-
mary judgment. Humane Soc’y v. Locke, 626 F.3d 1040, 1047
9532      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
(9th Cir. 2010). We review an agency’s compliance with the
MMPA, ESA, and NEPA under the Administrative Procedure
Act. Id. (MMPA and NEPA); City of Sausalito v. O’Neill, 386
F.3d 1186, 1205-06 (9th Cir. 2004) (NEPA and ESA). We
may not set aside an agency decision unless it is “arbitrary,
capricious, an abuse of discretion, or otherwise not in accor-
dance with law.” 5 U.S.C. § 706(2)(A). A decision is arbitrary
and capricious if the agency

    relied on factors Congress did not intend it to con-
    sider, entirely failed to consider an important aspect
    of the problem, or offered an explanation that runs
    counter to the evidence before the agency or is so
    implausible that it could not be ascribed to a differ-
    ence in view or the product of agency expertise.

Lands Council v. McNair, 537 F.3d 981, 987 (9th Cir. 2008)
(en banc) (internal quotation marks omitted), overruled on
other grounds by Winter v. Natural Res. Def. Council, Inc.,
555 U.S. 7 (2008).

   We review an agency’s interpretation of a statute it is
charged with administering under the familiar two-step frame-
work set forth in Chevron, U.S.A., Inc. v. Natural Res. Def.
Council, Inc., 467 U.S. 837 (1984). We first determine
whether “Congress has directly spoken to the precise question
at issue. If the intent of Congress is clear, that is the end of
the matter; for the court, as well as the agency, must give
effect to the unambiguously expressed intent of Congress.” Id.
at 842-43. However, “if the statute is silent or ambiguous with
respect to the specific issue, the question for the court is
whether the agency’s answer is based on a permissible con-
struction of the statute.” Id. at 843. “If a statute is ambiguous,
and if the implementing agency’s construction is reasonable,
Chevron requires a federal court to accept the agency’s con-
struction of the statute, even if the agency’s reading differs
from what the court believes is the best statutory interpreta-
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR        9533
tion.” Nat’l Cable & Telecomm. Ass’n v. Brand X Internet
Servs., 545 U.S. 967, 980 (2005).

                       III.   Discussion

   Plaintiffs allege that the Service’s incidental take regula-
tions and accompanying environmental review documents fail
to comply with the MMPA, ESA, and NEPA. We take the
arguments under each statute in turn.

                         A.    MMPA

   Plaintiffs make three arguments under the MMPA. First,
they argue that the Service relied on an impermissible regula-
tory definition that conflates the question whether an autho-
rized take is for “small numbers” of mammals with the
separate question whether the take will result in a “negligible
impact” on the species or stock. Second, they argue that the
Service improperly authorized incidental take without quanti-
fying how many polar bears and Pacific walruses would be
taken. Third, they argue that the Service’s qualitative “small
numbers” finding was based on false assumptions and bad
science. The arguments challenge the Service’s “small num-
bers” interpretation as applied in the 2008 rule, as well as the
substance of the Service’s “small numbers” determination.

            1.    “Small Numbers” Interpretation

   [1] Under Section 101(a)(5)(A) of the MMPA, citizens
who engage in a specified activity (other than commercial
fishing) within a specified geographical region may request
authorization for incidental, but not intentional, take of “small
numbers” of marine mammals pursuant to that activity for a
period of no more than five years. 16 U.S.C.
§ 1371(a)(5)(A)(i). The Service shall allow such take if it
determines inter alia that the total incidental take during the
five-year period will have only a “negligible impact” on the
relevant species or stock. Id. § 1371(a)(5)(A)(i)(I).
9534      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
   [2] The 1983 regulations implementing Section 101(a)(5)
define “small numbers” as “a portion of a marine mammal
species or stock whose taking would have a negligible impact
on that species or stock.” 50 C.F.R. § 18.27(c). The imple-
menting regulations, as amended, define “negligible impact”
as an impact that is not reasonably likely or expected to “ad-
versely affect the species or stock through effects on annual
rates of recruitment or survival.” Id.

   Plaintiffs argue that the 1983 regulatory definition is an
impermissible construction of the statute because it renders
the “small numbers” language superfluous by conflating it
with the separate “negligible impact” standard. Plaintiffs point
to Natural Res. Def. Council, Inc. v. Evans, 279 F. Supp. 2d
1129, 1150-53 (N.D. Cal. 2003), decided by Magistrate Judge
Laporte, which held precisely this in a challenge to incidental
take regulations promulgated by the National Marine Fish-
eries Service (“NMFS”) under Section 101(a)(5)(A). NMFS,
through the Secretary of Commerce, administers the MMPA
with respect to cetaceans (whales, dolphins, and porpoises)
and pinnipeds (seals and sea lions) other than walruses. 16
U.S.C. § 1362(12)(A)(i). The Service, through the Secretary
of the Interior, administers the MMPA with respect to all
other marine mammals, including polar bears and Pacific wal-
ruses. Id. § 1362(12)(A)(ii).

   It is “a cardinal principle of statutory construction” that a
statute should be construed, if possible, so that “no clause,
sentence, or word shall be superfluous, void, or insignificant.”
TRW Inc. v. Andrews, 534 U.S. 19, 31 (2001) (quoting Dun-
can v. Walker, 533 U.S. 167 (2001)) (internal quotation marks
omitted); see also Nevada v. Watkins, 939 F.2d 710, 715 (9th
Cir. 1991) (“It is a fundamental rule of statutory construction
that we should avoid an interpretation of a statute that renders
any part of it superfluous and does not give effect to all of the
words used by Congress.” (internal alteration and quotation
marks omitted)). Section 101(a)(5)(A) of the MMPA provides
that the Service shall allow incidental take of “small num-
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR       9535
bers” of marine mammals if the Service “finds that the total
of such taking . . . will have a negligible impact on [the rele-
vant] species or stock.” 16 U.S.C. § 1371(a)(5)(A)(i)(I). The
district court observed in Evans, “The plain language indi-
cates that ‘small numbers’ is a separate requirement from
‘negligible impact.’ To treat them as identical would appear
to render the reference to ‘small numbers’ mere surplusage.”
279 F. Supp. 2d at 1150. We agree. That is, by defining
“small numbers” as any amount that would have a “negligible
impact,” the implementing regulations allow the Service to
authorize the incidental take of large numbers of mammals, so
long as that take did not have more than a negligible impact
on the relevant species or stock. This authorization, while
complying with the 1983 regulatory definition, violates the
plain language of the statute.

   [3] Legislative history confirms our reading of the statute
if such confirmation is needed. The House Report accompa-
nying Section 101(a)(4)-(5) of the MMPA indicates that Con-
gress intended “small numbers” and “negligible impact” to
serve as two separate standards. The Report explains:

       The taking authorized under these new provisions
    is the taking of small numbers of marine mammals.
    The Committee recognizes the imprecision of the
    term ‘small numbers’, but was unable to offer a more
    precise formulation because the concept is not capa-
    ble of being expressed in absolute numerical limits.
    The Committee intends that these provisions be
    available for persons whose taking of marine mam-
    mals is infrequent, unavoidable, or accidental.

       It should also be noted that these new provisions
    of the Act provide an additional and separate safe-
    guard in that the Secretary must determine that the
    incidental takings of small numbers of marine mam-
    mals have a ‘negligible’ impact upon the species
    from which such takings occur. This additional test
9536     CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
    is meant to serve as a separate standard restricting
    the authority of the Secretary. . . . Unless a particular
    activity takes only small numbers of marine mam-
    mals, and that taking has a negligible impact on the
    species, the new provisions of sections 101(a)(4) and
    (5) are not applicable to that activity.

Id. at 1150-51 (quoting H.R. Rep. No. 97-228 (1981),
reprinted in 1981 U.S.C.C.A.N. 1458, 1469) (emphasis in
Evans). As a result, incidental take permitted under Section
101(a)(5)(A) “must be small and have [only] a negligible
impact on the affected species or stock of marine mammals.”
Id. at 1152 (emphasis in original).

   [4] The Service dismisses the district court’s opinion in
Evans in a footnote and suggests that the court failed to apply
the proper Chevron framework. The Service is mistaken. The
court in Evans properly relied on “Congress’ intent” and the
“plain language” of the MMPA to hold that the agency’s
interpretation was an impermissible construction of the stat-
ute. Id. at 1153. As Chevron recognized,

    The judiciary is the final authority on issues of statu-
    tory construction and must reject administrative con-
    structions which are contrary to clear congressional
    intent. If a court, employing traditional tools of statu-
    tory construction, ascertains that Congress had an
    intention on the precise question at issue, that inten-
    tion is the law and must be given effect.

467 U.S. at 843 n.9 (internal citations omitted). The Evans
court quoted from this Chevron passage in its analysis. Evans,
279 F. Supp. 2d at 1153 (quoting Chevron, 467 U.S. at 843
n.9). We agree with Evans that “[t]o effectuate Congress’
intent, ‘small numbers’ and ‘negligible impact’ must be
defined so that each term has a separate meaning.” Id.

   The Service and Association contend that any facial chal-
lenge to the 1983 regulatory definition of “small numbers” is
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR       9537
barred by the six-year statute of limitations for civil actions
against the United States. 28 U.S.C. § 2401(a). The Evans
court addressed the same argument and held that “plaintiffs
are time-barred from challenging the [1983] regulation itself,
but are not time-barred from challenging the application of
that regulation to them.” Evans, 279 F. Supp. 2d at 1148. We
agree. Although Plaintiffs cannot challenge facially the 1983
regulatory definition, they can challenge the Service’s alleged
application of that definition in the 2008 Chukchi Sea regula-
tions as exceeding the agency’s statutory authority. See Nw.
Envtl. Advocates v. U.S. Envtl. Prot. Agency, 537 F.3d 1006,
1018-19 (9th Cir. 2008); Wind River Mining Corp. v. United
States, 946 F.2d 710, 715 (9th Cir. 1991) (a plaintiff may con-
test “an agency decision as exceeding constitutional or statu-
tory authority . . . later than six years following the decision
by filing a complaint for review of the adverse application of
the decision to the particular challenger”). Accordingly, we
must determine whether the Service applied the 1983 regula-
tory definition, as opposed to some other permissible defini-
tion, in promulgating the contested 2008 incidental take
regulations.

   The Chukchi Sea regulations that were initially proposed in
2007 clearly applied the impermissible regulatory definition
by conflating the “small numbers” and “negligible impact”
standards. See 72 Fed. Reg. at 30,690-92. However, in the
spring of 2008, based in part on criticisms of the proposed
regulations made in comments filed by Plaintiffs, Service
officials voiced internal concerns about the legal defensibility
of the proposed “small numbers” analysis. The Service signif-
icantly redrafted the analysis in its preparation of the final
regulations. Compare id., with 73 Fed. Reg. at 33,233-37. The
2008 final rule for the Chukchi Sea incidental take regulations
cites the 1983 “small numbers” regulatory definition in its
preamble, 73 Fed. Reg. at 33,212 (citing 50 C.F.R. § 18.27),
and asserts in response to comments that “[t]he Service’s
analysis of ‘small numbers’ complies with the agency’s
[1983] regulatory definition,” id. at 33,244. However, the
9538     CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
2008 final rule analyzes the “small numbers” and “negligible
impact” standards separately under different headings. It
determines that “the number of walrus and polar bear taken by
this activity will be small and the effect on their respective
populations negligible.” Id. at 33,235 (emphasis added). The
final rule, challenged in this case, is thus different from the
incidental take regulations struck down in Evans, which
applied the 1983 regulatory definition and conflated the two
standards. See 67 Fed. Reg. 46,712, 46,764 (July 16, 2002)
(“NMFS continues to believe that its [1983] regulatory defini-
tion is consistent with Congressional intent.”); id. at 46,780
(analyzing “small numbers” and “negligible impact”
together).

   Plaintiffs argue that even though the Service analyzes the
two standards under separate headings in its 2008 final rule,
the agency applies a concept of “relatively small numbers”
that eviscerates any distinction from the “negligible impact”
standard. In the rule, the Service interprets “small numbers”
to mean small relative to the size of the mammals’ larger pop-
ulation. See, e.g., 73 Fed. Reg. at 33,233 (“[O]nly small num-
bers of Pacific walrus and polar bears are likely to be taken
incidental to the described Industry activities relative to the
number of walruses and polar bears that are expected to be
unaffected by those activities.” (emphasis added)); id. at
33,232 (“[W]e conclude that the proposed exploration activi-
ties, as mitigated through the regulatory process, will impact
relatively small numbers of animals . . . .” (emphasis added));
id. at 33,245 (“Although a numerical estimate . . . could not
be practically obtained, the Service deduced that only small
numbers of Pacific walruses and polar bears, relative to their
populations, have the potential to be impacted by the pro-
posed Industry activities described in these regulations.”
(emphasis added)). Plaintiffs argue that “while ‘negligible
impact’ may be a relative concept, ‘small numbers’ is an
absolute limit that may not be defined in relation to popula-
tion size, distribution, or other demographics.” They contend
that the “small numbers” language in Section 101(a)(5)(A)
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR       9539
requires the Service to quantify in absolute terms the number
of mammals that would be taken by the covered activities pur-
suant to the incidental take regulations. The Service does not
provide a numerical estimate of the take that would occur
under the 2008 Chukchi Sea regulations.

   We agree with the Service that Congress has not spoken
directly to the question whether “small numbers” can be ana-
lyzed in relative or proportional terms. As the Service
observed in its brief, “ ‘Small numbers’ in this context does
not have a plain meaning that unambiguously forbids use of
a proportional approach.” Legislative history reveals that
Congress recognized “the imprecision of the term ‘small num-
bers,’ but was unable to offer a more precise formulation
because the concept is not capable of being expressed in
absolute numerical limits.” H.R. Rep. No. 97-228, reprinted
in 1981 U.S.C.C.A.N. at 1469 (emphasis added). Nor is there
anything in Section 101(a)(5)(A) that requires the Service,
when promulgating incidental take regulations, to quantify or
estimate the number of mammals that would be taken. In con-
trast, Congress expressly required numerical estimates in
other provisions of the MMPA. See, e.g., 16 U.S.C.
§ 1374(b)(2)(A) (requiring that any permit for take of marine
mammals for purposes like scientific research shall “specify
the number and kind of animals which are authorized to be
taken or imported”); id. § 1386(a)(2) (requiring that the Secre-
tary of Commerce prepare an assessment specifying “the
minimum population estimate” for each marine mammal
stock in U.S. waters); id. § 1387(f)(4)(B) (requiring that take
reduction plans include “an estimate of the total number . . .
of animals from the stock that are being incidentally lethally
taken or seriously injured each year during the course of com-
mercial fishing operations, by fishery”). Plaintiffs note that
the 1983 implementing regulations require parties requesting
an incidental take authorization to submit, along with their
request, “[a]n estimate of the species and numbers of marine
mammals likely to be taken.” 50 C.F.R. § 18.27(d)(1)(iii)(A).
However, a regulation requiring that private parties submit
9540      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
estimates is immaterial to whether the statute requires that the
Service quantify estimates of its own.

   Because we find that the statute is silent or ambiguous on
the precise question at issue, Chevron commands that we
accept the agency’s interpretation so long as it is reasonable,
even if it is not the reading that we would have reached on our
own. 467 U.S. at 843 & n.11. The key interpretative require-
ment of the Section 101(a)(5)(A) language is that “small num-
bers” and “negligible impact” remain distinct standards. The
Service explains in its brief to us how “relatively small num-
bers” can have a meaning distinct from “negligible impact.”
Specifically, the “small numbers” determination focuses on
the portion of a species or stock subject to incidental take,
whereas the “negligible impact” analysis focuses on the
impact of the anticipated take — that is, on whether the type
and duration of take or harassment may adversely affect the
species’ annual rates of recruitment or survival. The Service
explains that “even if a proposed activity affects only a small
number of animals, the Service could still find[ ] that the tak-
ing will have more than a negligible effect on the species or
stock, particularly if the impact on those particular animals is
severe, or if those animals are of great importance to the spe-
cies or stock.” For example, anticipated harassment of even
small numbers of mammals might prevent mating or repro-
duction during key parts of the year, or might result in lethal
take of newborn mammals. These circumstances could pose
more than a negligible impact on the relevant species or stock,
even if they directly affect only small numbers of mammals
relative to the population as a whole. Likewise, a proposed
activity might harass a large portion of the relevant mammal
population, but have only a negligible impact on the species
or stock because the harassment is merely trivial and fleeting.
The Service still could not allow such a take under Section
101(a)(5)(A) because it would result in a take of more than
“small numbers” of mammals. As the House Report
explained, “Unless a particular activity takes only small num-
bers of marine mammals, and that taking has a negligible
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR          9541
impact on the species, the new provisions of sections
101(a)(4) and (5) are not applicable to that activity.” H.R.
Rep. No. 97-228, reprinted in 1981 U.S.C.C.A.N. at 1469
(emphasis added).

   We find this interpretation of Section 101(a)(5)(A) both
reasonable and persuasive. The Service’s “small numbers”
analysis in the 2008 final rule focuses primarily on the loca-
tion of the exploration activities in relation to the mammals’
larger populations, whereas the “negligible impact” analysis
considers the likely effects of interactions on the mammals’
recruitment and survival. Thus, in making its “small numbers”
determination, the final rule concludes that “given the spatial
distribution, habitat requirements, and observed and reported
data, the number of animals coming in contact with the indus-
try activity will be small by an order of magnitude to the [rel-
evant walrus and] polar bear populations.” 73 Fed. Reg. at
33,235. In making its “negligible impact” determination, the
rule notes that “[t]he predicted effects of proposed activities
on walruses and polar bears will be nonlethal, temporary pas-
sive takes of animals.” Id.

   We note that the Service relies on many of the same factors
in making the two determinations. For example, the final rule
does not limit its “small numbers” analysis to the portion of
the polar bear and walrus populations subject to incidental
take. It also looks at the nature of the anticipated take and the
mammals’ behavioral response — factors that more appropri-
ately address the “negligible impact” standard. See id. at
33,236 (“[T]he number of animals likely to be affected is
small, because . . . the response by those animals will likely
be minimal changes in behavior.”); see also, e.g., id. at 33,234
(“The behavioral responses and the effects were limited to
short-term, minor behavioral changes, primarily dispersal or
diving. None of the take that occurred would have affected
reproduction, survival, or other critical life functions.”); id. at
33,235 (“[T]he behavioral response observed [from prior
interactions] was a very passive form of take. . . . Such
9542      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
response would not have affected reproduction, survival, or
other critical life functions. This same level of behavioral
response is expected if encounters occur during future opera-
tions[.]”). We recognize, as the Service argues in its brief, that
there will inevitably be “some overlap” between the two stan-
dards. The Service can (and should) do a better job of keeping
the standards distinct when promulgating future incidental
take regulations under Section 101(a)(5)(A). However, we
uphold the “small numbers” interpretation as applied in the
2008 rule because the Service’s “small numbers” and “negli-
gible impact” analyses are sufficiently distinct to survive our
deferential review.

   [5] In sum, we hold that “small numbers” and “negligible
impact” are distinct standards that the Service must satisfy
when promulgating incidental take regulations under Section
101(a)(5)(A) of the MMPA. The Service need not quantify
the number of marine mammals that would be taken under the
regulations, so long as the agency reasonably determines
through some other means that the specified activity will
result in take of only “small numbers” of marine mammals.
The Service can analyze “small numbers” in relation to the
size of the larger population, so long as the “negligible
impact” finding remains a distinct, separate standard. Because
the Service analyzed it as a distinct standard in the 2008 final
rule, we uphold the agency’s “small numbers” interpretation
as applied in the challenged regulations.

            2.    “Small Numbers” Determination

   Plaintiffs argue that, even if the Service applied a permissi-
ble “small numbers” interpretation in the 2008 final rule, the
substance of the agency’s “small numbers” analysis is arbi-
trary and capricious because (1) it accounts for only some of
the proposed oil and gas exploration activities, and (2) it relies
on “unproven” or “inadequate” monitoring and mitigation
techniques. Notably, however, Plaintiffs do not challenge the
Service’s “negligible impact” finding, as they did in their
         CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR          9543
unsuccessful challenge to the Service’s 2006 Beaufort Sea
incidental take regulations. See Kempthorne, 588 F.3d at 711
(upholding the Service’s “negligible impact” finding because
the agency “made scientific predictions within the scope of its
expertise, the circumstance in which we exercise our greatest
deference”).

   [6] First, Plaintiffs argue that the Service’s “small num-
bers” determination is arbitrary and capricious because the
analysis ignores expected impacts from oil and gas support
operations and onshore activities. Plaintiffs concede that the
Service discusses these impacts elsewhere in the final rule.
See, e.g., 73 Fed. Reg. at 33,224 (“[N]oise and disturbance
from aircraft and vessel traffic associated with exploration
projects are expected to have relatively localized, short-term
effects.”); id. at 33,227 (“Onshore activities will have the
potential to interact with polar bears mainly during the fall
and ice-covered season when bears come ashore to feed, den,
or travel.”). It is true that the final rule’s “small numbers”
analysis focuses primarily on offshore, open-water explora-
tion activities. However, this focus is not irrational because
the analysis notes that these are the areas “where the majority
of the proposed activities would occur.” 73 Fed. Reg. at
33,234. Moreover, the “small numbers” analysis does refer to
onshore activities, expressly noting that “[w]here terrestrial
activities may occur in coastal areas of Alaska in polar bear
denning habitat, specific mitigation measures will be required
to minimize Industry impacts.” Id. The final rule also
explains, in response to comments:

    [W]e expect industry operations will only interact
    with small numbers of these animals in open water
    habitats. Of course, some of the proposed explor-
    atory activities will occur on land as well. However,
    we have reviewed the proposed activities, both on
    land and at sea, and . . . . [t]his review leads us to
    conclude that, while some incidental take of wal-
    ruses and polar bears is reasonably expected to
9544      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
    occur, these takes will be limited to non-lethal dis-
    turbances, affecting a small number of animals . . . .

Id. at 33,244 (emphasis added).

   [7] Second, Plaintiffs argue that the Service’s “small num-
bers” determination relies on mitigation and monitoring mea-
sures “that are either unproven or that have been shown to be
inadequate.” However, the overall record supports the Ser-
vice’s conclusion that the mitigation and monitoring measures
are effective. The 2008 Chukchi Sea rule notes that “[t]he
mitigation measures associated with the Beaufort Sea inciden-
tal take regulations have proven to minimize human-bear
interactions and will be part of the requirements of future
LOAs associated with the Chukchi Sea incidental take regula-
tions.” Id. at 33,229. The Service’s rule listing the polar bear
as threatened under the ESA notes that the “mitigative regula-
tions” imposed on oil and gas activities “have proven to be
highly successful in providing for polar bear conservation in
Alaska.” 73 Fed. Reg. 28,212, 28,265-66 (May 15, 2008).
Indeed, we implicitly endorsed the Service’s mitigation mea-
sures when we analyzed Plaintiffs’ challenge to the 2006
Beaufort Sea incidental take regulations under NEPA. See
Kempthorne, 588 F.3d at 712 (“[T]he EA provides convincing
reasons to believe that incidental take regulations will amelio-
rate the impact of takes. LOAs include mitigating guidelines
that minimize disturbances to, among other things, denning
females.”). Accordingly, we hold that the Service’s “small
numbers” determination is not arbitrary and capricious.

                           B.   ESA

   Plaintiffs’ arguments under the ESA echo some of their
arguments under the MMPA. First, Plaintiffs fault the Ser-
vice’s BiOp for relying on allegedly unproven and ineffective
mitigation. In its conclusion that the 2008 Chukchi Sea regu-
lations are not likely to jeopardize the continued existence of
the polar bear, the BiOp notes, “Although Industry activities
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR         9545
may adversely affect a small number of polar bears within the
action area, mitigating measures included in the proposed
action reduce the potential for exposure to adverse effects
. . . .” Plaintiffs’ argument fails here for the same reason it
failed under the MMPA: the record supports the Service’s
determination that the mitigation measures are effective.

   Second, Plaintiffs argue that the Incidental Take Statement
(“ITS”) included in the Service’s BiOp fails to comply with
the ESA because it does not provide a numerical limit on the
amount of permissible take or provide an adequate surrogate
measure for such a limit. Because the relevant ESA provisions
differ from those in the MMPA, we briefly review the statu-
tory and regulatory background before addressing this argu-
ment.

   The ESA contains both substantive and procedural require-
ments. Substantively, Section 9 of the ESA prohibits “take”
of endangered species. 16 U.S.C. § 1538(a)(1)(B). The ESA’s
definition of “take” is similarly broad, but slightly different
from, the MMPA’s definition. For example, whereas harass-
ment under the ESA requires a “likelihood of injury to [a
listed species] by annoying it to such an extent as to signifi-
cantly disrupt normal behavioral patterns,” 50 C.F.R. § 17.3
(emphasis added), the MMPA requires only that harassment
have the “potential to injure . . . or . . . disturb a marine mam-
mal . . . by causing disruption of behavioral patterns,” 16
U.S.C. § 1362(18)(A)(i)-(ii) (emphasis added).

  Procedurally, Section 7 of the ESA requires that federal
agencies consult with the Service or NMFS for any agency
action that “may affect” a listed species or its critical habitat.
16 U.S.C. § 1536(a)(2); 50 C.F.R. § 402.14(a). Formal con-
sultation results in a BiOp that determines whether the pro-
posed action is likely to jeopardize the continued existence of
a listed species or adversely modify its critical habitat. 16
U.S.C. § 1536(b)(3)(A); 50 C.F.R. § 402.14(h). If the BiOp
concludes that the action is not likely to jeopardize the spe-
9546      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
cies, but is likely to result in some take, the Service will pro-
vide an ITS along with the BiOp. 50 C.F.R. § 402.14(i). An
ITS specifies the impact (i.e., the “amount or extent”) of the
incidental take on the listed species, contains terms and condi-
tions designed to minimize the impact, and, in the case of
marine mammals, specifies measures that are necessary to
comply with Section 101(a)(5) of the MMPA. 16 U.S.C.
§ 1536(b)(4); 50 C.F.R. § 402.14(i)(1). Take that complies
with the terms and conditions of an ITS is not a prohibited
take under Section 9. 16 U.S.C. § 1536(o)(2); 50 C.F.R.
§ 402.14(i)(5). If the amount or extent of take specified in the
ITS is exceeded, the Service reinitiates Section 7 consultation
to ensure that the “no jeopardy” determination remains valid.
50 C.F.R. §§ 402.14(i)(4), 402.16(a).

             1.   Whether an ITS Was Required

   As a preliminary matter, the Service and Association assert
that the agency was not required to issue an ITS in this case.
The Service, citing Arizona Cattle Growers’ Association v.
U.S. Fish & Wildlife Service, 273 F.3d 1229, 1243 (9th Cir.
2001), argues that an ITS need not have accompanied the
BiOp for the Chukchi Sea incidental take regulations because
it was not “reasonably certain” that take would occur until the
Service issued LOAs. The Service states that it issued an ITS
along with the BiOp in this case “out of an abundance of cau-
tion.” This argument fails for at least two reasons.

   First, Arizona Cattle Growers is inapposite. We held in that
case that the Service could not attach binding conditions on
permittees via an ITS where no listed species were present in
the area and thus the agency “ha[d] no rational basis to con-
clude that a take will occur incident to the otherwise lawful
activity.” 273 F.3d at 1242-44. Here, threatened polar bears
are present in the Chukchi Sea area, and the oil and gas explo-
ration activities are reasonably certain to result in at least
some nonlethal harassment. Indeed, that is the very purpose
of issuing incidental take regulations under the MMPA.
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR        9547
  Second, the Evans court considered and rejected a similar
argument that NMFS did not have to prepare an ITS along
with its BiOp for incidental take regulations until it issued
LOAs. 279 F. Supp. 2d at 1182-83. That court noted that the
ITS provision in Section 7(b)(4) of the ESA specifically refer-
ences Section 101(a)(5) of the MMPA, rather than the MMPA
implementing regulations referring to LOAs, and thus “clearly
contemplates the promulgation of a Final Rule, not letters of
authorization,” as the trigger for producing an ITS. Id. at
1182. We agree with this reasoning.

   The Association argues further that an ITS was not required
in this case because the ESA Section 9 take prohibitions do
not apply here. The Section 9 prohibitions apply expressly to
endangered, rather than threatened, species. 16 U.S.C.
§ 1538(a)(1)(B). For threatened species like the polar bear,
Section 4(d) provides that the Service or NMFS shall promul-
gate regulations that they deem “necessary and advisable to
provide for the conservation of such species,” including, pos-
sibly, applying some or all of the Section 9 prohibitions to the
threatened species. Id. § 1533(d); see also id. § 1538(a)(1)(G)
(making it unlawful for any person to violate regulations pro-
mulgated under Section 4(d) for threatened species). When
the Service listed the polar bear as threatened in 2008, it
issued a Section 4(d) rule that applied most of the Section 9
prohibitions to the polar bear. 73 Fed. Reg. 28,306, 28,306
(May 15, 2008) (interim final rule); 50 C.F.R. § 17.40(q)(1).
However, because the Service concluded that MMPA restric-
tions are at least as protective as those under the ESA, it
exempted from those prohibitions “any activity conducted in
a manner that is consistent with the requirements of the
Marine Mammal Protection Act.” 50 C.F.R. § 17.40(q)(2).
Accordingly, the ITS accompanying the BiOp for the Chukchi
Sea regulations notes that “the activities covered by this con-
sultation are exempt from any take prohibitions that might
otherwise apply under the ESA.”

   [8] Plaintiffs contend that exemption from Section 9 take
liability is irrelevant to the Service’s Section 7 obligations to
9548         CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
prepare a BiOp and ITS. We agree. The ESA requires an ITS
for “the taking of an endangered species or a threatened spe-
cies incidental to the agency action,” 16 U.S.C.
§ 1536(b)(4)(B) (emphasis added), not the prohibited taking.
The polar bear Section 4(d) rule expressly states that
“[n]othing in this special rule affects the issuance or contents
of the biological opinions for polar bears or the issuance of an
incidental take statement, although incidental take resulting
from activities that occur outside of the current range of the
polar bear is not subject to the taking prohibition of the ESA.”
73 Fed. Reg. at 76,252. The Association’s argument fails to
recognize that exemption from Section 9 take liability “is not
the sole purpose of the ITS. If the amount or extent of taking
specified in the ITS is exceeded, reinitiation of formal consul-
tation is required. . . . Thus, the ITS serves as a check on the
agency’s original decision that the incidental take of listed
species resulting from the proposed action will not [jeopardize
the continued existence of the species].” Evans, 279 F. Supp.
2d at 1182. Accordingly, exemption from Section 9 take pro-
hibitions does not negate the separate requirement that the
Service “will provide” an ITS along with its BiOp. 50 C.F.R.
§ 402.14(i)(1).

        2.      Numerical or Surrogate Take in the ITS

   As discussed above, Section 101(a)(5)(A) of the MMPA
does not require that the Service quantify in absolute terms
the number of marine mammals that would be taken pursuant
to incidental take regulations, so long as the agency reason-
ably determines through some other means that the specified
activity would result in take of only “small numbers” of mam-
mals. Legislative history of the MMPA reveals that Congress
recognized that the “small numbers” concept “is not capable
of being expressed in absolute numerical limits.” H.R. Rep.
No. 97-228, reprinted in 1981 U.S.C.C.A.N. at 1469. By con-
trast, the legislative history of the ESA reveals that Congress
“clearly declared a preference for expressing take in numeri-
cal form” with respect to ITSs under Section 7. Or. Natural
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR         9549
Res. Council v. Allen, 476 F.3d 1031, 1037 (9th Cir. 2007)
(citing H.R. Rep. No. 97-567, at 27 (1982), reprinted in 1982
U.S.C.C.A.N. 2807, 2827). Section 7(b)(4) of the ESA
requires that an ITS specify the “impact” of the incidental
take on the listed species. 16 U.S.C. § 1536(b)(4)(C)(i). ESA
implementing regulations clarify that “impact” in this context
means the “amount or extent” of incidental take. 50 C.F.R.
§ 402.14(i)(1)(i). The House Report accompanying Section
7(b)(4) states, “Where possible, the impact should be speci-
fied in terms of a numerical limitation . . . .” H.R. Rep. No.
97-567, at 27, reprinted in 1982 U.S.C.C.A.N. at 2827.

   “Accordingly, we have recognized that the permissible
level of take [in an ITS] ideally should be expressed as a spe-
cific number.” Allen, 476 F.3d at 1037 (citing Ariz. Cattle
Growers, 273 F.3d at 1249); accord Miccosukee Tribe of
Indians of Fla. v. United States, 566 F.3d 1257, 1274-75 (11th
Cir. 2009). However, “while Congress indicated its preference
for a numerical value, it anticipated situations in which [the
amount of take] could not be contemplated in terms of a pre-
cise number.” Ariz. Cattle Growers, 273 F.3d at 1250; see
also H.R. Rep. No. 97-567, at 27, reprinted in 1982
U.S.C.C.A.N. at 2827 (“The Committee does not intend that
the Secretary will, in every instance, interpret the word
‘impact’ to be a precise number. . . . [I]t may not be possible
for the Secretary to specify a number in every instance.”). As
a result, we have held that the Service need not specify
numerical take in an ITS if it establishes “that no such numer-
ical value could be practically obtained.” Ariz. Cattle Grow-
ers, 273 F.3d at 1250. In such circumstances, an ITS may
“utilize[ ] a surrogate instead of a numerical cap on take,” so
long as it “explain[s] why it was impracticable to express a
numerical measure of take.” Allen, 476 F.3d at 1037. The cho-
sen surrogate “must be able to perform the functions of a
numerical limitation” by “set[ting] forth a ‘trigger’ that, when
reached, results in an unacceptable level of incidental take . . .
and requir[es] the parties to re-initiate consultation.” Id. at
1038 (internal quotation marks omitted). The ITS also “must
9550      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
articulate a rational connection between the surrogate and the
taking of the species.” Wild Fish Conservancy v. Salazar, 628
F.3d 513, 531 (9th Cir. 2010) (citing Ariz. Cattle Growers,
273 F.3d at 1250-51).

   In Arizona Cattle Growers, we rejected a surrogate as too
vague because it did not contain measurable guidelines and
thus failed to “provide a clear standard for determining when
the authorized level of take has been exceeded.” 273 F.3d at
1250-51. In Allen, we struck down an ITS that “offer[ed] no
explanation of why the [Service] was unable numerically to
quantify the level of take.” 476 F.3d at 1038. In that case, the
BiOp for timber sales in suitable habitat for the threatened
northern spotted owl noted that owl survey data was out of
date, and that surveys had been discontinued or reduced. Id.
We noted that the Service “never states that it is not possible
to update the survey data in order to estimate the number of
takings, only that it has not actually done the surveys. This
does not establish the numerical measure’s impracticality.” Id.
We also rejected the ITS for using an improper surrogate that
authorized the take of all spotted owls associated with the
project and thus did not set forth an adequate trigger for reini-
tiating consultation. Id. at 1038-39. We explained that “[e]ven
if the actual number of takings of spotted owls that occurred
during the project was considerably higher than anticipated
[in the BiOp], the Incidental Take Statement would not permit
the [Service] to halt the project and reinitiate consultation.”
Id. at 1039.

   Here, the ITS does not specify a numerical measure of the
“amount or extent” of anticipated incidental take. The ITS
states:

    In the accompanying [BiOp], the Service determined
    that total take anticipated as a result of the issuance
    of the Regulations under section 101(a)(5)(A) of the
    MMPA is not likely to result in jeopardy to the polar
    bear. No lethal take is anticipated. While the Service
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR          9551
    cannot anticipate the specific amount or extent of
    other types of take that may result from activities
    that may be authorized under the Regulations until
    they are proposed and the specific activities and
    location is known, the negligible effects finding and
    the small numbers determination articulates the
    anticipated amount of take with respect to effect on
    the population.

Borrowing from a draft of the Chukchi Sea final rule, the Ser-
vice explains elsewhere in the BiOp that

    The dynamic nature of sea ice habitats and its influ-
    ence on the seasonal and annual distribution and
    abundance of polar bears and walruses in the speci-
    fied geographical region (eastern Chukchi Sea), lim-
    its the Service’s ability to provide a priori numerical
    estimates of the number of Pacific walruses and
    polar bears that might potentially be impacted in any
    given year.

The final rule elaborates on this explanation in its response to
comments. See 73 Fed. Reg. at 33,243-44.

   [9] The ITS is not very illuminating regarding the feasibil-
ity of providing a specific numerical estimate of take under
the ESA. The Service at oral argument contended that the
explanation, while short, adequately summarizes the reasons,
described in greater detail in the final rule, why a numerical
measure was impracticable. Although it is a close question,
we conclude that the ITS, as supplemented by the explanation
elsewhere in the BiOp, sufficiently “explain[s] why it was
impracticable to express a numerical measure of take.” Allen,
476 F.3d at 1037. This is not a case, as in Allen, where “the
BiOp offers no explanation of why the [Service] was unable
numerically to quantify the level of take.” Id. at 1038.

   A surrogate measure of take in an ITS “must be able to per-
form the functions of a numerical limitation” by setting forth
9552      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
“a clear standard for determining when the authorized level of
take ha[s] been exceeded.” Id. at 1038-39 (an adequate surro-
gate must contain “measurable guidelines to determine when
incidental take would be exceeded” and “not be so general
that the applicant or the action agency cannot gauge its level
of compliance”). Here, the ITS states that the “negligible
effects finding and the small numbers determination [in the
2008 Chukchi Sea rule] articulates the anticipated amount of
take with respect to effect on the population.” In most circum-
stances, such a statement in an ITS would not serve as an ade-
quate surrogate because it does not specify a clear standard
for determining when the anticipated level of take would be
exceeded. See Ctr. for Biological Diversity v. Bureau of Land
Mgmt., 422 F. Supp. 2d 1115, 1138-39 (N.D. Cal. 2006) (“To
the extent this sentence is meant to act as a surrogate for a
numerical estimate of take, it is too vague and confusing to
act as any meaningful standard upon which compliance with
the ITS can be measured.”). However, given the interplay
between the ESA and MMPA in this case, we conclude that
the ITS, incorporating by reference the 2008 Chukchi Sea
rule, satisfies the requirement that it specify “the impact, i.e.,
the amount or extent,” of the incidental take. 50 C.F.R.
§ 402.14(i)(1)(i).

   We base this conclusion on several factors. First, as dis-
cussed above, a primary purpose of the ITS and its measure
of permissible take is to provide a trigger for reinitiating con-
sultation under Section 7(a)(2) of the ESA. See Allen, 476
F.3d at 1040; Evans, 279 F. Supp. 2d at 1182 (“[T]he ITS
serves as a check on the agency’s original decision that the
incidental take of listed species resulting from the proposed
action will not violate section 7(a)(2) of the ESA.”). However,
the relevant MMPA standard at issue here is more conserva-
tive than the ESA standard. The Service, when promulgating
incidental take regulations under Section 101(a)(5) of the
MMPA, must determine that the specified activity will have
no more than a “negligible impact” on the relevant stock of
polar bears, 16 U.S.C. § 1371(a)(5)(A)(i)(I), whereas the stan-
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR              9553
dard under Section 7(a)(2) of the ESA is whether the agency
action would “jeopardize the continued existence” of the spe-
cies as a whole, id. § 1536(a)(2). As the BiOp explains

    [I]f an action meets the MMPA standard of negligi-
    ble impact . . . , there should be little potential for the
    action to jeopardize the species. . . . It is reasonable
    to expect that a proposed action being independently
    evaluated under the MMPA and the ESA would be
    determined to have more than a negligible impact
    before, and in some cases well before, a jeopardy
    determination would be made.

Thus, so long as the amount and extent of take remains con-
sistent with the Service’s “small numbers” and “negligible
impact” findings in the MMPA incidental take regulations,
there should be no need for reinitiating consultation under the
ESA.

   Second, Section 101(a)(5)(B) of the MMPA provides that
the Service “shall withdraw, or suspend[,]” its incidental take
authorization if the agency finds that take from the specified
activity “is having, or may have, more than a negligible
impact on the species or stock concerned.” Id.
§ 1371(a)(5)(B). The Service highlighted this provision in its
response to comments in the final rule. See 73 Fed. Reg. at
33,240. The “may have[ ] more than a negligible impact”
standard would necessarily operate as a trigger to reinitiate
consultation under the ESA. See 50 C.F.R. § 402.16(b)
(“Reinitiation of formal consultation is required . . . [i]f new
information reveals effects of the action that may affect listed
species or critical habitat in a manner or to an extent not pre-
viously considered.”). Accordingly, this is not a case, as in
Allen, where “[e]ven if the actual number of takings . . . was
considerably higher than anticipated [in the BiOp], the Inci-
dental Take Statement would not permit the [Service] to halt
the project and reinitiate consultation.” 476 F.3d at 1039.
9554      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
   The Service will not always be able to rely on findings
under Section 101(a)(5)(A) of the MMPA to specify the
“amount or extent” of take permitted under the ESA. Such
reliance is permissible only where, as here, the Service first
establishes that it is impracticable to quantify a numerical
measure of take. If the Service fails to establish that a numeri-
cal measure is impracticable, then the ESA requires that the
agency provide a numerical limit in the ITS, even though Sec-
tion 101(a)(5)(A) of the MMPA does not require it in the inci-
dental take regulations themselves. Such reliance is also
permissible only where, as with the polar bear here, the listed
species at issue is also a marine mammal subject to the inci-
dental take regulations under the MMPA. If the specified
activity pursuant to the incidental take regulations could affect
other listed species, like threatened or endangered fish, as in
Evans, 279 F. Supp. 2d at 1180-81, a “small numbers” and
“negligible impact” finding as to marine mammals under the
MMPA would be irrelevant to the Service’s obligations to
protect those other species under the ESA. In such an
instance, the Service could not rely on findings in the MMPA
incidental take regulations to provide a surrogate for the
“amount or extent” of take under the ESA.

   Third, Plaintiffs have failed to articulate a feasible, alterna-
tive surrogate measure of take. Given the nature of the spe-
cies, the geographic region, and the proposed activities at
issue here, we recognize that it may be impossible for the Ser-
vice to develop an adequate surrogate based on other potential
measures, such as habitat or ecological conditions. Here, the
Service is dealing with about 3,500 widely distributed polar
bears that travel thousands of miles per year, a dynamically
changing geographic area of about 90,000 square miles, pro-
posed oil and gas activities without specific locations, and a
type of anticipated take that results in only short-term, mini-
mal changes in behavior. We do not hold that it is generally
a plaintiff’s burden to propose alternative, surrogate measures
of take. But Plaintiffs’ inability to propose such measures here
— even when specifically questioned about it at oral argu-
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR        9555
ment — influences our view of the adequacy of the Service’s
proffered surrogate in the ITS.

   [10] In sum, although it is a close question, we agree with
the Service that “[t]he ITS in this case reasonably relies on the
negligible impact and small numbers findings of the MMPA
incidental take regulation to articulate the anticipated amount
of take and the effect on the polar bear population.”

                          C.   NEPA

   Congress enacted NEPA “to protect the environment by
requiring that federal agencies carefully weigh environmental
considerations and consider potential alternatives . . . before
the government launches any major federal action.” Barnes v.
U.S. Dep’t of Transp., 655 F.3d 1124, 1131 (9th Cir. 2011)
(internal quotation marks omitted). NEPA requires that fed-
eral agencies prepare an Environmental Impact Statement
(“EIS”) for any “major Federal actions significantly affecting
the quality of the human environment.” 42 U.S.C.
§ 4332(2)(C). As a preliminary step, an agency may first pre-
pare a less exhaustive EA, which is a “concise public docu-
ment” that “[b]riefly provide[s] sufficient evidence and
analysis for determining whether to prepare an [EIS].” 40
C.F.R. § 1508.9(a). If the agency concludes in an EA that the
federal action will not have significant environmental
impacts, it may issue a Finding of No Significant Impact
(“FONSI”) in lieu of preparing an EIS. Id. §§ 1508.9(a)(1),
1508.13.

   Plaintiffs here do not challenge the Service’s FONSI or its
decision to prepare an EA instead of an EIS, as they did in
their unsuccessful challenge to the Service’s 2006 Beaufort
Sea incidental take regulations. See Kempthorne, 588 F.3d at
711-12. Instead, Plaintiffs challenge the Service’s EA for the
2008 Chukchi Sea regulations on two grounds: first, that it
fails to consider a reasonable range of alternatives; and sec-
9556      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
ond, that it fails to address the potential impacts of a large oil
spill.

                  1.   Range of Alternatives

   NEPA requires federal agencies to “study, develop, and
describe appropriate alternatives to recommended courses of
action.” 42 U.S.C. § 4332(2)(E). This provision applies
whether an agency is preparing an EIS or an EA. Native Eco-
systems Council v. U.S. Forest Serv., 428 F.3d 1233, 1245
(9th Cir. 2005); see also Bob Marshall Alliance v. Hodel, 852
F.2d 1223, 1228-29 (9th Cir. 1988) (“[C]onsideration of alter-
natives is critical to the goals of NEPA even where a pro-
posed action does not trigger the EIS process.”). However,
“an agency’s obligation to consider alternatives under an EA
is a lesser one than under an EIS.” Native Ecosystems, 428
F.3d at 1246. “[W]hereas with an EIS, an agency is required
to ‘[r]igorously explore and objectively evaluate all reason-
able alternatives,’ see 40 C.F.R. § 1502.14(a), with an EA, an
agency only is required to include a brief discussion of rea-
sonable alternatives. See 40 C.F.R. § 1508.9(b).” N. Idaho
Cmty. Action Network v. U.S. Dep’t of Transp., 545 F.3d
1147, 1153 (9th Cir. 2008) (second alteration in original).

   The Service’s EA in this case analyzes two alternatives: a
no-action alternative and the proposed incidental take regula-
tions. The EA describes the projected impacts of the no-action
alternative as follows:

       If this alternative is implemented, no [incidental
    take regulations] would be issued. Consequently, any
    takes resulting from the proposed exploration activi-
    ties would not be authorized and any incidental takes
    would be a violation of the MMPA. However,
    because the [regulations] do not explicitly permit or
    prohibit oil and gas activities, Industry could con-
    tinue to conduct exploration activities as planned
    without the benefit of mitigation measures proposed
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR        9557
    by the Service. In that event, the Service would have
    no formal means of communicating with Industry or
    have the ability to require monitoring and mitigation
    of specific activities and any form of “take” would
    be a violation of the Act.

   [11] Plaintiffs argue that the no-action alternative fails to
comply with NEPA because it “assumes that industry will act
in bad faith and proceed to take marine mammals in blatant
violation of the law.” That is, Plaintiffs fault the EA for
assuming that oil and gas exploration would continue under
the no-action alternative. However, the EA plainly states that
the MMPA “prohibits Industry from ‘taking’ marine mam-
mals,” and that any incidental take pursuant to the no-action
alternative “would be a violation of the MMPA” for which
“Industry would be liable.” The EA further notes that “be-
cause the [regulations] do not explicitly permit or prohibit oil
and gas activities, Industry could continue to conduct explora-
tion activities.” (Emphasis added.) These are correct state-
ments. As the 2008 final rule explains, the incidental take
regulations “do not authorize, or ‘permit,’ the actual activities
associated with oil and gas exploration in the Chukchi Sea”;
they simply shield the proposed activities from take liability
under the MMPA. 73 Fed. Reg. at 33,213. Other federal and
state agencies are responsible for permitting oil and gas activ-
ities on waters and lands within their jurisdiction. Id.; see
also, e.g., Native Village of Point Hope v. Salazar, 680 F.3d
1123 (9th Cir. 2012) (upholding Bureau of Ocean Energy
Management’s approval of Shell Offshore Inc.’s plan for
exploratory drilling in Beaufort Sea under the Outer Conti-
nental Shelf Lands Act). As we explained in upholding the
Service’s EA for its 2006 Beaufort Sea regulations, “The pur-
pose of the Service’s EA in this context was not to evaluate
the impact of industry on polar bears and Pacific walrus . . .
but rather to evaluate the impact of issuing incidental take
regulations as opposed to permitting industrial activities in the
absence of such regulation.” Kempthorne, 588 F.3d at 706
(alteration and quotation marks omitted). Here, the EA use-
9558      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
fully could have acknowledged that MMPA take liability
would deter industry from pursuing at least some of the explo-
ration activities under the no-action alternative, but its failure
to do so does not make its alternatives analysis arbitrary and
capricious.

   [12] Plaintiffs argue further that even if the no-action alter-
native was appropriately described, the EA fails to analyze
other reasonable alternatives, such as imposing additional mit-
igation measures recommended by Service scientists, or
excluding key habitat areas from the geographic scope of the
regulations. The Service initially considered other action
alternatives, but explains in the EA why it concluded that they
were not feasible. The Service also explains in the 2008 final
rule why the EA did not examine in greater detail some of the
alternatives suggested by Plaintiffs. 73 Fed. Reg. at 33,239.
We have previously upheld EAs that gave detailed consider-
ation to only two alternatives. N. Idaho Cmty., 545 F.3d at
1154 (“[W]e hold that the Agencies fulfilled their obligations
under NEPA’s alternatives provision when they considered
and discussed only two alternatives in the 2005 EA.”); Native
Ecosystems, 428 F.3d at 1246 (“To the extent that Native Eco-
systems is complaining that having only two final alternatives
— no action and a preferred alternative — violates the regula-
tory scheme, a plain reading of the regulations dooms that
argument.”). Because an EA need only include a “brief dis-
cussion[ ]” of reasonable alternatives, 40 C.F.R. § 1508.9(b),
and an agency’s “obligation to consider alternatives under an
EA is a lesser one than under an EIS,” Native Ecosystems,
428 F.3d at 1246, the Service’s alternatives analysis here is
not arbitrary and capricious.

            2.    Potential Impacts of an Oil Spill

   NEPA requires that we determine whether the agency took
a “hard look” at the likely effects of the proposed action.
Native Ecosystems, 428 F.3d at 1239. Taking a “hard look”
includes “considering all foreseeable direct and indirect
          CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR          9559
impacts.” N. Alaska Envtl. Ctr. v. Kempthorne, 457 F.3d 969,
975 (9th Cir. 2006) (internal quotation marks omitted). An
EA also “must fully assess the cumulative impacts of a proj-
ect.” Barnes, 655 F.3d at 1141; Te-Moak Tribe of W. Sho-
shone of Nev. v. U.S. Dep’t of Interior, 608 F.3d 592, 602-03
(9th Cir. 2010).

   [13] Plaintiffs argue that the EA “fails to analyze the sig-
nificant foreseeable impacts of oil spills.” The EA discusses
the possible severe, even lethal, impacts of oil spills on polar
bears, Pacific walruses, and their prey. However, the EA
focuses primarily on the risk of “small operational spills”
because it considers the likelihood of a large spill to be very
low. Plaintiffs point to a comment from the Marine Mammal
Commission, citing a Minerals Management Service
(“MMS”) estimate that the likelihood of a large oil spill in the
Chukchi Sea was somewhere between 33 to 51 percent “over
the life of the development and production activity.” The Ser-
vice discussed this estimate in its rule listing the polar bear,
but explains in the EA that the scope of its analysis was more
narrow because the Chukchi Sea incidental take regulations
cover only exploration activities and only for a period of five
years.

  In its 2008 final rule, the agency explains:

    These regulations are of a finite duration (i.e., five
    years) and authorize incidental take associated with
    specified exploration activities only. The analyses
    did not assess the potential for spills from full-scale
    development and production because that was
    beyond the scope of analysis. . . . In the event of a
    large spill, we would reassess the impacts to the
    polar bear and walrus populations and reconsider the
    appropriateness of authorizations for taking through
    Section 101(a)(5)(A) of the MMPA.

73 Fed. Reg. at 33,246. The final rule cites another MMS esti-
mate that “during exploratory activities, the probability of a
9560      CENTER FOR BIOLOGICAL DIVERSITY v. SALAZAR
large oil spill occurring throughout the duration of these pro-
posed regulations (five years) is very small.” Id. at 33,232.
The EA refers to this same MMS estimate in stating that “the
chance of a large . . . oil spill from exploratory activities in
the Chukchi Sea is very low.” The EA’s failure to mention the
other MMS estimate, regarding the likelihood of a large spill
over the life of development and production activities, is not
arbitrary and capricious given the relatively narrow scope of
the activity contemplated in the incidental take regulations.

                          Conclusion

   [14] Section 101(a)(5)(A) of the MMPA requires the Ser-
vice to determine separately that a specified activity will take
only “small numbers” of marine mammals, and that the take
will have only a “negligible impact” on the species or stock.
We hold that the Service permissibly determined that only
“relatively small numbers” of polar bears and Pacific walruses
would be taken in relation to the size of their larger popula-
tions, because the agency separately determined that the antic-
ipated take would have only a “negligible impact” on the
mammals’ annual rates of recruitment or survival. The “small
numbers” determination was consistent with the statute and
was not arbitrary and capricious. We also hold that the Ser-
vice’s accompanying BiOp and EA comply with the ESA and
NEPA.

  AFFIRMED.
