                  FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

TROUT UNLIMITED; AMERICAN              
RIVERS; PACIFIC RIVERS COUNCIL
WILD STEELHEAD COALITION;
NATIVE FISH SOCIETY; SIERRA CLUB,
               Plaintiffs-Appellees,
                v.
D. ROBERT LOHN, in his official
capacity as Regional Administrator          No. 07-35623
of National Marine Fisheries
Service Northwest Regional
Office; NATIONAL MARINE FISHERIES
                                             D.C. No.
                                           CV-06-00483-JCC
SERVICE,
       Defendants-cross-claimants-
                          Appellees,
BUILDING INDUSTRY ASSOCIATION OF
WASHINGTON; WASHINGTON STATE
FARM BUREAU; COALITION FOR
IDAHO WATER; IDAHO WATER USERS
ASSOCIATION,
 Defendant-intervenors-Appellants.
                                       




                            3261
3262        TROUT UNLIMITED v. BUILDING INDUSTRY



TROUT UNLIMITED; AMERICAN              
RIVERS; PACIFIC RIVERS COUNCIL
WILD STEELHEAD COALITION;
NATIVE FISH SOCIETY; SIERRA CLUB,
               Plaintiffs-Appellees,
                v.
BUILDING INDUSTRY ASSOCIATION OF
WASHINGTON; WASHINGTON STATE                No. 07-35750
FARM BUREAU; COALITION FOR
IDAHO WATER; IDAHO WATER USERS               D.C. No.
                                           CV-06-00483-JCC
ASSOCIATION,
                        Intervenors,          OPINION
D. ROBERT LOHN, in his official
capacity as Regional Administrator
of National Marine Fisheries
Service Northwest Regional
Office; NATIONAL MARINE FISHERIES
SERVICE,
            Defendants-Appellants.
                                       
        Appeal from the United States District Court
          for the Western District of Washington
       John C. Coughenour, District Judge, Presiding

                  Argued and Submitted
          October 20, 2008—Seattle, Washington

                   Filed March 16, 2009

 Before: Diarmuid F. O’Scannlain, Pamela Ann Rymer, and
            Andrew J. Kleinfeld, Circuit Judges.

               Opinion by Judge O’Scannlain
3266         TROUT UNLIMITED v. BUILDING INDUSTRY
                         COUNSEL

Damien M. Schiff, Pacific Legal Foundation, Sacramento,
California, argued the cause for the defendant-intervenors-
appellants. Brian T. Hodges and Sonya D. Jones, Pacific
Legal Foundation, Bellevue, Washington, and James S. Burl-
ing, Pacific Legal Foundation, Sacramento, California, filed
the briefs.

Patti Goldman, Earthjustice, Seattle, Washington, argued the
cause for the plaintiffs-appellees and filed the brief. Jan Has-
selman, Earthjustice, Seattle, Washington, was on the brief.

Ellen J. Durkee and David C. Shilton, Environmental & Natu-
ral Resources Division, U.S. Department of Justice, argued
the cause for the defendants-appellees-appellants and filed the
briefs. Michael Bancroft and Chris McNulty, NOAA Office
of General Counsel, Seattle, Washington, and Ronald J. Ten-
pas, Assistant Attorney General, and Lisa Russell, U.S.
Department of Justice, were on the brief.


                          OPINION

O’SCANNLAIN, Circuit Judge:

   We must decide whether the National Marine Fisheries Ser-
vice may distinguish between natural and hatchery-spawned
salmon and steelhead when determining the level of protec-
tion the fish should be afforded under the Endangered Species
Act.

                               I

                               A

   Pacific Coast salmon are anadromous fish, meaning that
they can survive both in saltwater and in freshwater. The
              TROUT UNLIMITED v. BUILDING INDUSTRY                 3267
salmon hatch out of eggs laid in freshwater rivers and
streams, then migrate often hundreds of miles to the ocean,
where they live for years before returning to their natal
streams to spawn and to die. Steelhead, a closely related spe-
cies, perform the same migration but are able to spawn multi-
ple times. In the Pacific Northwest, anadromous salmon and
steelhead populate the Columbia River and its tributaries,
including the Willamette River, the Snake River, the Oka-
nogan River, and the Yakima River.

   Pacific salmon have a long and turbulent evolutionary his-
tory. Salmon have survived geological disruptions such as the
rotation of the Cascade Mountains, which caused coastal riv-
ers to change their patterns; the most recent ice age, which
covered the present location of Seattle with a sheet of ice
4,000 feet thick; and the warming and frequent floods atten-
dant on the thawing of that glacier. Such natural challenges
have resulted in a set of genetically diverse salmon popula-
tions. Accordingly, salmon populations can vary greatly even
if geographically close, depending on their adaptations to con-
ditions in the natal stream.

    Human development in the Pacific Northwest has long
threatened many salmon and steelhead species with extinction.1
“[F]orestry, agricultural, mining, and urbanization activities
. . . have resulted in the loss, degradation, simplification, and
fragmentation of habitat.” Final Listing Determinations for 10
Distinct Population Segments of West Coast Steelhead, 71
Fed. Reg. 834, 856 (Jan. 5, 2006). In particular, “logging,
road construction, [and] urban development” have caused
“declines in [steelhead populations] in the past several dec-
ades.” Id. These declines have caused concern to environmen-
tal organizations and fisheries alike.
  1
   In this opinion, we use the term “species” to include “any subspecies
of fish or wildlife or plants, and any distinct population segment of any
species of vertebrate fish or wildlife which interbreeds when mature.” 16
U.S.C. § 1532(16).
3268         TROUT UNLIMITED v. BUILDING INDUSTRY
   To compensate for reduced natural salmon populations,
“extensive hatchery programs have been implemented
throughout . . . the West Coast.” Id. at 857. Such programs
artificially increase salmon abundance by capturing and kill-
ing returning adult females, harvesting their eggs, and fertiliz-
ing them with the sperm of returning adult males. After being
kept in the hatchery during their youth, hatchery salmon are
released into the wild, where most complete the same migra-
tion to and from the ocean as natural salmon do. After hatch-
ery salmon return to their natal stream, they are killed and the
assisted fertilization process is repeated. Not all hatchery fish
return to the hatchery, however; some stray from the hatchery
to mate and spawn in the wild.

   Hatchery programs generally have two goals which can
conflict with one another: to increase the number of salmon
available for fishing, and to prevent natural salmon from
becoming extinct. “While some of the programs . . . have
been successful in providing fishing opportunities, many such
programs have posed risks to the genetic diversity and long-
term reproductive fitness of local natural steelhead popula-
tions.” Id. The risks hatchery programs pose to natural fish
include:

    excessive mortality of natural steelhead in fisheries
    targeting hatchery-origin steelhead; competition for
    prey and habitat; predation by hatchery-origin fish
    on younger natural fish; genetic introgression by
    hatchery-origin fish that spawn naturally and inter-
    breed with local natural populations; disease trans-
    mission; degraded water quality and quantity, and
    impediments to fish passage imposed by hatchery
    facilities.

Id. Interbreeding poses particular risks to natural salmon pop-
ulations because it can result in decreased genetic differentia-
tion. On the other hand, “the use of conservation hatcheries
may play an important role, under appropriate circumstances,
             TROUT UNLIMITED v. BUILDING INDUSTRY           3269
in reestablishing depressed West Coast [salmon and] steel-
head stocks.” Id.; see also Proposed Listing Determinations
for 27 ESUs of West Coast Salmonids, 69 Fed. Reg. 33,102,
33,142 (June 14, 2004).

                               B

   Congress enacted the Endangered Species Act (“ESA”) in
1973 “to provide a means whereby the ecosystems upon
which endangered species and threatened species depend may
be conserved.” 16 U.S.C. § 1531(b). The ESA’s “primary pur-
pose . . . is to prevent animal and plant species endangerment
and extinction caused by man’s influence on ecosystems, and
to return the species to the point where they are viable compo-
nents of their ecosystems.” H.R. Rep. No. 95-1625, at 5
(1978), reprinted in 1978 U.S.C.C.A.N. 9453, 9455.

   As part of this mandate, the ESA requires the National
Marine Fisheries Service (“NMFS”) to do three things that are
at issue in these appeals. First, NMFS must decide whether a
population of fish or wildlife constitutes a “species” or a “dis-
tinct population segment” within the meaning of the ESA.
The ESA defines “species” to include “any subspecies of fish
or wildlife or plants, and any distinct population segment of
any species of vertebrate fish or wildlife which interbreeds
when mature. ” 16 U.S.C. § 1532(16) (emphasis added). “The
ability to designate and list [distinct population segments]
allows the [agency] to provide different levels of protection to
different populations of the same species.” Nat’l Ass’n of
Home Builders v. Norton, 340 F.3d 835, 842 (9th Cir. 2003).
The ESA does not define the term “distinct population seg-
ment.”

   Second, after deciding whether a population of fish or wild-
life constitutes a “species” or a “distinct population segment,”
NMFS must decide whether to “list” the species or distinct
population segment. A species or distinct population segment
may be listed as either “endangered” or “threatened.” 16
3270           TROUT UNLIMITED v. BUILDING INDUSTRY
U.S.C.§ 1533(a)(1). An “endangered” species “is in danger of
extinction throughout all or a significant portion of its range.”
Id. § 1532(6). A “threatened” species “is likely to become an
endangered species within the foreseeable future.” Id.
§ 1532(20). A species may be considered “threatened” or “en-
dangered” because of “(A) the present or threatened destruc-
tion, modification, or curtailment of its habitat or range; (B)
overutilization for commercial, recreational, scientific, or edu-
cational purposes; (C) disease or predation; (D) the inade-
quacy of existing regulatory mechanisms; or (E) other natural
or manmade factors affecting its continued existence.” Id.
§ 1533(a)(1)(A)-(E). The ultimate listing determinations must
be based “solely on . . . the best scientific and commercial
data available after conducting a review of the status of the
species.” Id. § 1533(b)(1)(A).

   Third, if NMFS decides to list a species or a distinct popu-
lation segment as “endangered” or “threatened,” it must
accord the species or the distinct population segment various
legal protections. For example, an endangered species may
not be “take[n],” meaning that no one may “harass, harm, pur-
sue, hunt, shoot, wound, kill, trap, capture, or collect” it. Id.
§§ 1538(a)(1)(B), 1532(19). A threatened species, on the
other hand, is subject to NMFS’s discretionary protection. See
id. § 1533(d) (“Whenever any species is listed as a threatened
species . . . the Secretary shall issue such regulations as he
deems necessary and advisable to provide for the conservation
of such species.”). “Conservation” measures “may include
regulated taking” in “the extraordinary case where population
pressures within a given ecosystem cannot be otherwise
relieved.” Id. § 1532(3).2
  2
    Regulations regarding taking are referred to as “§ 4(d) regulations,” a
reference to the original section in the ESA.
             TROUT UNLIMITED v. BUILDING INDUSTRY          3271
                               C

   Over the past two decades, NMFS has adopted regulations
applying the ESA’s mandates to salmon and steelhead popula-
tions in the Pacific Northwest. These regulations have been
modified over the years in response to scientific evidence and
to legal challenges. To provide context for the present
appeals, we describe how the modern regulations at issue here
developed.

   In 1991, NMFS issued a Policy Statement defining a “dis-
tinct population segment” (“DPS”), and hence a “species”
under the ESA, as an “evolutionarily significant unit”
(“ESU”) “of the biological species.” Policy on Applying the
Definition of Species, 56 Fed. Reg. 58,612, 58,618 (Nov. 20,
1991) (the “ESU Policy”). An ESU “must satisfy two criteria
. . . (1) It must be substantially reproductively isolated from
other conspecific population units; and (2) It must represent
an important component in the evolutionary legacy of the spe-
cies.” Id. The first criterion—reproductive isolation—is based
upon “movements of tagged fish, recolonization rates of other
populations, measurements of genetic differences between
populations, and evaluations of the efficacy of natural barri-
ers.” Id. The second criterion—the population’s contribution
to the evolutionary legacy of the species—is based upon “the
ecological/genetic diversity of the species as a whole. In other
words, if the population became extinct, would this event rep-
resent a significant loss to the ecological/genetic diversity of
the species?” Id.

   Five years later, NMFS and the U.S. Fish and Wildlife Ser-
vice adopted a joint policy which slightly modified the three
factors to be considered in a listing decision for a “distinct
population segment.” The joint policy (the “1996 DPS poli-
cy”) describes these factors as (1) the “[d]iscreteness of the
population segment in relation to the remainder of the species
to which it belongs,” (2) “[t]he significance of the population
segment to the species to which it belongs,” and (3) “[t]he
3272         TROUT UNLIMITED v. BUILDING INDUSTRY
population segment’s conservation status in relation to the
[ESA’s] standards for listing (i.e., is the population segment,
when treated as if it were a species, endangered or threat-
ened?).” Policy Regarding the Recognition of Distinct Verte-
brate Population Segments, 61 Fed. Reg. 4,722, 4,725 (Feb.
7, 1996). NMFS applies the joint policy to steelhead popula-
tions but the original 1991 policy to Pacific salmon popula-
tions. Id. at 4,722.

   In 1993, NMFS concluded that hatchery fish could be part
of the same ESU as natural fish. The agency issued an Interim
Hatchery Policy which reasoned that “[g]enetic resources
important to the species’ evolutionary legacy may reside in
hatchery fish as well as in natural fish.” Interim Policy on
Artificial Propagation of Pacific Salmon, 58 Fed. Reg.
17,573, 17,574 (Apr. 5, 1993). Hatchery fish otherwise meet-
ing the two criteria for a natural population’s ESU would nev-
ertheless be excluded from that ESU, and not included in the
listed species, if information indicated that:

    (1) the hatchery population in question is of a differ-
    ent genetic lineage than the listed natural popula-
    tions, (2) artificial propagation has produced
    appreciable changes in the hatchery population in
    characteristics that are believed to have a genetic
    basis, or (3) there is substantial uncertainty about the
    relationship between existing hatchery fish and the
    natural population.

Id. at 15,575.

   Although hatchery fish could be part of the same ESU as
natural fish, NMFS decided that, absent exceptional circum-
stances, only natural fish could be listed as endangered or
threatened. See id. at 17,575 (“In general, [hatchery] fish will
not be included as part of the listed species.”). NMFS decided
that only those hatchery fish “considered to be essential for
recovery [of the natural population]” could be listed alongside
               TROUT UNLIMITED v. BUILDING INDUSTRY                     3273
the natural fish.3 Id. According to the Interim Hatchery Policy,
hatchery fish might be considered “essential to recovery” if
“the natural population faces a high, short-term risk of extinc-
tion, or if the hatchery population is believed to contain a sub-
stantial proportion of the genetic diversity remaining in the
species.” Id.

   The Interim Hatchery Policy lasted only until 2001, when
Alsea Valley Alliance (“Alsea Valley”), the appellant in a
companion case,4 successfully challenged NMFS’s decision to
distinguish between natural and hatchery fish for listing pur-
poses, after finding both to be within the same ESU. Applying
the Interim Hatchery Policy, NMFS had included nine hatch-
ery populations of Oregon coast coho salmon within the same
ESU as natural coho salmon, but had listed only the natural
portion of the ESU as threatened. See 50 C.F.R. § 227.4
(1999); 63 Fed. Reg. 42,587, 42589 (Aug. 10, 1998). Alsea
Valley challenged that distinction in the United States District
Court for the District of Oregon, arguing that “the ESA does
not allow the Secretary to make listing distinctions below that
of species, subspecies or a distinct population segment of a
species.” Alsea Valley Alliance v. Evans (Alsea I), 161 F.
Supp. 2d 1154, 1161 (D. Or. 2001).

   The district court agreed with Alsea Valley, concluding that
“NMFS may consider listing only an entire species, subspe-
cies or distinct population segment.” Id. at 1162. Because
NMFS had placed the hatchery coho and the natural coho in
the same ESU, the district court reasoned, NMFS was
required to list both hatchery and natural coho as endangered,
  3
     The offspring of artificially propagated salmon that are born in the wild
are counted as natural salmon. See 58 Fed. Reg. at 17,575 (“Under any
scenario, progeny of fish from the listed species that are propagated artifi-
cially are considered part of the listed species and are protected under the
ESA.”)
   4
     Alsea Valley’s claims are addressed in a memorandum disposition
filed concurrently with this opinion. See Alsea Valley Alliance v. Lautenb-
acher, No. 07-35824 (9th Cir. 2009).
3274         TROUT UNLIMITED v. BUILDING INDUSTRY
or neither hatchery nor natural coho as endangered. Accord-
ingly, the district court struck down the Oregon coast coho
listing as arbitrary and capricious.

   Rather than appeal the district court’s decision, NMFS
revised its Interim Hatchery Policy to eliminate the distinction
between natural and hatchery fish in listing determinations.
After 162 days of public comment, NMFS issued a Final
Hatchery Listing Policy on June 28, 2005. See Policy on the
Consideration of Hatchery-Origin Fish, 70 Fed. Reg. 37,204
(June 28, 2005). That policy is a central subject of the present
appeals.

   The 2005 Hatchery Listing Policy reaffirms that hatchery
fish may be part of the same ESU as natural fish, but alters
NMFS’s listing practices in several ways. To comply with
Alsea I, the Hatchery Listing Policy provides that hatchery
fish that are part of the same ESU as natural fish “will be
included in any listing of the ESU.” Id. at 37,215. In addition,
the Hatchery Listing Policy requires NMFS to consider the
status of the ESU as a whole rather than the status of only the
natural fish within the ESU when determining whether an
ESU should be listed as endangered or threatened. Id. How-
ever, under the policy, a listing determination still places pri-
mary importance on the viability of natural, self-sustaining
populations, providing that “[h]atchery fish will be included
in assessing an ESU’s status in the context of their contribu-
tions to conserving natural self-sustaining populations.” Id.
The Hatchery Listing Policy also requires status determina-
tions to be based upon abundance, productivity, genetic diver-
sity, and spatial distribution of the ESU. Id. Noting that
hatchery fish can be both helpful and harmful to natural fish,
the policy also allows NMFS to use its discretionary authority
via § 4(d) regulations to provide for the take of certain hatch-
ery fish, even if the ESU to which they belong is listed as
threatened. Id. at 37,215-16.
               TROUT UNLIMITED v. BUILDING INDUSTRY                   3275
   Around this time, NMFS issued § 4(d) regulations with
respect to the taking of ESUs deemed threatened. Under the
regulations, naturally spawned and hatchery salmon with
intact adipose fins may not be taken. See Endangered and
Threatened Species: Final Listing Determinations for 16
ESUs of West Coast Salmon, 70 Fed. Reg. 37,160, 37,194-95
(June 28, 2005). On the other hand, hatchery fish with clipped
adipose fins may be taken.5 Id.

                                    D

   These appeals primarily involve NMFS’s decision, in
accordance with the 2005 Hatchery Listing Policy, to down-
list a population of Upper Columbia River steelhead from
endangered to threatened.

   The Upper Columbia River steelhead is an inland steelhead
ESU in the Columbia River Basin upstream from the Yakima
River, Washington to the United States-Canada border, which
suffered major population declines due to dam construction in
the 1930s and 1940s. During the following decades, this steel-
head ESU became homogenized due to hatchery practices and
from the proliferation of hatchery fish. By 1997, NMFS had
listed the steelhead as endangered based on low abundance,
both in absolute numbers and in relation to the numbers of
hatchery fish sharing the habitat. Listing of Several Evolutio-
narily Significant Units (ESUs) of West Coast Steelhead, 62
Fed. Reg. 43,937 (Aug. 18, 1997). In the 1997 “endangered”
listing, NMFS referenced the effect of hatchery and harvest
management as factors affecting the endangerment of the
ESU.

  In 2004, however, NMFS added hatchery populations to the
Upper Columbia River steelhead ESU. Applying the 1993
  5
    The regulations’ purpose is to allow the management of the hatchery
fish population. Prior to release, hatcheries may clip adipose fins to indi-
cate such fish may be taken. See supra note 1.
3276        TROUT UNLIMITED v. BUILDING INDUSTRY
ESU Policy, which focuses on “reproductive isolation” and
the species’ “evolutionary legacy,” NMFS created a ESU
composed of natural steelhead and various stocks of hatchery
steelhead. 69 Fed. Reg. at 33,102. To determine which hatch-
ery fish to include in the ESU, NMFS relied on a Salmon and
Steelhead Hatchery Assessment Group which used available
information to assess the relatedness of each hatchery stock to
the natural population on the basis of stock origin and the
degree of genetic divergence between the hatchery stock and
the natural population. Id. at 33,111. The result was a modi-
fied Upper Columbia River steelhead ESU that included six
hatchery stocks as well as resident rainbow trout populations
that co-occur with the anadromous populations. NMFS
rejected petitions filed by Trout Unlimited seeking to split
natural and hatchery fish into separate ESUs. Then, partly
because the modified ESU contained hatchery fish as well as
natural fish, NMFS downlisted the Upper Columbia River
steelhead from “endangered” to “threatened.”

   Trout Unlimited and other environmental conservation
organizations subsequently brought this action. They chal-
lenge (1) NMFS’s rejection of Trout Unlimited’s petitions to
separate natural fish and hatchery fish into different ESUs,
and (2) the downlisting of the Upper Columbia River steel-
head ESU from “endangered” to “threatened.” As part of its
second claim, Trout Unlimited argues that the 2005 Hatchery
Listing Policy impermissibly requires NMFS to consider the
status of the entire ESU rather than just the natural compo-
nents of the ESU when making listing determinations. Trout
Unlimited maintains that both NMFS decisions are arbitrary
and capricious and thus unlawful under the Administrative
Procedure Act and that both fail to employ “the best scientific
and commercial data available” in violation of the ESA. 16
U.S.C. § 1533(b)(1)(A).

  The Building Industry Association of Washington and
other trade associations (the “Building Industry”) intervened,
challenging NMFS’s listing policies on opposite grounds.
             TROUT UNLIMITED v. BUILDING INDUSTRY          3277
Relying on Alsea I, the Building Industry’s core claim is that
the ESA does not allow NMFS to make any distinctions
between hatchery fish and natural fish once NMFS has
included them in the same ESU. In particular, the Building
Industry challenges (3) NMFS’s policy, when making listing
determinations, to assess hatchery fish “in the context of their
contributions to conserving natural self-sustaining popula-
tions.” The Building Industry also objects to (4) NMFS’s
decision to prohibit the “take” of only natural fish and hatch-
ery fish with intact adipose fins. The Building Industry’s chal-
lenge does not involve the initial question (raised by Trout
Unlimited) of whether natural and hatchery fish can belong to
the same ESU. The Building Industry’s basic contention is
that once NMFS defines an ESU, it may not further distin-
guish between members of that ESU when making its listing
determinations.

   All parties cross-moved for summary judgment on all
claims. The district court granted summary judgment (1) to
NMFS on Trout Unlimited’s claim that NMFS impermissibly
included natural fish and hatchery fish as part of the same
ESU, (2) to Trout Unlimited on its claim that the Hatchery
Listing Policy and the corresponding downlisting of the
Upper Columbia River steelhead ESU from “endangered” to
“threatened” violated the ESA, (3) to NMFS on the Building
Industry’s challenge to NMFS’s policy, when making listing
determinations, to assess hatchery fish “in the context of their
contributions to conserving natural self-sustaining popula-
tions,” and (4) to NMFS on the Building Industry’s claim that
NMFS’s decision to prohibit the “take” of only natural fish
and hatchery fish with intact adipose fins violated the ESA.
These timely appeals followed.

                               II

  Before turning to the merits of Trout Unlimited’s claims,
we must resolve a disputed threshold issue: whether the
Hatchery Listing Policy is entitled to deference under Chev-
3278           TROUT UNLIMITED v. BUILDING INDUSTRY
ron v. Natural Resources Defense Council, 467 U.S. 837
(1984).

   We defer to the “administrative implementation of a partic-
ular statutory provision . . . when it appears that Congress del-
egated authority to the agency generally to make rules
carrying the force of law, and that the agency interpretation
claiming deference was promulgated in the exercise of that
authority.” United States v. Mead Corp., 533 U.S. 218,
226-27 (2001). “Delegation of such authority may be shown
in a variety of ways, as by an agency’s power to engage in
adjudication or notice-and-comment rulemaking, or by some
other indication of a comparable congressional intent.” Id. at
227.6

   If these conditions are met, we then ask “whether Congress
has directly spoken to the precise question at issue.” Chevron,
467 U.S. at 842. “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 Con-
gress.” Id. at 842-43. If the statutory provision at issue is sus-
ceptible to multiple interpretations, “the question for the court
is whether the agency’s answer is based on a permissible con-
struction of the statute.” Id. at 843.

   We have previously held that NMFS’s 1996 policy inter-
preting the statutory term “distinct population segment,” see
supra at 3271, is entitled to Chevron deference. See Nw.
Ecosys. Alliance v. U.S. Fish & Wildlife Serv., 475 F.3d 1136
(9th Cir. 2007). There, we pointed out that Congress “ex-
pressly delegated authority to the Service to develop criteria
  6
    If Congress has not delegated interpretive authority or if the agency’s
action is not taken pursuant to that interpretive authority, the agency’s
views are still entitled to deference depending on “the degree of the agen-
cy’s care, its consistency, formality, and relative expertness, and . . . the
persuasiveness of the agency’s position.” Mead, 533 U.S. at 228 (internal
footnotes omitted).
               TROUT UNLIMITED v. BUILDING INDUSTRY                  3279
for evaluating petitions to list endangered species. Under 16
U.S.C. § 1533(h)(2), the Service is required to publish, in the
Federal Register, guidelines on ‘criteria for making findings
. . . with respect to petitions.’ ” Id. at 1141-42. We also
emphasized that the formal process required by § 1533(h)
weighs in favor of according Chevron deference: NMFS must
“provide to the public notice of, and opportunity to submit
written comments on, any guideline . . . proposed to be estab-
lished under this subsection.” Id. at 1142 (internal citation and
quotation marks omitted).

   Northwest Ecosystem is instructive. Here, as there, Con-
gress delegated authority to NMFS to “make rules carrying
the force of law.” See 16 U.S.C § 1533(h) (“The Secretary
shall establish, and publish in the Federal Register, agency
guidelines to insure that the purposes of this section are
achieved efficiently and effectively.”). Furthermore, the
Hatchery Listing Policy at issue in these appeals went through
the same formal notice-and-comment process as the 1996
DPS policy. We are satisfied that the Hatchery Listing Policy
is entitled to Chevron deference.7

   We review NMFS’s rejection of Trout Unlimited’s peti-
tions to split natural and hatchery fish into separate ESUs as
well as the downlisting of the Upper Columbia River steel-
head under the “arbitrary and capricious” standard. See 5
U.S.C. § 706(2)(A) (“[A]gency action . . . found to be . . .
arbitrary, capricious, an abuse of discretion, or otherwise not
in accordance with law” must be set aside). In applying this
  7
   NMFS argues that the Building Industry’s claims are not justiciable
because the Building Industry is challenging the Hatchery Listing Policy
on its face rather than as-applied. In response, however, the Building
Industry clarifies that it also is challenging the downlisting of the Upper
Columbia River steelhead ESU, which is a concrete implementation of the
policy.
  In passing, NMFS also contends that the Building Industry lacks stand-
ing. In response, the Building Industry points to allegations in its cross-
complaint which, we are satisfied, establish standing.
3280         TROUT UNLIMITED v. BUILDING INDUSTRY
standard, we defer to the informed exercise of agency discre-
tion, especially where that discretion is exercised in an area
where the agency has special “technical expertise.” Marsh v.
Or. Natural Res. Council, 490 U.S. 360, 377 (1989) (internal
quotation marks and citation omitted); see also The Lands
Council v. McNair, 537 F.3d 981, 993 (9th Cir. 2008) (en
banc); Greenpeace Action v. Franklin, 14 F.3d 1324, 1330-32
(9th Cir. 1992).

                                III

   Keeping the appropriate standard of review in mind, we
turn to Trout Unlimited’s contention that NMFS improperly
rejected its petitions to split natural and hatchery fish into sep-
arate ESUs. According to Trout Unlimited, lumping natural
and hatchery fish together is contrary to the “best available
science” because hatchery fish pose threats to wild fish: “[n]o
hatchery has ever been shown to promote the long-term
recovery of wild salmon, and countless studies document the
harm that hatcheries have caused to wild populations.” Trout
Unlimited also argues that “NMFS ignored scientific evidence
that hatchery fish exhibit important differences from wild
fish.” According to Trout Unlimited, “hatchery stocks are
functionally distinct and reproductively isolated from natu-
rally spawned populations.” 69 Fed. Reg. at 33,112. It con-
cludes that NMFS should have separated natural salmon and
hatchery salmon into different ESUs in order to advance the
ESA’s main goal of conserving natural populations.

   [1] We disagree. In our view, Trout Unlimited’s emphasis
on the threats hatchery fish pose to natural fish collapses two
analytically distinct phases of agency action: the initial deci-
sion regarding the composition of the ESU, and the subse-
quent decision whether to list the ESU. The composition
phase is not influenced by whether hatchery fish threaten wild
fish; it is concerned only with the “neutral” task of defining
a species. See ESU Policy, 56 Fed. Reg. at 58,618; see also
70 Fed. Reg. at 37,215. In contrast, the second phase—the
             TROUT UNLIMITED v. BUILDING INDUSTRY           3281
listing decision—takes into account the effects, positive or
negative, of hatchery salmon on natural fish to determine
whether the ESU is endangered or threatened. Trout Unlimit-
ed’s argument regarding the threats posed by hatchery fish
should be directed at NMFS’s listing decisions rather than at
its definitional decisions.

   Indeed, NMFS recognized this distinction when it denied
Trout Unlimited’s petitions to split natural and hatchery fish
into separate ESUs:

    NMFS recognizes that artificial propagation under
    certain circumstances can pose threats to natural
    populations. However, it is not appropriate to
    include a consideration of the threats faced by an
    ESU (such as any risks posed by artificial propaga-
    tion) when determining what constitutes a species
    under the ESA. Rather, such an evaluation of threats
    is conducted after the “species” has been defined,
    and the likelihood of extinction for the defined spe-
    cies is being assessed.

69 Fed. Reg. at 33,112.

   [2] To the extent that Trout Unlimited contends that the
“definitional” phase should be influenced by the threats pres-
ented by hatchery salmon, we see nothing in the ESA that
requires such a modification to NMFS’s policies. NMFS has
not abdicated its statutory duty to consider the effects of
hatchery salmon on natural populations; it merely conducts
that inquiry at a different stage. Indeed, the ESA itself implies
that NMFS should evaluate the impact of hatchery fish at the
listing stage rather than at the definitional phase. See 16
U.S.C. § 1533(a)(1) (“The Secretary shall . . . determine
whether any species is an endangered species or a threatened
species [based upon] natural or manmade factors [presumably
including manmade hatcheries] affecting its continued exis-
tence.”). NMFS’s policies on this point are therefore a “per-
3282         TROUT UNLIMITED v. BUILDING INDUSTRY
missible construction” of the ESA and entitled to Chevron
deference. Chevron, 467 U.S. at 843.

  Nor are we persuaded that NMFS ignored scientific evi-
dence regarding the genetic and behavioral differences
between natural and hatchery fish. NMFS gave a reasoned
explanation for rejecting Trout Unlimited’s petitions, relying
on its own science to conclude that:

    [T]he petitioners’ argument that hatchery stocks are
    functionally distinct and reproductively isolated
    from naturally spawned populations is unsubstan-
    tiated. The derivation of hatchery stocks from local
    natural populations and the established practice of
    incorporating natural fish as hatchery broodstock
    results in hatchery and natural populations that share
    the same evolutionary genetic and ecological leg-
    acy. . . . The shared evolutionary legacy of certain
    hatchery stocks with natural populations does not
    support the exclusion of these hatchery stocks from
    ESUs containing natural fish.

69 Fed. Reg. at 33,112. NMFS supported its decision by refer-
encing the two expert reports that formed the basis of its 2004
proposed listing determinations.

   We will not second-guess NMFS’s resolution of this scien-
tific question. See Marsh, 490 U.S. at 377 (“Because analysis
of the relevant documents requires a high level of technical
expertise, we must defer to the informed discretion of the
responsible federal agencies.” (internal quotation marks and
citation omitted)). Nothing in the record suggests that
NMFS’s decision was uninformed or was arbitrary and capri-
cious. Trout Unlimited and NMFS are engaged in a good faith
disagreement that is supported by science on both sides;
indeed, the amicus brief filed in this case argues that there is
no scientific consensus concerning the relationship between
               TROUT UNLIMITED v. BUILDING INDUSTRY                   3283
hatchery and natural fish. In such situations, we stay our hand.8
See The Lands Council, 537 F.3d at 993.

   [3] Accordingly, we agree with the district court’s conclu-
sion that the denial of Trout Unlimited’s petitions to split nat-
ural and hatchery fish into separate ESUs was not arbitrary
and capricious.9

                                    IV

   We turn now to Trout Unlimited’s claim that NMFS’s list-
ing policies, which resulted in the downlisting of the Upper
Columbia River steelhead, violate the ESA because they base
listing determinations on the status of the entire ESU (includ-
ing hatchery fish) rather than the status of only the natural fish
within the ESU. According to Trout Unlimited, basing listing
determinations on the status of the entire ESU is arbitrary and
capricious because the ESU’s central purpose is to preserve
natural populations rather than artificial ones. Trout Unlim-
ited maintains that hatchery fish can give the misleading
appearance of a well-stocked fishery when in fact the natural
fish are endangered, resulting in improper downlistings. Trout
Unlimited suggests that that is precisely what happened when
  8
     In addition, although Trout Unlimited does not explicitly challenge the
1991 and 1993 ESU policies, we note that those policies are subject to
Chevron deference because they interpret the statutory terms “species”
and “distinct population segment.” NMFS’s 1991 and 1993 ESU policies
are plainly “based on a permissible construction of the statute.” Chevron,
467 U.S. at 843; cf. Nw. Ecosystem, 475 F.3d at 1140-43 (holding that
NMFS’s 1996 DPS policy was entitled to Chevron deference).
   9
     Trout Unlimited also contends that because NMFS must list all compo-
nents of an ESU as endangered or threatened, including hatchery fish in
the same ESU as natural fish “creat[es] the anomaly of affording ESA pro-
tection to the very hatchery fish that pose a threat to wild salmon viabili-
ty.” This argument ignores the positive impact NMFS found hatchery fish
could have on natural populations. In addition, although NMFS lists all
components of a ESU, it protects only those hatchery fish with an intact
adipose fin. 69 Fed. Reg. at 33,167-68.
3284         TROUT UNLIMITED v. BUILDING INDUSTRY
NMFS downlisted the Upper Columbia River steelhead from
“endangered” to “threatened.”

   The district court agreed with Trout Unlimited, holding that
the ESA is primarily concerned with preserving natural popu-
lations. The court thought that “the record does not support a
conclusion that an assessment of the status of an entire ESU
is an appropriate proxy for assessing the status of natural pop-
ulations.” Accordingly, the district court concluded that list-
ing determinations must be based on the status of only the
natural components of the ESU.

   [4] We agree with Trout Unlimited, with NMFS, and with
the district court that the ESA’s primary goal is to preserve
the ability of natural populations to survive in the wild. As the
district court put it, “[t]hat the purpose of the ESA is to pro-
mote populations that are self-sustaining without human inter-
ference can be deduced from the statute’s emphasis on the
protection and preservation of the habitats of endangered and
threatened species.” See, e.g., 16 U.S.C. § 1531(b) (“The pur-
poses of this [Act] are to provide a means whereby the eco-
systems upon which endangered species and threatened
species depend may be conserved, [and] to provide a program
for the conservation of such endangered species and threat-
ened species . . . .” ). The statute mentions artificial propaga-
tion merely as a means “to bring any endangered species or
threatened species to the point at which the measures pro-
vided pursuant to this [Act] are no longer necessary.” 16
U.S.C. § 1532(3). The ESA’s legislative history also confirms
that the ESA is primarily focused on natural populations. See
H.R. Rep. No. 95-1625, at 5, reprinted in 1978 U.S.C.C.A.N.
at 9455.

   [5] Despite Trout Unlimited’s assertions, however, we are
satisfied that the Hatchery Listing Policy is consistent with
both the plain language of the ESA and with the statutory goal
of preserving natural populations. The ESA requires NMFS to
“determine whether any species is an endangered species or
             TROUT UNLIMITED v. BUILDING INDUSTRY           3285
a threatened species.” 16 U.S.C. § 1533(a)(1) (emphasis
added). A species, in turn, includes “any subspecies of fish or
wildlife or plants, and any distinct population segment.” Id.
§ 1532(16). Consistent with the plain language of the statute,
the Hatchery Listing Policy conducts a status review of the
entire “species”—no more, and no less.

   [6] We are also persuaded that the Hatchery Listing Policy
does not flout the statutory goal of preserving natural popula-
tions. Trout Unlimited casts the policy in an overly simplistic
light, suggesting that, under the policy, an abundance of
hatchery fish can transform an endangered species into a
threatened or an unlisted one, even when the hatchery fish
actually harm the natural fish. In fact, the policy does not
operate in that simplistic way. Rather, it mandates a more
complex evaluation process that considers both the positive
and the negative effects of hatchery fish on the viability of
natural populations. Indeed, the policy explicitly states that
“NMFS will apply this policy in support of the conservation
of naturally-spawning salmon and the ecosystems upon which
they depend.” 70 Fed. Reg. at 37,215. The policy further
states that “[h]atchery fish will be included in assessing an
ESU’s status in the context of their contributions to conserv-
ing natural self-sustaining populations.” Id.

  The policy lists a number of ways in which hatchery fish
can positively or negatively affect the status of an entire ESU:

    The presence of hatchery fish within the ESU can
    positively affect the overall status of the ESU, and
    thereby affect a listing determination, by contribut-
    ing to increasing abundance and productivity of the
    natural populations in the ESU, by improving spatial
    distribution, by serving as a source population for
    repopulating unoccupied habitat, and by conserving
    genetic resources of depressed natural populations in
    the ESU. Conversely, a hatchery program managed
    without adequate consideration of its conservation
3286         TROUT UNLIMITED v. BUILDING INDUSTRY
    effects can affect a listing determination by reducing
    adaptive genetic diversity of the ESU, and by reduc-
    ing the reproductive fitness and productivity of the
    ESU. In evaluating the effect of hatchery fish on the
    status of an ESU, the presence of a long-term hatch-
    ery monitoring and evaluation program is an impor-
    tant consideration.

Id. Therefore, the policy’s review of the status of the entire
ESU is consistent with the ESA’s overall focus on preserving
natural populations.

   We are also convinced that the Hatchery Listing Policy’s
method of assessing the status of an entire ESU, and NMFS’s
corresponding downlisting of the Upper Columbia River
steelhead, were decisions based upon the best scientific evi-
dence available.

   “Where scientific and technical expertise is necessarily
involved in agency decision-making, . . . a reviewing court
must be highly deferential to the judgment of the agency.”
Nat’l Wildlife Fed’n v. U.S. Army Corps of Eng’rs, 384 F.3d
1163, 1174 (9th Cir. 2004). An agency’s decision may be
based on the best scientific evidence available even if the
administrative record contains evidence for and against its
decision. When not dictated by statute or regulation, the man-
ner in which an agency resolves conflicting evidence is enti-
tled to deference so long as it is not arbitrary and capricious.
See The Lands Council, 537 F.3d at 993.

   [7] Here, the process that led to the development of the
Hatchery Listing Policy plainly involved “scientific and tech-
nical expertise.” Id. To evaluate the extinction risk faced by
a particular ESU, the agency used the criteria identified by
experts in the Viable Salmonid Populations Technical Memo-
randum. The agency also relied upon an expert report pre-
pared by a Biological Review Team (“BRT”), which
considered the extinction risk faced by the natural members
            TROUT UNLIMITED v. BUILDING INDUSTRY          3287
of the ESU without reference to the effects of hatchery popu-
lations. To develop a more complete picture of the risks faced
by the naturally spawning portion of the ESU, NMFS also
considered a Salmonid Hatchery Inventory and Effects Evalu-
ation Report (the “SHIEE Report”). That report found that
“[t]he abundance of naturally produced steelhead in the
Wenatchee, Methow, and Okanogan basin has increased since
the endangered listing in 1997, at least in part because of
steelhead produced by hatchery programs. In addition, recent
changes in the operation of hatchery programs are reducing
potential adverse impacts on the ESU.” NMFS completed its
review by relying on the findings of an Artificial Propagation
Evaluation Workshop (“APEW”), which concluded that
hatchery programs “substantially mitigated the immediacy of
extinction risk” for the Upper Columbia River steelhead ESU.

   The record shows that NMFS approached the listing deci-
sion in a thoughtful, comprehensive manner that balanced the
agency’s concerns and goals. Because the downlisting
occurred as a result of “substantial—though not dispositive—
scientific data, and not on mere speculation,” Greenpeace
Action, 14 F.3d at 1333, we are satisfied that the downlisting
was not “arbitrary and capricious.” NMFS is entitled to decide
between conflicting scientific evidence. See id. (“To set aside
the Service’s determination in this case would require us to
decide that the views of Greenpeace’s experts have more
merit than those of the Service’s experts, a position we are
unqualified to take.”). It is not our role to ask whether we
would have given more or less weight to different evidence,
were we the agency. Assessing a species’ likelihood of extinc-
tion involves a great deal of predictive judgment. Such judg-
ments are entitled to particularly deferential review. The
Lands Council, 537 F.3d at 993.

   [8] Accordingly, the district court’s grant of summary judg-
ment to Trout Unlimited on its claim that the Hatchery Listing
Policy and the downlisting of the Upper Columbia River
steelhead violated the ESA was erroneous. On remand, the
3288         TROUT UNLIMITED v. BUILDING INDUSTRY
district court should grant NMFS’s motion for summary judg-
ment.

                               V

   We now turn to the arguments of the intervenors, who
attack NMFS’s policies from the opposite angle. While Trout
Unlimited contends that NMFS impermissibly conflates
hatchery and naturally spawned salmon, the Building Industry
argues that NMFS impermissibly distinguishes between
hatchery and naturally spawned salmon.

                               A

   The lynchpin of the Building Industry’s argument is that
the ESA’s plain language bars NMFS from making any dis-
tinction, at any stage of the listing process, between hatchery
and naturally spawned fish. Once an ESU is defined to
include both hatchery and naturally spawned fish, the Build-
ing Industry contends that any further differentiation within
the ESU is error. Thus, the Building Industry maintains that
considering hatchery and naturally spawned fish separately
during the listing process violates the ESA—a violation exac-
erbated by the fact that analysis of the hatchery fish is limited
to their contributions to “conserving natural self-sustaining
populations.” 70 Fed. Reg. at 37,215.

   The Building Industry supports its claim in several ways.
First, it points to the ESA’s text, arguing that under that stat-
ute’s “clear terms,” any distinction between hatchery and nat-
urally spawned salmon is impermissible. The “clear terms”
referenced by the Building Industry are the ESA’s definition
of a “species,” see 16 U.S.C. § 1532(16), the ESA’s require-
ment that listing determinations be made upon a review of the
status of a species as a whole, see id. § 1533(b)(1)(A), and the
ESA’s failure to reference “natural populations.” This lan-
guage (or lack thereof) purportedly demonstrates that there is
no legal basis for treating hatchery and naturally spawned
             TROUT UNLIMITED v. BUILDING INDUSTRY          3289
salmon differently when reviewing the listing status of the
population.

   Second, the Building Industry relies upon the ESA’s legis-
lative history. The original text of the ESA defined “species”
to include “any subspecies of fish . . . and any other group of
fish . . . of the same species or smaller taxa in common spatial
arrangement that interbreed when mature.” Endangered Spe-
cies Act of 1973, Pub. L. No. 93-205, § 3(11), 87 Stat. 884,
886. When Congress amended the definition of species in
1978 to replace “smaller taxa” with “distinct population seg-
ment,” legislative reports indicated that the definition now
“would exclude taxonomic categories below subspecies [i.e.,
smaller taxa] from the definition.” H.R. Rep. No. 95-1804, at
17 (1978) (Conf. Rep.), reprinted in 1978 U.S.C.C.A.N. 9484,
9485. According to the Building Industry, this change meant
that while NMFS could no longer list portions of a species
based on genetics, listing portions of a species on the basis of
geographic range was permissible. The Building Industry con-
cludes that this change eliminated NMFS’s ability to differen-
tiate among members of a species which swim side-by-side in
the same streams.

   Third, the Building Industry repeatedly draws on the dis-
trict court’s opinion in Alsea I, which held that NMFS may
not distinguish between natural fish and hatchery fish when
listing an ESU. Essentially, the Building Industry contends
that the Hatchery Listing Policy is flawed for the same reason
as the Interim Policy: “distinction between members of the
same ESU/DPS is arbitrary and capricious because NMFS
may consider listing only an entire species, subspecies, or
[DPS/ESU] of any species.” Alsea I, 161 F. Supp. 2d at 1162.

   [9] We reject each of these arguments. We do not agree that
the Building Industry’s arguments are grounded in the “clear
terms” of the ESA. The two provisions relied upon—16
U.S.C. § 1532(16) and 16 U.S.C. § 1533(b)(1)(A)—make no
reference to how NMFS is to conduct its listing determina-
3290           TROUT UNLIMITED v. BUILDING INDUSTRY
tions. Section 1532(16) simply defines “species.” While
§ 1533(b)(1)(A) does require NMFS to conduct status
reviews, it provides no instruction on how such reviews
should be conducted. Thus, the Building Industry’s demand
for “equal treatment” of hatchery and naturally spawned fish
during the review process simply finds no grounding in the
statutory text of the ESA. Cf. Or. Trollers Ass’n v. Gutierrez,
452 F.3d 1104, 1117-19 (9th Cir. 2006) (deciding, under a
different statute and applying a different term—“stock,”
rather than “species”—that NMFS could treat hatchery and
naturally spawned salmon differently).

   Likewise, the legislative history does not establish the
“clear intent” of Congress. Nothing in the reports cited by the
Building Industry addresses how biological distinctions—
such as those between hatchery and naturally spawned salmon
—should affect the process by which NMFS makes its listing
determinations. If NMFS were attempting to list something
less than a ESU, the Building Industry’s arguments might
have some merit. However, no party in this case claims that
NMFS has listed anything but an entire ESU.

   Nor do we find persuasive the Building Industry’s claim
that these cases are controlled by what the district court cor-
rectly deemed an “expanded interpretation of the Alsea [I]
decision.” Alsea I does not stand for the “equal treatment”
requirement proposed by the Building Industry.10 Rather,
Alsea I stands for the separate and distinct proposition that
once NMFS determines that hatchery and naturally spawned
salmon belong to the same ESU, it may not list the naturally
spawned portion to the exclusion of the hatchery portion of
  10
    At the outset, that contention is undermined by the fact that the very
judge who decided Alsea I found it inapplicable to the claims the Building
Industry brings here. Compare Alsea I, 161 F. Supp. 2d at 1156, with
Alsea Valley Alliance v. Lautenbacher, No. 06-6093-HO, 2007 WL
2344927, at *5 (D. Or. Aug. 14, 2007) (noting that “nothing in . . . Alsea
I prohibits the aspects of the status review challenged by plaintiffs in this
case”).
             TROUT UNLIMITED v. BUILDING INDUSTRY           3291
the ESU. See Alsea I, 161 F. Supp. 2d at 1162. In this case,
no party disputes that when NMFS lists an ESU, it must list
the entire ESU—including both hatchery and naturally
spawned fish.

   [10] The Chevron deference we owe to the agency’s posi-
tion confirms our conclusion. The ESA requires that the
agency “shall by regulation . . . determine whether any spe-
cies is an endangered species or a threatened species.” 16
U.S.C. § 1533(a)(1). By leaving an “explicit gap” for agency-
promulgated regulations, the ESA expressly delegates author-
ity to the NMFS to decide how such listing determinations
should be made. Mead, 533 U.S. at 227. In addition,
§ 1532(16) and § 1533(b)(1)(A) are “silent,” or at best “am-
biguous,” as to how a status review should be conducted. Id.
at 843. Accordingly, we should not substitute our judgment
for that of the agency, but rather limit ourselves to asking
“whether the agency’s answer is based on a permissible con-
struction of the statute.” Id.; see also The Lands Council, 537
F.3d at 993.

   [11] Here, the agency’s construction is eminently reason-
able and in accord with the statutory text. The ESA requires
only that status reviews evaluate an entire species (here,
ESU). See 16 U.S.C. § 1533(b)(1)(A). The Hatchery Listing
Policy entails such comprehensive review. While the process
initially considers the contributions of hatchery and naturally
spawned salmon to the ESU’s viability, ultimately, ESUs are
assessed as a whole. Naturally spawned fish are not evaluated
or listed to the exclusion of hatchery fish. Rather, the contri-
bution of both populations is analyzed with an eye to their
respective impact on the likelihood of extinction of the entire
ESU. Listing determinations are made on the basis of abun-
dance, productivity, genetic diversity, and spatial distribution.
That hatchery fish reproduce with less frequency than natu-
rally spawned salmon and can contribute to a loss of genetic
diversity makes it reasonable initially to consider them sepa-
rately. Indeed, failing to account for these and other distinc-
3292           TROUT UNLIMITED v. BUILDING INDUSTRY
tions between hatchery and naturally spawned salmon might
violate the ESA’s mandate that status reviews be conducted
“on the basis of the best scientific and commercial data avail-
able.” Id.

   Moreover, the Hatchery Listing Policy complies with the
express purpose of the ESA to preserve “the ecosystems upon
which endangered and threatened species depend,” id.
§ 1531(b), and to restore any such “species to the point at
which the measures provided pursuant to this [Act] are no
longer necessary,” id. § 1532(3). Though not dispositive, this
alignment with the statute’s express purpose lends credence to
NMFS’s position that its policy is a reasonable application of
the ESA’s mandate.

  [12] Accordingly, the district court’s ruling that NMFS per-
missibly distinguishes between hatchery and naturally
spawned salmon during the status review process was correct.

                                     B

   For many of the reasons discussed above, the Building
Industry also argues that NMFS may not distinguish between
hatchery and naturally spawned fish when issuing § 4(d) pro-
tective regulations. Again, the Building Industry asserts that
the language of the ESA, legislative history, and Alsea I man-
date equal treatment among members of the same ESU. If one
portion of a ESU is protected from taking, the Building Indus-
try maintains, all portions must be protected.

   NMFS’s authority to issue protective regulations is found
in 16 U.S.C. § 1533(d), which authorizes the issuance of regu-
lations deemed “necessary and advisable . . . for the conserva-
tion of [the threatened] species.”11 As discussed above, the
  11
     As the statutory text indicates, these regulations only apply to species
deemed “threatened.” Species listed as “endangered” may not be subjected
to taking under § 1533(d).
               TROUT UNLIMITED v. BUILDING INDUSTRY                    3293
specific regulations at issue prohibit the taking of salmon with
intact adipose fins. See 70 Fed. Reg. at 37,215-16; see also 70
Fed. Reg. at 37,194-96. On the other hand, salmon with
clipped fins may be taken. 70 Fed. Reg. at 37,194. Therefore,
it is permissible to take some members of a threatened ESU,
but not others.

   The Building Industry’s challenge to these regulations fails
for essentially the same reasons as its attack on the listing
determinations. Nothing in the text or legislative history of the
ESA requires equal treatment for members of the same ESU.
See supra 3288-91. Moreover, nothing in the record indicates
that NMFS considered anything but the status of entire ESUs
in issuing these regulations.12

   [13] The flaws in the Building Industry’s insistence on
equal treatment between hatchery and naturally spawned
salmon are especially apparent in this context. The Building
Industry’s position fails to consider NMFS’s findings that
hatchery fish can help or hinder the viability of a ESU as a
whole. See 70 Fed. Reg. at 37,195 (describing the benefits and
adverse effects of hatchery fish and noting that “[n]ot all
hatchery stocks considered to be part of listed ESUs are of
   12
      The Building Industry makes one unique argument on this point. It
claims that there is tension between the language of 16 U.S.C. § 1533(d)
which authorizes regulations “necessary and advisable . . . for the conser-
vation of [the threatened] species” and the ESA’s definition of “conserva-
tion.” Conservation includes “the use of all methods and procedures which
are necessary to bring any . . . threatened species to the point at which the
measures provided pursuant to this chapter are no longer necessary.” Id.
§ 1532(3). The definition, however, foresees the use of “regulated taking”
only “in the extraordinary case where population pressures within a given
ecosystem cannot be otherwise relieved.” Id. Significantly, however,
§ 1533(d) does not require regulations protecting threatened species from
taking. The combination of the discretionary “may” and the phrase “neces-
sary and advisable” grant NMFS much leeway in crafting regulations. Id.
§ 1533(d). To the extent reading these provisions in concert creates any
ambiguity, the contested regulations are reasonable and entitled to defer-
ence under Chevron.
3294        TROUT UNLIMITED v. BUILDING INDUSTRY
equal value for use in conservation in recovery”). In the same
way that pruning involves the destruction of some branches of
a tree to allow the remaining portions to grow, the § 4(d) reg-
ulations at issue here represent the agency’s reasonable judg-
ment that permitting the destruction of some members of an
ESU to enable the remaining portions to flourish. Given the
agency’s finding that conservation and recovery efforts may
be affected by the yearly variation in size and composition of
the ESUs, we cannot conclude that regulatory changes giving
the agency the flexibility to manage these variations are arbi-
trary and capricious.

   [14] Accordingly, the district court’s ruling that the chal-
lenged § 4(d) regulations permissibly distinguish between
hatchery and naturally spawned salmon was proper.

                              VI

 Based on the foregoing, the opinion of the district court is
AFFIRMED in part, REVERSED in part, and
REMANDED with instructions.
