Thompsonetal2015Fisheriesleastchub

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The Role of Introduced Populations in the
Management and Conservation of Least Chub
Paul D. Thompson, P. Aaron Webber & Cassie D. Mellon
To cite this article: Paul D. Thompson, P. Aaron Webber & Cassie D. Mellon (2015) The Role of
Introduced Populations in the Management and Conservation of Least Chub, Fisheries, 40:11,
546-556, DOI: 10.1080/03632415.2015.1093472
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546 Fisheries | Vol. 40 • No. 11 • November 2015
The Role of Introduced
Populations in the
Management and
Conservation of
Least Chub
FEATURE
Downloadedby[PaulThompson]at06:5623November2015

Fisheries | www.fisheries.org 547
Paul D. Thompson
Utah Division of Wildlife Resources, 515 East 5300 South, Ogden, UT 84405. E-mail: paulthompson@utah.gov
P. Aaron Webber
U.S. Fish and Wildlife Service, Bethel, AK
Cassie D. Mellon
U.S. Bureau of Land Management, West Valley, UT

Native fishes continue to decline in abundance and distribution. One common practice to ensure native fish persistence
has been to introduce fish into new habitats. Though these introductions reduce the risk of extinction, often the
introduced populations are not considered in the Endangered Species Act (ESA) listing status in the same manner as
extant populations. One instance where they were considered was when Least Chub Iotichthys phlegethontis were
introduced into 23 locations within the Bonneville Basin between 2005 and 2013. In 2014, 10 of these populations were
considered successful and were evaluated with the six remaining extant populations by the U.S. Fish and Wildlife Service.
In this instance, Least Chub were removed from the candidate list in part due to these introduced populations, which
increased the resiliency, redundancy, and representation for this species to persist now and into the foreseeable future.
The creation of introduced fish populations is a conservation practice that needs to be considered as a potential tool for
fisheries managers, not only to ensure persistence, but also to preclude the need for federal listing under ESA.
El papel de poblaciones introducidas en el manejo y conservación del Least Chub
La abundancia y distribución de los peces nativos continúan declinando. Una práctica común para asegurar la persistencia
de los peces nativos ha sido introducirlos en nuevos hábitats. Si bien estas introducciones reducen el riesgo de extinción,
a veces las poblaciones introducidas, en comparación a las poblaciones ya existentes, no son consideradas de la misma
forma dentro de los listados del AEA (Acta de Especies Amenazadas). La única instancia en la que fueron consideradas
así fue cuando el Least Chub (Iotichthys phlegethontis) fue introducido en 23 sitios dentro de la cuenca Bonneville entre
2005 y 2013. En 2014, 10 de estas poblaciones fueron consideradas como un éxito y el Departamento de Pesca y Vida
Salvaje (DPVS) de los EE.UU. se encargó de evaluarlas junto con las seis poblaciones restantes que existían. En este caso,
el Least Chub fue removido de la lista de especies amenazadas en parte debido a dichas poblaciones introducidas, las
cuales incrementaron la resiliencia, redundancia y representatividad de la especie en pos de su persistencia presente
y futura. La creación de poblaciones introducidas de peces es una práctica de conservación que debe ser considerada
como una potencial herramienta para los manejadores de pesquerías, no solo con el fin de asegurar la persistencia de las
especies sino también como medida preventiva para evitar la necesidad de listarlas en los elencos del AEA.
Le rôle des populations introduites dans la gestion et la conservation des Iotichthys
Phlegethontis (Least Chub)
L’abondance et la distribution de poissons indigènes continuent à décliner. Une pratique courante pour assurer la
persistance de poissons indigènes a été d’introduire les poissons dans de nouveaux habitats. Bien que ces introductions
réduisent le risque d’extinction, bien souvent les populations introduites ne sont pas prises en compte dans la liste
des espèces en danger de l’ESA de la même manière que les populations existantes. l’iotichthys phlegethontis (Least
Chub) a été introduit dans 23 endroits dans le bassin de Bonneville entre 2005 et 2013 et est l’un des cas où cela a été
pris en considération. En 2014, 10 de ces populations ont bien réussi et ont été évaluées avec les six autres populations
existantes par le Fish and Wildlife Service des États-Unis (USFWS). Dans ce cas, les Iotichthys phlegethontis (Least Chub)
ont été retirés de la liste des candidats, en partie parce que ces populations introduites ont augmenté leur résilience,
leur redondance et leur représentation et permis à cette espèce de persister maintenant et dans un avenir prévisible. La
création de populations de poissons introduites est une pratique de conservation qui doit être considérée comme un
outil potentiel pour les gestionnaires des pêches, non seulement pour assurer la persistance, mais aussi pour empêcher la
nécessité de tenir une liste fédérale en vertu de l’ESA.
INTRODUCTION
Native fishes have declined steadily in abundance and
distribution across western North America in the 20th
century (Williams et al. 1989; Moyle and Leidy 1992), and
approximately one-half of the fish species listed as threatened
or endangered, or being considered for listing by the U.S. Fish
and Wildlife Service (USFWS), occur west of the Continental
Divide (Minckley and Douglas 1991). These declines have been
especially pronounced with species occupying desert spring
habitats (Minckley and Douglas 1991). Factors attributed to the
decline of desert fishes include the alteration of environmental
conditions associated with water development, improper
grazing management, nonnative fish, and changes in land use
and management, which has resulted in habitat degradation.
Introduced nonnative fishes became established as they were
better adapted to these conditions or could outcompete the
native fish population. These introduced fishes contributed to the
decline of desert fish through competition, predation, and other
detrimental interactions (Meffe 1985; Minckley and Douglas
1991; Lydeard and Belk 1993; Stevens and Meretsky 2008).
Least Chub Iotichthys phlegethontis populations have
declined as a result of these same factors. The Least Chub is
a minnow endemic to the Bonneville Basin of Utah, and they
are the only species of the genus Iotichthys. Least Chub are
the smallest cyprinids in Utah, reaching a maximum size of
only 76 mm total length (TL; Sigler and Sigler 1996; Figure
1). The Least Chub are considered a short-lived species, and
some individuals have been aged up to six years (Mills et
al. 2004a). Least Chub mature at age 1 (Sigler and Sigler
1996) and spawn intermittently throughout the late spring and
summer (Crawford 1979). Least Chub feed primarily on algae
and small invertebrates (Sigler and Sigler 1996) and persist
in fish communities with low species diversity. The rarity and
imperilment of Least Chub has been recognized for more than
40 years (Table 1), and although once common in springs, lakes,
streams, marshes, rivers, and ponds within the Bonneville Basin
(Sigler and Sigler 1987; Page and Burr 1991), only six known
extant Least Chub populations remain (Figure 2; Bailey et al.
2005; Mock and Bjerregaard 2007). One of the most significant
factors attributed to their decline has been the introduction of
Western Mosquitofish Gambusia affinis. Western Mosquitofish

Table 1. Least Chub listing status and significant decisions/actions taken in relation to ESA listing.
Year Agency/groupa
Decision/action Reference
1972 AFS Classified as a threatened species Miller (1972)
1979 AFS Classified as a threatened species Deacon et al. (1979)
1980 USFWS Status review, insufficient data to be classified as threatened or endangered
species
Bailey et al. (2005)
1982 USFWS Classified as category 2 candidate species USFWS (1982)
1989 USFWS Classified as category 1 candidate species USFWS (1989)
1995 USFWS Proposed to list as endangered with critical habitat under the Endangered
Species Act (ESA)
USFWS (1995)
1995 Congress Listing moratorium suspended listing action Bailey et al. (2005)
1998 Signatories
of agreement
Developed Least Chub conservation agreement and strategy Perkins et al. (1998)
1998 Signatories
of agreement
Formed Least Chub Conservation Team (LCCT) Perkins et al. (1998)
2001 UDWR Considered Least Chub a conservation agreement species UDWR Sensitive Species Listb
2005 LCCT Revised 1998 Least Chub conservation strategy Bailey et al. (2005)
2007 Petitioners Petitioned Least Chub to be listed as threatened under ESA and critical habitat
be designated
2008 USFWS 90-Day finding concluded that petition presented substantial information in
support of listing
USFWS (2008)
2010 USFWS 12-Month finding identified Least Chub as a species for which listing as
endangered or threatened was warranted, but was precluded
USFWS (2010a)
2010 USFWS Classified as candidate species USFWS (2010b)
2011 USFWS Candidate notice of review (CNOR) maintained the species as a candidate USFWS (2011)
2012 USFWS CNOR maintained the species as a candidate USFWS (2012)
2013 USFWS CNOR maintained the species as a candidate USFWS (2013)
2014 LCCT Amended 2005 Least Chub conservation strategy LCCT (2014)
2014 USFWS Removed from list of candidates under ESA USFWS (2014b)
a
AFS = American Fisheries Society; USFWS = United States Fish and Wildlife Service; Congress = United States Congress; Signatories
of agreement = signatories to the Least Chub conservation agreement and strategy as of 2014 include Utah Department of Natural
Resources, Division of Wildlife Resources; Bureau of Land Management; United States Fish and Wildlife Service; Bureau of Reclamation;
Utah Reclamation Mitigation and Conservation Commission; Central Utah Water Conservancy District; Confederated Tribes of the Goshute
Reservation; and Southern Nevada Water Authority; UDWR = Utah Division of Wildlife Resources; LCCT = Least Chub Conservation Team;
Petitioners = Center for Biological Diversity, Confederated Tribes of the Goshute Reservation, Great Basin Chapter of Trout Unlimited, and
Utah Chapter of the Sierra Club.
b
The UDWR also considered the Least Chub a wildlife species of concern and a federal candidate species in past sensitive species lists.
Figure 1. Typical adult Least Chub with a female (left) and a male in spawning colors (right). Photo credit:
Paul Thompson.

are known to be aggressive and predate on eggs and young of
other fishes (Meffe 1985; Sigler and Sigler 1987; Mills et all.
2004b).
In 1998, a Least Chub Conservation Agreement and
Strategy (LCCAS) was developed that outlined specific actions
and strategies needed to ensure the persistence of the species
(Perkins et al. 1998). The Least Chub Conservation Team
(LCCT) was formed and revised the LCCAS in 2005 (Bailey
et al. 2005) and in 2014 (LCCT 2014; Table 1). The genetic
characterization of the six extant wild Least Chub populations
indicated minimal genetic differences between the populations
(Mock and Miller 2005; Mock and Bjerregaard 2007); however,
Mock and Miller (2005) recommended that Least Chub be
grouped into and managed within three geographic management
units (GMUs; Figure 2). The GMUs are managed independently
to maintain potential adaptive differences that were not
detectable within molecular markers.
One major action identified in the LCCAS was to establish
and maintain three introduced Least Chub populations per GMU
source with at least one per distinct extant population source for
a total of nine introduced populations (LCCT 2014). Though the
term “introduced” can reference the stocking of nonnative fish or
the stocking of fish outside of their known range (Hendrickson
and Brooks 1991; George et al. 2009), this was the terminology
utilized by the LCCT and the USFWS during the listing decision
process, so throughout the remainder of this article, introduced
refers to both the translocation of wild fish and the stocking of
hatchery Least Chub to establish new populations within their
historic range.
The goal of introducing additional Least Chub populations
was to maintain the genetic diversity and integrity of the wild
populations while reducing the extinction risk of Least Chub due
to environmental stochasticity or random catastrophes (Lande
1993). Echelle (1991) recommended that 200 or more fish be
utilized in a founding stock for intermittent breeders because
some losses of rare alleles can occur when smaller founding
stocks of less than 100 are utilized. Nuclear (amplified fragment
length polymorphism) and mitochondrial genetic markers from
two previous Least Chub introductions, Lucin Pond and Walter
Springs, used similar donor numbers of n = 131 and n = 230
fish, respectively. Mock and Miller (2005) found that these
populations were similar to their donor populations with respect
to genetic diversity measures and there was no evidence of
reduced diversity or severe bottlenecks in these populations.
The introduction of Least Chub into additional locations
began as early as 1979, with nine introductions documented
from 1979 to 2004. These early introductions were primarily
opportunistic and though some persisted for a couple years,
they were largely unsuccessful. Since 2005, 23 Least Chub
introductions have been attempted and 12 reproducing and
recruiting Least Chub populations have been established. The
objectives for introducing Least Chub populations were to
(1) increase redundancy and resiliency and prevent possible
extinction of the remaining six extant populations and (2)
preclude the need for federal listing under the Endangered
Species Act (ESA). Objectives specific to this article were to
demonstrate how introduced populations can be a conservation
tool for fisheries managers and review how the USFWS viewed
introduced populations in the Least Chub listing decision under
ESA.
METHODS
Study Site
Introduced populations of Least Chub were required to be
within the historic range of the species, the Bonneville Basin.
Many larger water bodies were excluded due to the prior
establishment of species that are known to be noncompatible
with Least Chub. This resulted in smaller spring and wetland
systems and manmade ponds being the focus for establishing
introduced populations (Figure 3). Landowner willingness,
potential threats, likelihood of success, impacts on the existing
native species community, and impacts on the source population
were factors that determined where Least Chub were introduced.
Because potential habitats were the limiting factor, the
translocation of extant populations did not have to occur in the
GMU from which they originated; recipient habitats only needed
to be located in the Bonneville Basin and have no potential for
Least Chub escapement into other extant populations or other
private lands.
Establishing Introductions
Utah Division of Wildlife Resources (UDWR) biologists
used their expert opinions to determine suitable habitats, and
though this process was subjective, these general factors were
considered: (1) stability of water source, (2) habitat protection
(e.g., minimal or no livestock grazing, remote, etc.), (3) presence
or diversity of fishes, and (4) size (e.g., small habitats are
more subject to catastrophic disturbances). Two habitats were
Figure 2. Location of extant and successful introduced Least Chub
populations within three geographic management units in the Bon-
neville Basin, Utah.
The introduction of Least Chub into additional
locations began as early as 1979.

Figure 3. Examples of habitats where Least Chub were introduced with SE Pilot (bottom) and Rosebud Pond (top).
Photo credits: Paul Thompson.

chemically treated with rotenone to remove nonnative fish
prior to the introduction of Least Chub. Other sites contained
nonnative fish such as sterile Rainbow Trout Oncorhynchus
mykiss, which were determined not to threaten Least Chub
success, but the majority of introduction sites were free of
nonnative fishes. Least Chub were collected with mesh minnow
traps or seines from wild and hatchery populations. All hatchery
populations were created from wild-caught broodstock that were
placed in earthen ponds or cement raceways with spawning
structure (e.g., submerged Christmas trees) and allowed to
reproduce and recruit in the absence of nonnative fishes and
without human intervention. Least Chub were transported
in UDWR hatchery trucks or coolers with battery-powered
aerators and, if needed, the fish were temperature acclimated to
the recipient waters prior to stocking. A minimum of 200 Least
Chub was chosen to establish new Least Chub populations with
a target of 500 fish stocked in larger habitats (e.g., >1 ha). If the
donor population was believed to be less than 1,000 individuals,
transplants occurred over multiple years to minimize impacts to
the donor population.
The UDWR followed Utah code and agency policies
during the collection, transportation, and stocking of wild or
hatchery Least Chub. Least Chub could only be moved if (1)
the donor population was certified pathogen free as defined
in the American Fisheries Society Fish Health Section Blue
Book (Mitchell and Goowin 2004), (2) the donor and recipient
location were surveyed for aquatic invasive species to ensure
there was no transport of invasive species with the fish to the
recipient site, and (3) the introduction was supported. Support
for a Least Chub introduction was obtained from the LCCT and
UDWR as well as the owner of the property and county where
the introduction would occur. In all cases the landowner was
made fully aware of the current federal candidate status for
Least Chub. Initially, some private landowners were asked to
sign an agreement with UDWR, establishing the conditions for
the introduction and what both parties agreed to. One of these
stipulations allowed landowners to request UDWR to remove
Least Chub from their property with sufficient notice.
Monitoring
A new population of Least Chub was considered a successful
introduced population if the population remained secure with
no or minimal threats (e.g., minimal livestock grazing, secure
water source, and few or no nonnative fish present), there was
a current agreement in place if the population was on private
land, and the population had successfully spawned for at least
the previous two years. Successful spawning was determined by
the presence of age-0 Least Chub (<35 mm TL; modified from
Mills et al. 2004b) during annual monitoring with mesh minnow
traps or seines. Monitoring occurred in mid-August through
September and required the capture and measurement of 100
Least Chub per population and followed the same procedure
as monitoring of the extant populations (LCCT 2010). The
population status for all introduced populations of Least Chub
was reviewed annually by the LCCT to determine whether a
population should be considered successful.
RESULTS
Successful Introductions
Between 2005 and 2013, Least Chub were introduced
into 23 locations (not including UDWR hatchery facilities
and educational facilities) within the Bonneville Basin, Utah
(Table 2). As of the 2014 USFWS decision, insufficient time
had elapsed to determine whether Least Chub had become
established at the six locations that were stocked from 2012
to 2013. For the 17 locations that were evaluated, 10 of the
introduced populations met the introduced population criteria
and were considered successful introduced Least Chub
populations (Table 2; Figure 2). The three introductions into
lotic habitats were unsuccessful because Least Chub were not
documented again at these sites (Table 2). The population at
Willow was considered a success for four years; however, a
wildfire in 2013 and subsequent ash flow into the pond from
rain events severely reduced the number of Least Chub at this
location. Least Chub were salvaged from this site following the
fire and subsequent monitoring documented their persistence,
but as of 2014, Willow was not considered a successful
introduced population because post-fire threats remained
elevated. The Least Chub population at Stokes was considered
unsuccessful because there was no formal agreement in place
with the private landowner. Annual Least Chub monitoring at
Chambers indicated that this population was small with minimal
recruitment, and there was no formal agreement with the private
landowner. Shortly after the introduction of Least Chub into
Fish Springs National Wildlife Refuge, Western Mosquitofish
invaded and this was attributed to the failure of this introduction.
Listing Decision
In the 2014 USFWS decision to remove Least Chub from the
candidate list, each of the 10 successful introduced populations
were evaluated using the five factor threat assessment in
the same manner as the extant populations. The five factors
USFWS evaluates in every listing decision are as follows: (1)
damage to or destruction of habitat; (2) overutilization of the
species for commercial, recreational, scientific, or educational
purposes; (3) disease or predation; (4) inadequacy of existing
protection; and (5) other natural or manmade factors. Based on
this analysis in the 2010 decision that determined that Least
Chub were warranted for listing (USFSW 2010b), the threats
that were significant for Least Chub were as follows: factor
A—specifically livestock grazing and groundwater withdrawal;
factor C—introduction of nonnative fish; factor D—inadequacy
of existing regulatory mechanisms, specifically relating to
groundwater regulation; and factor E—the cumulative effects of
drought and climate change.
Threats Analysis
No significant threats were identified for the 10 introduced
Least Chub populations that were considered successful by
the LCCT. Livestock grazing was completely excluded from
6 of the 10 sites, and 4 allowed some level of grazing that was
not considered detrimental to Least Chub habitat. All of the
introduced populations had a water right associated with the
site. Though the water right was not exclusively for maintaining
Least Chub habitat, this provided a mechanism for ensuring
that water levels were maintained and that additional water
use in the future would not impact these habitats. With regard
to factor C, there were only two introduced populations that
had any nonnative species present. These were Common Carp
Cyprinus carpio and Goldfish Carassius auratus at Atherly
and sterile Rainbow Trout that were stocked at Rosebud Pond.
The introduced populations where these nonnative fish occur
were two of the larger sites with complex habitat. Common
Carp in large numbers can have a significant impact on habitat
(Parkos et al 2003; Matsuzuki et al. 2007), but they occurred
in low densities and were not negatively impacting the Least

Table 2. Least Chub introductions in the Bonneville Basin, Utah, 2005–2013. Other species: UC = Utah Chub Gila atraria, MF = Western
Mosquitofish, CP = Common Carp, GF = Goldfish, SD = Speckled Dace Rhinichthys osculus, RT = Rainbow Trout, BT = Brook Trout
Salvelinus fontinalis, US = Utah Sucker Catostomus ardens, MT = Mountain Sucker C. platyrhynchus, and LB = Largemouth Bass
Micropterus salmoides. NWR = National Wildlife Refuge, WD = West Desert, SR = Sevier River, WF = Wasatch Front. RI = recently
introduced.
Site
Source popu-
lation/GMU
Year
introduced
(# of Least
Chub)
Ownership
Habitat
type
Estimated
water
volume
(m3
)
Other
species
present
Least Chub
recruitment
documented
Population met
introduced criteria of
LCCT (if not, reason)
Red Knolls Bishop/WD 2005
(250)
Federal Pond 220 Nonea
Yes Yes
Fish Springs
NWR
Leland/WD 2006
(170)
Federal Marsh 1,466,610 MF, UC No No (not biologically
successful)
Atherly Mills Valley/SR 2006
(19,000)
State Reservoir 162,865 UC, CP, GF Yes Yes
Escalante Mona/WF 2006
(175)
Local
government
Pond 52 None Yes Yes
Willow Clear Lake/SR 2007
(340)
Private Pond 330 None Yes No (2013 wildfire
threatens population)
Pilot Leland/WD 2008
(147)
2013
(208)
Federal Pond 640 Nonea
Yes Yes
SE Pilot Mills Valley/SR 2008
(700)
Federal Pond 670 None Yes Yes
Rosebud
Pond
Mills Valley/SR 2008
(3,000)
Private Reservoir 24,260 SD, RT Yes Yes
Rosebud
Spring
(500)
Federal,
private
Flowing
spring
n/a SD No No (not biologically
successful)
Cluster
(two ponds)
(400)
Stokes Mills Valley/SR 2008
(700)
Private Pond 1,320 UC Yes No (no agreement with
landowner)
Chambers Mills Valley/SR 2008
(1,000)
Private Pond 4,570 SD, CP Yes No (small population,
limited recruitment)
Mountain
Valley
(1,200)
Private Flowing
spring
n/a SD, RT No No (not biologically
successful)
AW
Heritage
(1,700)
Private Flowing
spring
n/a US No No (not biologically
successful)
Keg Gandy/WD 2009
(200)
Upper
Garden
Mona/WF 2011
(1,000)
State Pond 385 None Yes Yes
Deseret Mona/WF 2011
(500)
West
Locomotive
Bishop/WD 2012
(341)
2013
(216)
State Pond 28,330 MF, CP, UC No No (RI)
Sparks Mona/WF 2012
(2,000)
Teal Clear Lake/SR 2013
(500)
State Pond 60,700 MF, CP,
UC, LB
No No (RI)
Baker Gandy/WD 2013
(496)
Bar M Leland/WD 2013
(605)
State Pond 42,490 MF, CP,
UC, MT
No No (RI)
Big Springs Mona/WD 2013
(15,200)
Private Pond 8,250 BT No No (RI)
a
Chemically treated with rotenone prior to the introduction of Least Chub.

Chub populations. Sterile Rainbow Trout were stocked into one
of the privately owned introduced populations as an incentive
for the landowner that allowed the UDWR to establish a Least
Chub population on their property. This stocking was initially
considered experimental; however, the habitat complexity at
this site allowed sufficient habitat separation between Rainbow
Trout and Least Chub and did not negatively impact the Least
Chub population. Inadequacy of existing regulatory mechanisms
also was not determined to be a threat at any of the introduced
populations because there was often a water right associated
with the site. All introduced populations were determined to be
managed by state or federal agencies or under an agreement with
an agency. These agreements and the management authority
helped ensure that sufficient regulatory mechanisms were in
place at the introduced populations. Cumulative effects of
drought and climate change were determined to be a threat in
the 2010 decision for the extant Least Chub populations. The
introduced populations were a significant factor in addressing
this threat because they provided mitigation by providing
redundancy and resiliency.
DISCUSSION
Establishing introduced populations for sensitive fish species
has been a common conservation practice. Such an approach
frequently is considered when the species is endemic to a small
range or has considerable threats to its existence in its current
occupied range or simply as a precaution to avoid extinction
from catastrophic events. Managers also need to consider the
potential impacts of introducing the species on the native species
already existing in that location, as well as consider the impact
if the introduced species escapes or expands beyond its intended
location. Other factors to consider are the impacts of removing
fish on the source population. If the species is long-lived or slow
to reproduce, it may take many years before the success of the
introduction can be determined; in addition, multiple transfers
may be needed and this could impact the source populations
(Minckley 1995; George et al. 2009). These factors were
considered in Least Chub introductions but were determined
not to be significant barriers. Many of the introduced locations
were artificially created ponds and any native species present
also coexisted with Least Chub in the extant populations. The
impact on the source populations was considered, but for two
extant populations, a successful hatchery population already
had been established and was utilized as a source of fish for the
introductions.
Introduced or conservation populations have been created
for several species in the western United States, including the
Apache Trout Oncorhynchus apache, Big Bend Gambusia
Gambusia gaigei, Desert Pupfish Cyprinodon macularius,
and Gila Topminnow Poeciliopsis occidentalis (Simons 1987;
Johnson and Hubbs 1989; Williams 1991). Translocating fishes
from the wild to new locations has saved some species from
extinction, but establishing introduced populations for a species
does not guarantee persistence (e.g., Amistad Gambusia G.
amistadensis; Hubbs and Jensen 1984). The establishment of
introduced populations is not specific to the western United
States. In Australia, the Murray Hardyhead Craterocephalus
fluviatilis was taken into captivity and subsequently stocked
into several locations with some success (Ellis et al. 2013).
In addition, the Pedder Galaxias Galaxias pedderensis was
found only in Lake Pedder in Tasmania, Australia. This species
was removed from Lake Pedder and now exists only in two
introduced populations located outside its historical range
(Lintermans 2013a).
The creation of self-sustaining introduced Least Chub
populations proved to be relatively simple, likely because they
are generalists and have a broad tolerance limit to many water
quality parameters (Sigler and Sigler 1987). Because Least Chub
become reproductively mature in one year and are intermittent
spawners (Crawford 1979), releasing eggs throughout the
majority of late spring/summer months, populations often
became well established within two years from the introduction.
Least Chub that were introduced into smaller ponds in the
absence of other fish species generally became established. Of
the 10 successful introduced populations, Least Chub were the
only species present in eight of the habitats (Table 2). Although
Least Chub historically occupied lotic habitats (Sigler and
Sigler 1987; Page and Burr 1991), the three introductions into
lotic habitats were unsuccessful (Table 2) for reasons unknown.
Hickman (1989) considered the Least Chub to be constantly
threatened by the introduction and presence of nonnative fish
species, and Western Mosquitofish pose a direct threat because
of their known aggressive predation on eggs and young of other
fishes (Meffe 1985; Sigler and Sigler 1987). Mills et al. (2004b)
found that Least Chub juveniles were vulnerable to Western
Mosquitofish predation and no Least Chub introductions were
considered successful when Western Mosquitofish were present
(Table 2). The invasion or illegal introduction of nonnative fish
species into extant and introduced Least Chub populations will
be a continual threat; however, the majority of these populations
occur in remote locations, which should lessen this threat.
The establishment of introduced populations is often an
integral part of ESA recovery plans because range expansion
and establishment of new populations are typically a criterion
for down-listing or delisting (USFWS 2000; Paragamian and
Beamesderfer 2004); however, this goal is not always achieved.
In Australia, no freshwater fish has had its conservation
status down-listed or delisted as a result of conservation
management, including the establishment of introduced
populations (Lintermans 2013b). Hendrickson and Brooks
(1991) also provide many examples of imperiled fishes in the
western United States where introduced populations have been
successfully established, yet their official regulatory status was
not changed as a result. In 2014, however, the Oregon Chub
Oregonichthys crameri was removed from protection under
ESA (USFWS 2014a). The Oregon Chub is similar to the Least
Chub in that it is a small minnow and shares some life history
similarities. The Oregon Chub Recovery Plan (USFWS 1998)
outlined the establishment of refuge populations to preserve
genetic variability among populations (DeHaan et al. 2012)
and was a requirement for delisting (USFWS 1998). The
successful delisting of the Oregon Chub not only provides hope
for the delisting of additional threatened or endangered fishes
but highlights the important role that establishing introduced
populations can serve in recovery.
The Least Chub was not a federally listed species under
ESA; however, in 2007, this fish was petitioned to be listed as
threatened or endangered under ESA including the designation
of critical habitat. The USFWS concluded in a 90-day finding
that the petition did present substantial information in support
of listing (USFWS 2008). The 90-day finding resulted in a
12-month status review that identified the Least Chub in 2010
as a species for which listing as endangered or threatened was
warranted but precluded (USFWS 2010a). The Least Chub
remained a candidate species for four years until the USFWS

ruled in 2014 that federal protection under ESA was not needed
and the Least Chub was removed from the candidate list
(USFWS 2014b).
The 2014 decision cited that 10 successful Least Chub
introduced populations had been established and, when
combined with the naturally occurring populations, there was a
high likelihood of persistence even under higher probabilities
of catastrophic events. The introduced populations were
determined to provide habitat heterogeneity and redundancy
and a buffer against environmental effects (e.g., cumulative
effects from drought and changing climate conditions in the
Bonneville Basin), and the geographic distribution of the
introduced populations was representative of the known genetic
diversity of the species (e.g., each natural population and
GMU was represented in at least one introduced population).
Therefore, if the Least Chub continued to persist in its current
distribution, there would be sufficient resiliency, redundancy,
and representation to persist now and in the foreseeable future
(USFWS 2014a).
The introduced Least Chub populations that likely played
the most significant role in the decision not to list Least Chub
were for the extant population at Mona (Least Chub were
discovered in Mona Springs in 1995). The Mona population was
not considered self-sustaining in the USFWS decision. Western
Mosquitofish and other nonnative fish also were documented
in 1995, and by 1998 the Least Chub population had declined
substantially. Least Chub were collected from Mona at that
time and taken into captivity. Multiple efforts to remove
nonnative fish, improve habitat, and reestablish Least Chub were
attempted, but habitat complexity did not allow for the complete
removal of Western Mosquitofish. Recently, barriers have been
established to create smaller habitats where nonnative removal
and Least Chub stocking efforts have been targeted. As of 2014,
Least Chub were successfully recruiting at Mona but they were
still being stocked annually to maintain this population. Three
introduced populations for the wild Mona population have been
established in addition to a hatchery population. The success and
stability of these introduced populations has ensured that any
unique genetic characteristic of the Mona Least Chub population
will not be lost if a population cannot be maintained in the
extant habitat.
This was the first USFWS listing decision where introduced
fish populations were evaluated as discrete populations; hence,
the consideration of these introduced populations was a key
factor in precluding the need to list Least Chub under ESA.
Fisheries managers are faced with multiple threats to habitat
that are difficult to address on site. Climate change receives
consideration as a threat in most listing determinations and,
consequently, the establishment of introduced populations
should be considered and may be one of the few tools for
fisheries managers to prevent federal listings or recover listed
species.
ACKNOWLEDGMENTS
We thank the highly functional Least Chub Conservation
Team for dedicating the past 16 years to providing solid
guidance for the conservation of the Least Chub and for helping
preclude the need for federal listing of this species under the
ESA. The work to establish Least Chub by UDWR biologists
was funded by UDWR through Endangered Species Mitigation
Funds and State Wildlife Grants, as well as the Bureau of Land
Management. Kevin Wheeler, Richard Fridell, Chris Crockett,
Mark Grover, Krissy Wilson, Paul Thompson, Aaron Webber,
and Samuel McKay were the biologists and managers who
worked diligently to find suitable habitats and introduce Least
Chub. We thank Phaedra Budy and Mark Belk for reviewing
earlier drafts of this article. Reference to trade names does not
imply endorsement by the U.S. Government.
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