Download Exploring Crowded Trophic Niche Space in a Novel Many?

Transactions of the American Fisheries Society
ISSN: 0002-8487 (Print) 1548-8659 (Online) Journal homepage: http://www.tandfonline.com/loi/utaf20
Exploring Crowded Trophic Niche Space in a Novel
Reservoir Fish Assemblage: How Many is Too
Many?
Lisa K. Winters & Phaedra Budy
To cite this article: Lisa K. Winters & Phaedra Budy (2015) Exploring Crowded Trophic Niche
Space in a Novel Reservoir Fish Assemblage: How Many is Too Many?, Transactions of the
American Fisheries Society, 144:6, 1117-1128, DOI: 10.1080/00028487.2015.1083475
To link to this article: http://dx.doi.org/10.1080/00028487.2015.1083475
Published online: 06 Oct 2015.
Submit your article to this journal
Article views: 38
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
http://www.tandfonline.com/action/journalInformation?journalCode=utaf20
Download by: [Utah State University Libraries]
Date: 13 October 2015, At: 08:14
Transactions of the American Fisheries Society 144:1117–1128, 2015
Ó American Fisheries Society 2015
ISSN: 0002-8487 print / 1548-8659 online
DOI: 10.1080/00028487.2015.1083475
ARTICLE
Exploring Crowded Trophic Niche Space in a Novel
Reservoir Fish Assemblage: How Many is Too Many?
Lisa K. Winters1
Department of Watershed Sciences and The Ecology Center, Utah State University, 5210 Old Main Hill,
Logan, Utah 84322, USA
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
Phaedra Budy*
U.S. Geological Survey, Utah Cooperative Fish and Wildlife Research Unit,
Department of Watershed Sciences, Utah State University, 5210 Old Main Hill, Logan,
Utah 84322, USA; and Department of Watershed Sciences and The Ecology Center,
Utah State University, 5210 Old Main Hill, Logan, Utah 84322, USA
Abstract
In highly managed reservoir systems, species interactions within novel fish assemblages can be difficult to
predict. In high-elevation Scofield Reservoir in Utah the unintentional introduction of Utah Chub Gila atraria and
subsequent population expansion prompted a shift from stocking exclusively Rainbow Trout Oncorhynchus mykiss
to include tiger trout (female Brown Trout Salmo trutta £ male Brook Trout Salvelinus fontinalis) and Bonneville
Cutthroat Trout O. clarkii utah, which composed a novel suite of top predators and potential competitors. We
examined the interspecific interactions among Scofield Reservoir piscivores using a multifaceted approach
including gut analyses, stable isotopes, and gape limitation. Large Cutthroat Trout consumed 50–100% Utah Chub
and tiger trout consumed 45–80%. In contrast, small and large Rainbow Trout consumed primarily invertebrate
prey and exhibited significant overlap with small tiger trout, Cutthroat Trout, and Utah Chub. Large Cutthroat
Trout and tiger trout occupy a top piscivore trophic niche and are more littoral, while Rainbow Trout occupy an
omnivore niche space and are more pelagic. Both Cutthroat and tiger trout varied in niche space with respect to
size-class, demonstrating an ontogenetic shift to piscivory at approximately 350 mm TL. Cutthroat Trout and tiger
trout are capable of consuming prey up to 50% of their own size, which is larger than predicted based on their
theoretical gape limit. Because it appears food resources (Utah Chub) are not limited, and performance metrics are
high, competition is unlikely between Cutthroat Trout and tiger trout. In contrast, apparent survival of Rainbow
Trout has recently declined significantly, potentially due to shared food resources with Utah Chub or negative
behavioral interactions with other members of the community. Collectively, this research aids in understanding
biotic interactions within a top-heavy and novel fish community and assists towards developing and implementing
suitable management strategies to control nuisance species.
In an era when many aquatic systems and fisheries are manmade, artificial, and highly manipulated, we are continually
challenged with understanding the interactions that define
food webs and community structure of aquatic systems (Polis
and Strong 1996; Terra and Araujo 2011). Fish communities
and food web interactions in impounded riverine systems
probably differ from those in natural lakes because these communities often include both native stream species that persist
in the reservoir and nonnative species intentionally or accidentally introduced (Miranda and DeVries 1996; Matthews et al.
*Corresponding author: [email protected]
1
Present address: Arizona Game and Fish Department, 5000 West Carefree Highway, Phoenix, Arizona 85086, USA.
Received April 24, 2015; accepted August 6, 2015
1117
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
1118
WINTERS AND BUDY
2004; Clavero et al. 2013). These novel fish assemblages can
be very dynamic and unstable (Stein et al. 1995; Havel et al.
2005), may have more trophic positions than before manipulations (Walsworth et al. 2013), and can exhibit decoupled or
new predator–prey interactions (Noble 1986; Denlinger et al.
2006). Consequently, there is increased potential for competition for resources among species, as well as intensified predation (Ney 1996; Wuellner et al. 2010).
When considerable numbers of large-bodied fish are
stocked in reservoirs to meet angling desires, there is potential
for strong competition among apex species. Overlapping food
habits may reduce prey resources for the predator community
(Bacheler et al. 2004), leading to reduced growth and survival
(Teuscher and Luecke 1996; Ruzycki et al. 2001; Correa et al.
2012). Competition among top predators may also affect
behavior, habitat use, diet, or fitness (Werner and Hall 1977;
Marrin and Erman 1982; Turek et al. 2013). In addition,
because these fish assemblages are novel (i.e., do not occur in
nature), there is potential for unanticipated interspecific and
intraspecific predation at top trophic levels (Wahl 1999). This
strong potential for competition and even predation can limit
predator performance (Kitchell and Crowder 1986; Ney 1990)
and decrease angler success (Marrin and Erman 1982; Wuellner et al. 2010).
Apex predators are introduced into lakes and reservoirs not
only to support sport fisheries (Wuellner et al. 2010), but also
as a management tool to control undesired species (Kohler and
Courtenay 1986). Ultimately, the growth and survival of these
piscivores depends on prey fish availability (Ney and Orth
1986; Lundvall et al. 1999). However, the overall impact of
these introduced piscivores on food webs also depends, in
part, on prey life history traits, such as habitat choices (Tabor
and Wurtsbaugh 1991; Romare and Hansson 2003; Carey and
Wahl 2010), prey behavior (Hays 2003; Sih et al. 2010), or
prey defenses (Dodds and Whiles 2010). In addition, predator
body morphology is a major factor determining foraging performance of fish. For example, gape size relative to prey body
size determines the maximum size prey that can be eaten by
piscivores (Mittelbach and Persson 1998). While the selection
of prey fish of larger sizes increases proportionally with predator size (Juanes et al. 2002), sizes of prey consumed often
appear to be smaller than theoretical predation based on gape
size measurements (Truemper and Lauer 2005). Finally, predator feeding is not solely dependent on prey, but also on a suite
of characteristics related to the optimal foraging by predators
(Werner and Hall 1974; Werner and Gilliam 1984; Dodds and
Whiles 2010). Due to the complexities of these predator–prey
interactions, introductions of novel predators may have unintended or unanticipated impacts on the fish community and
fishery (Knapp et al. 2005; McDowall 2006; Sih et al. 2010).
There are many systems worldwide where a nuisance species threaten the existence of a popular sport fishery, and in
response, a variety of apex predators are simultaneously or
sequentially stocked in an attempt to control the prey (Irwin
et al. 2003). Scofield Reservoir, in Utah, is one of those typical
reservoir systems. Scofield Reservoir is a high-elevation reservoir that is home to a very popular blue-ribbon (defined as
“quality angling experiences in exquisite settings”; UDWR
2015) fishery for Rainbow Trout Oncorhynchus mykiss. This
reservoir now contains a novel assemblage of top predators
including Rainbow Trout, Bear Lake strain Bonneville Cutthroat Trout O. clarkii utah, and tiger trout (female Brown
Trout Salmo trutta £ male Brook Trout Salvelinus fontinalis);
these additional species were more recently stocked in an
effort to suppress Utah Chub Gila atraria, an unwelcome and
rapid invader that reappeared in 2005. Following the Utah
Chub invasion, Rainbow Trout experienced apparent reductions in abundance and survival. Though each of the predator
species is known for their individual piscivorous nature, this
combination of multiple potential top predators is unique, so
relatively little was known of the likely interspecific interactions between Cutthroat Trout, Rainbow Trout, and tiger trout.
In addition, because tiger trout are a relatively new hybrid,
there is little basic information describing their performance,
feeding behavior, and piscivorous potential (Winters 2014),
albeit popularity among anglers and managers has been
growing.
Using Scofield Reservoir as a model system, our overall
goal was to better understand how multiple top predators in a
novel food web interact in reservoir systems. Specifically, our
study objectives were to (1) explore the food habits, diet overlap, and trophic niche space among the three potential top
predators, (2) identify potential for competition, and (3) characterize potential predation and competition linkages. To do
this, we used a multifaceted approach including diet analyses,
stable isotope analysis, and a gape limitation study including
predator morphology and prey body size.
METHODS
Study site.—Scofield Reservoir is a high elevation
(2,322 m) impoundment on the Price River in southeast Utah.
The reservoir was created by Scofield Dam in 1926 and is predominantly used for irrigation water storage; recreation and
flood control provide additional benefits (U.S. Bureau of Reclamation 2011). The current reservoir has a capacity of 73,600
acre-feet at full pool, mean surface area of 1,139 ha, and a
mean depth of 8 m (U.S. Bureau of Reclamation 2009). Scofield Reservoir is classified as eutrophic, with “excessive” total
phosphorous enrichment (Utah DEQ 2010), typical of reservoirs, as high loads of organic materials and nutrients correspond to a proportionally large watershed area relative to
volume (Wetzel 1990). Blue-green algae predominate the phytoplankton community, indicative of poor water quality.
Blooms typically occur in summer, along with thermal stratification, creating hypolimnetic oxygen deficits that historically
have led to partial fish kills in some years (Hart and Birdsey
2008).
1119
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
CROWDED RESERVOIR FISH ASSEMBLAGE
Scofield Reservoir is managed as a put-grow-and-take sport
fishery, and historically, around 600,000 Rainbow Trout (150–
250 mm TL) were stocked every year. However, in 2005, in
response to the appearance of Utah Chub and fear of a population expansion, the fish stocking program has been adjusted
nearly every year since (Hart and Birdsey 2008). Sterile tiger
trout and Bonneville Cutthroat Trout have been stocked in the
fishery as a potential biological control for the Utah Chub, as
well as alternative sport fishes. Utah Division of Wildlife
Resources began stocking tiger trout in 2005 and continued to
stock an average of 110,000 (about 150 mm TL) each fall
since. Additionally, 85,000 Cutthroat Trout were stocked each
year beginning in spring of 2007 (200 mm TL). Rainbow
Trout were stocked at high numbers in the spring (400,000 fish
at 80 mm) and fall (120,000 at 175 mm) from 2005 to 2009,
but that effort was later reduced (80,000 at 180 mm TL once
yearly). Non-game, native species also present include the
Redside Shiner Richardsonius balteatus and Mountain Sucker
Catostomus platyrhynchus; however, Utah Chub are by far the
predominant nongame fish present in number and biomass
(Winters 2014).
Field sampling.—We sampled fishes in spring (April–
May), summer (July–August), and autumn (October) of 2012
in Scofield Reservoir. In this type of fixed-station sampling,
index sites were selected to be representative of the reservoirs’
longitudinal axis, i.e., from the upper riverine zone to the
lower lacustrine zone (McMahon et al. 1996), and maintained
consistency with long-term Utah Division of Wildlife Resources annual monitoring (Figure 1). We set horizontal sinking
gill nets (24 £ 1.8 m with eight monofilament mesh-size panels of 38, 57, 25, 44, 19, 64, 32, and 51 mm) according to standard gill-net methods to capture a representative size
distribution of all fish in the reservoir (Beauchamp et al.
2009). We placed two nets at each of eight sample sites in littoral areas perpendicular to shore. Most nets were set approximately 5 m from shore at 3 m deep and ending at 10 m deep;
shorelines had consistently steep bathymetry. We set nets
before dusk and pulled them at dawn to span two crepuscular
periods.
Diet analysis.—We collected Cutthroat Trout, Rainbow
Trout, tiger trout, and Utah Chub diets from each gill-net survey. We immediately preserved trout and chub stomachs in
95% ethanol for later analysis. We identified all organisms
from stomach contents to the lowest taxonomic level possible
(Brooks 1957; Edmonson 1959; Merritt and Cummins 1996)
to determine abundance of species and composition. We
grouped stomach contents by prey fish (identified to species),
zooplankton, organic matter, crayfish, aquatic invertebrates by
taxa (Amphipoda, Chironomidae, Coleoptera, Diptera,
Ephemeroptera, Hemiptera, Isopoda, Mollusca, and Trichoptera), and terrestrial invertebrates (classified to order). We
counted and weighed (blot-dry wet weight to nearest 0.001 g)
individual prey fish, and weighed invertebrate prey collectively by taxonomic group. We measured intact prey fish to
the nearest millimeter by backbone and standard lengths. We
then calculated the contribution of each prey category to the
diet of each predator species as the mean proportion by weight
(g) for each stomach individually and then averaged across all
nonempty stomachs (Chipps and Garvey 2007):
MW i D 1=N
N
X
W ij =
jD1
Q
X
!
W ij ;
iD1
where N is the total number of fish with nonempty stomachs,
Wij is the weight of prey i in the stomach of predator j, Q is the
total number of prey categories, and MW i is the mean proportion by weight.
We used Schoener’s index of diet overlap to calculate the
percent diet overlap between species:
/ D 1 ¡ 0:5
n
X
!
jpxi ¡ pyi j :
iD1
where / is the degree of overlap, n is the number of food categories, pxi is the proportion of food category i in the diet of
species x, and pyi is the proportion of food category i in the
diet of species y (Schoener 1970). Index values range from 0
to 1, where a value approaching 0 means the species share no
prey resources and a value closer to 1 means the species have
identical prey utilizations. Values exceeding 0.6 are considered “biologically significant” in terms of overlap in resource
use (Wallace 1981). All trout were pooled into small
(<350 mm TL) and large (350 mm TL) size-classes for
each species, and their diet overlap values were calculated for
each season of the study period. Utah Chub were pooled into
small (<250 mm TL) and large (250 mm TL) size-classes.
We based size-classes on initial observations of diet differences, sample size limitations (we could not add additional sizeclasses), and most importantly on the approximate size at
which Cutthroat and tiger trout demonstrated a switch to
piscivory.
Trophic position analysis.—From all gill-netted trout and
chub, we removed a small dorsal muscle tissue sample for isotopic diet analysis. We chose to utilize a representative group
of all tissues from October 2011 for analysis, due to the costly
nature of isotope preparation and analysis. Fall isotope tissues
encompass both the summer and fall growing season, as tissue
turnover rates allow for temporal integration of dietary information (Bearhop et al. 2002). We quantified longer-term dietary habits of a subset of Cutthroat Trout (n D 22), Rainbow
Trout (13), tiger trout (10), and Utah Chub (10) using stable
isotope analysis (Vinson and Budy 2011). Specifically, we
assessed fish trophic position and dietary carbon source based
on the respective d15N and d13C signatures of dorsal muscle
tissue (e.g., Post 2002). Tissue samples were dried in an oven
for 48 h at 70 C, ground to a powder, encapsulated in 8.5 mm
WINTERS AND BUDY
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
1120
FIGURE 1. Map of Scofield Reservoir, Utah, showing the eight locations where fish were sampled in 2011 and 2012, denoted as black circles.
tin capsules, and analyzed at the Washington State University
Stable Isotope Core for a mass spectrometry-based determination of isotopic signatures. Signatures are an expressed ratio
(15N:14N and 13C:12C), as per mille (%) values relative to
ratios of the standard atmospheric N2 and Pee Dee Belemnite,
respectively.
We then plotted isotopic signature values as coordinates in
niche space to determine both resource and habitat use of each
species (Newsome et al. 2007). We used the program SIBER
(Stable Isotope Bayesian Ellipses in R) to fit standard ellipses
based on multivariate normal distributions and maximum likelihood estimators. We then calculated standard ellipse area
CROWDED RESERVOIR FISH ASSEMBLAGE
1121
and overlap, corrected for small sample size (Jackson et al.
2011).
Gape limit.—To determine the gape size of predators,
which indicates the size of fish prey consumable, we measured
several morphometric features of each predator, including
total length (mm), and using a digital caliper, measured length
of the lower (LLJ) and upper (LUJ) jaw (0.1 mm). We then calculated gape size (G) assuming a maximum mouth opening of
60 during food uptake, as per Jensen et al. (2004):
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
Gpred D [.LLJ sin60/2 C .LUJ ¡ LLJ cos60/2 ]0:5 ;
where Gpred is the gape size of the predator, LLJ is length of the
lower jaw (mm), and LUJ is the length of the upper jaw (mm).
Second, we also measured the stretched width (laterally left to
right) of the predator’s mouth via a digital caliper (0.1 mm;
Truemper and Lauer 2005).
To determine actual sizes of prey consumed by piscivores,
we examined diet contents of predators. From fish prey found
in diets, when possible, we obtained prey backbone lengths,
standard lengths, and total length measurements. Backbone
length was measured as the full vertebrae when the fish musculature was digested. To determine the size of prey vulnerable to predators based on their gape, we also measured body
depth and total length of prey (i.e., Utah Chub) found in the
reservoir, and created a relationship relating body depth to
total length: BD D 0:27.TL/ ¡ 2:81, where BD is the body
depth of Utah Chub and TL is the total length of Utah Chub
(R2 D 0.90, n D 40, P < 0.0001).
RESULTS
Diet Composition
Food habits varied substantially among trout species and
Utah Chub in Scofield Reservoir. We found Utah Chub in the
diets of large (350 mm TL) Cutthroat Trout (n D 69 total;
23% empty) throughout every season in 2012 (Figure 2). In
the spring, large Cutthroat Trout (n D 28) stomachs contained
fish (56%) and chironomids (24%). In summer (n D 12), consumption of fish increased (69%), and by fall (n D 13), 100%
of large Cutthroat Trout diets contained fish. All fish identified
in stomach samples were Utah Chub. Small Cutthroat Trout
(n D 75 total; 13% empty) relied more heavily on aquatic
invertebrates. In the spring (n D 22) small Cutthroat Trout
consumed mostly chironomids (39%) but also Coleoptera,
Diptera, Ephemeroptera, Isopoda, and Mollusca. In the summer (n D 17), we found a variety of items in small Cutthroat
Trout diets, including chironomids (26%), terrestrial invertebrates (mostly flying ants; 20%), and crayfish (18%). Small
Cutthroat Trout in the fall (n D 26) shifted to a diet predominated by zooplankton (54%).
Tiger trout displayed similar food habits to Cutthroat Trout;
large tiger trout (n D 89; 36% empty) were distinguished by
FIGURE 2. Seasonal diet composition (proportion of diet by wet weight, g)
of small (<350 mm TL) and large (350 mm TL) Cutthroat Trout, Rainbow
Trout, and tiger trout captured in Scofield Reservoir. We did not catch any
small tiger trout or large Rainbow Trout in autumn 2011, therefore these columns are blank. All prey in the “fish” category was identified as Utah Chub.
their piscivorous diet, containing a high proportion of Utah
Chub that varied by season (Figure 2). In the spring, large
tiger trout (n D 29) consumed fish (46%), crayfish (22%), and
isopods (10%). In the summer, large tiger trout (n D 14) diet
was composed of only fish (79%) and crayfish (21%). In the
fall (n D 14), large tiger trout diets switched back to include
fish (55%), crayfish (10%), and chironomids (15%). Small
tiger trout (n D 44; 11% empty) consumed large numbers of
aquatic invertebrates, in addition to a substantial proportion of
crayfish (probably virile crayfish Orconectes virilis) throughout all seasons. Small tiger trout in the spring (n D 5) consumed 80% aquatic invertebrates (including molluscans and
chironomids). In the summer, small tiger trout (n D 16) consumed primarily crayfish (42%), isopods (16%), and terrestrial
invertebrates (13%), and in the fall (n D 18), their diet was
predominantly composed of chironomids (33%), crayfish
(22%), and zooplankton (21%).
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
1122
WINTERS AND BUDY
Rainbow Trout diets differed considerably from the trends
described above. Diets of small (n D 69; 3% empty) and large
Rainbow Trout (n D 15; 13% empty) were characterized by
low proportions of prey fish (Figure 2). Small Rainbow Trout
in the spring (n D 28) consumed primarily dipterans (28%),
terrestrial invertebrates, and organic matter. Large Rainbow
Trout (n D 3) included crayfish (33%) in their diet. Similarly,
in the summer, small Rainbow Trout (n D 35) continued to
consume dipteran (25%) and organic matter (22%). Large
Rainbow Trout (n D 8) consumed a variety of items, including
chironomids, crayfish, terrestrial invertebrates, and organic
matter. Noting the relatively small sample size, small Rainbow
Trout in the fall (n D 4) had a broad diet of crayfish (25%),
zooplankton (25%), fish (25%), and terrestrial invertebrates
(25%). We found stomachs of large Rainbow Trout (n D 2) at
the same time to have chironomids (31%) and crayfish.
Utah Chub diets consisted primarily of aquatic invertebrates, organic matter, and zooplankton at both smaller (n D
55; 45% empty) and larger (n D 33; 39% empty) size-classes.
In the spring small Utah Chub (n D 16) diets contained primarily organic matter (30%), chironomids, and zooplankton. This
trend was similar with small Utah Chub in the summer (n D
10), when diets consisted of mostly of zooplankton (40%) and
organic matter (30%). Large Utah Chub (n D 13) diets in the
spring were predominantly dipterans (58%), switching to a
variety of crayfish (31%), organic matter (30%), and zooplankton (20%) in the summer (n D 5). Both small (n D 4) and
large Utah Chub (n D 2) relied heavily on zooplankton in the
fall (72–100%).
Diet Overlap
Based on Schoener’s index, there was significant diet overlap between small Cutthroat Trout (a D 0.61) and small Rainbow Trout (a D 0.63) with large Utah Chub, probably due to
large proportions of chironomids in both fishes diets. Similarly, during the summer season, there was significant diet
overlap between small Cutthroat Trout with large Rainbow
Trout (a D 0.68) and with small tiger trout (a D 0.61),
highlighting a shared reliance on chironomids, isopods, and
crayfish. Additionally, there was overlap between small and
large Rainbow Trout (a D 0.62) and small and large Utah
Chub (a D 0.70). In autumn, we observed significant diet
overlap between small Cutthroat Trout and small Utah Chub
(a D 0.68) due to a shared reliance on zooplankton, between
small tiger trout and large Rainbow Trout (a D 0.65), probably
because of similar chironomid and crayfish consumption, and
between small and large Utah Chub (a D 0.72).
Isotopic Signature
Fish tissues collected for isotopic analyses displayed carbon
and nitrogen signatures indicative of their habitat use and feeding position. Tiger trout (n D 10) demonstrated the most
enriched carbon isotopic signature, corresponding with their
use of more littoral primary carbon sources. Utah Chub (n D
10), however, demonstrated the most negative carbon isotopic
signature, reflecting usage of pelagic habitat at all sizes. Utah
Chub were significantly more depleted in carbon than tiger
trout (ANOVA: df D 50, P < 0.001), as well as Rainbow
Trout (ANOVA: df D 50, P D 0.049; n D 13). Tiger trout
were significantly enriched in carbon compared with Cutthroat
Trout (ANOVA: df D 50, P D 0.005; n D 22). There were no
statistical differences between carbon signatures among sizeclasses (i.e., small versus large).
Based on d15N results, large Cutthroat Trout hold a
position at the top of the food web and higher than all
small fish: Cutthroat Trout (ANOVA: df D 50, P < 0.001),
Rainbow Trout (df D 50, P < 0.001), tiger trout (df D 50,
P < 0.001), and Utah Chub (df D 50, P < 0.001). Large
tiger trout share a similar trophic position to Cutthroat
Trout as predators at the top of the food web, having significantly higher nitrogen signatures than small Rainbow
Trout (ANOVA: df D 50, P D 0.003), small tiger trout (df
D 50, P < 0.001), and small Utah Chub (df D 50, P D
0.01). The nitrogen signature of large Rainbow Trout is
somewhat intermediary, and only significantly higher than
small Rainbow Trout (df D 50, P < 0.001).
Based on isotopic signatures, small and large Utah Chub
appeared to feed most pelagically. Small Cutthroat Trout
fed at nearly the same position as both small and large
Utah Chub, a mid-level trophic position. All three trout
species varied in niche space with respect to size-class,
demonstrating an ontogenetic shift around 350 mm TL, to
higher trophic positions (i.e., switch to piscivory). These
results suggest small tiger trout and Rainbow Trout fed in
littoral areas, whereas the diet of larger individuals
appeared to originate in more pelagic areas. Cutthroat
Trout sampled appeared to shift to feeding more in littoral
areas as body size increased.
When the isotopic niche of these species is plotted in twodimensional niche space, large tiger trout demonstrated a
very broad niche (ellipse area [EA] D 10.4; Figure 3). They
consumed food across a wide range of trophic positions, and
their isotopic niche space varied with respect to their
basal resources. In contrast, large Cutthroat Trout (EA D 2.2)
and large Rainbow Trout (EA D 2.7) both demonstrated relatively smaller, more focused isotopic niche areas, which
overlapped substantially with the tiger trout niche (78% and
64%, respectively), and overlapped with each other (31% and
26%, respectively). The isotopic niche space of tiger trout
only overlapped 17% with both Cutthroat Trout and Rainbow
Trout.
Small Rainbow Trout (EA D 3.2) and small tiger trout
(EA D 2.8) isotopic niche spaces overlapped substantially
(45% of the Rainbow Trout niche and 52% of the tiger trout
niche), indicating they shared similar prey resources at
an intermediate trophic position. Small Cutthroat Trout
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
CROWDED RESERVOIR FISH ASSEMBLAGE
1123
FIGURE 4. Cutthroat Trout, Rainbow Trout, and tiger trout body length versus length of Utah Chub found in diets of trout in Scofield Reservoir, Utah.
Gray circles represent actual sizes of prey found in trout diets. Lines show the
calculated gape limit for trout based on two gape measurements. Although
Rainbow Trout did consume fish prey, we found no whole measureable
fish prey in diets.
FIGURE 3. Two-dimensional isotopic (d15N and d13C) niche plots of four
fish species from Scofield Reservoir. Top panel depicts large (350 mm) Cutthroat Trout, Rainbow Trout, tiger trout, and Utah Chub (250 mm); bottom
panel depicts small (<350 mm) Cutthroat Trout, Rainbow Trout, tiger trout,
and Utah Chub (<250 mm). Dots represent individual isotopic signatures and
circles represent Stable Isotope Analysis in R (SIAR)-based isotopic ellipses.
(EA D 6.6), appeared to share a similar feeding niche space
with all sizes of Utah Chub, 19% of their niche overlapping
with the entirety of the small chub niche.
Gape Limit
Cutthroat Trout became piscivorous at approximately
320 mm TL, and consumed Utah Chub near and well above
both their estimated horizontal and vertical gape size
(Figure 4). Utah Chub found in Cutthroat Trout diets ranged
from 80 to 272 mm TL. In several instances, Cutthroat Trout
consumed chub greater than 50% of their body size. In fact,
one 425-mm Cutthroat Trout consumed a 272-mm chub (64%
of the trout’s body size). However, on average, Cutthroat
Trout consumed prey fish 30% of their body size. Horizontal
gape-width limit were very similar to the gape-size limit calculated using vertical gape measurements.
Tiger trout switched to piscivory at approximately 340 mm
TL and consumed fish very close to or just exceeding their
horizontal and vertical gape sizes (Figure 4). Utah Chub found
in tiger trout diets ranged from 37 to 234 mm TL. On average,
tiger trout consumed prey fish approximately 28% of their
body size. Notably, one 418-mm tiger trout consumed a 202mm chub (48% of its body size). Similar to Cutthroat Trout,
horizontal gape-width limit was very similar to gape-size
limit. Rainbow Trout, however, demonstrated limited piscivory in Scofield Reservoir, and we observed no measureable
prey fish in their diets.
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
1124
WINTERS AND BUDY
DISCUSSION
In our study, we presented new information describing the
feeding ecology and potential for interspecific interactions of a
novel assemblage of apex predators in a typical western reservoir. Overall, the fish composition in Scofield Reservoir was
predominated by the potentially nuisance species, Utah Chub
(Ward et al. 2008; Eilers et al. 2011), a species that consumed
large quantities of aquatic invertebrates and zooplankton
throughout the growing season. We caught consistently low
numbers of Rainbow Trout, which consumed few prey fish,
and relied substantially on aquatic and terrestrial invertebrates
for food resources. Cutthroat Trout and tiger trout were much
more abundant (Winters 2014), appeared to share the top trophic position in the food web, and consumed Utah Chub as a
large proportion of their diet. Both Cutthroat Trout and tiger
trout occasionally consumed Utah Chub at and above theoretical predictions of gape limit, demonstrating they were not
food-limited based on gape morphology or prey size. A high
degree of diet overlap was demonstrated among all small trout
and Utah Chub, indicating the potential for competition.
The potential for competition between top predators was
also demonstrated by similar trophic positions among Cutthroat Trout and tiger trout. Interestingly, there was no evidence of intraguild predation by one species on the other,
based on isotopes and an absence of identifiable trout in any
stomach contents (Polis and Holt 1992; Arim and Marquet
2004). The diets of Cutthroat Trout differed substantially from
diets reported from other lentic systems. Within Strawberry
Reservoir, Utah, for example, Daphnia were important prey
for juvenile Cutthroat Trout and were seasonally important to
adult fish; fish were only a minor contributor to adult diet
(Baldwin et al. 2000). In our study, zooplankton was important to small Cutthroat Trout only in summer, and adult fish
were consistently and strongly piscivorous.
Tiger trout are a relatively new hybrid, one for which there
is little empirical information; however, they are thought to
represent a voracious predatory trout. Because of their potential to utilize nuisance prey fish, they have been recently added
to a large number of stocking programs (Hepworth et al. 2009;
Hepworth et al. 2011). We confirmed that perception, given
that fish (Utah Chub) were their predominant prey, and Winters (2014) found tiger trout in Scofield Reservoir consumed
an extremely large number of Utah Chub across the year. In
contrast, tiger trout in eastern Washington appeared to rely
substantially on Daphnia and were only intermittently piscivorous (i.e., on Pumpkinseed Lepomis gibbosus and Redside
Shiner) and only in the summer months (Miller 2010). Miller
postulated that on lakes not dominated by zooplankton, tiger
trout had switched to a more benthic diet. While we did
observe tiger trout consuming a significant proportion of crayfish, a benthic prey item in this study, fish were still the predominant food source, particularly in summer.
Shared diet among Cutthroat Trout and tiger trout for Utah
Chub alone does not necessarily indicate competition for prey
resources (Matthews et al. 1982). Rather, resources must be
limited and fitness or performance-related factors such as
growth, condition, and fecundity must be negatively affected
in order for competition to occur. Consequently, since Cutthroat Trout and tiger trout are abundant and caught at large
sizes and Utah Chub prey are prolific, there is probably minimal competition between these trout (Winters 2014). However, interspecific competition or predation rates on other
organisms may increase if the Utah Chub population collapsed
under high predation pressure or if the trout population continued to expand via increases in abundance and survival (Bunnell et al. 2006; Svanback and Bolnick 2007).
Predator morphology and size, as well as prey size, strongly
influence rates of piscivorous predation (Hambright 1991;
Fritts and Pearsons 2006). Cutthroat Trout and tiger trout displayed an ontogenetic shift to piscivory around 330-mm TL
and consumed prey fish at an average of 30% of their own
size, increasing prey size as predator size increased (Juanes
et al. 2002). Additionally, the mouth size and morphometry of
these top piscivores does not appear to limit prey consumption
because both species consumed prey at and above their theoretical gape limits. This ability to consume larger prey may be
important to managers attempting to use trout as biological
control agents. While some studies suggest the vulnerability of
prey larger than predator gape size is reduced to zero (Hambright 1991; Hill et al. 2004), prey can be attacked and then
manipulated in such a way as to consume larger than expected
sizes (Nilsson and Bronmark 2000).
Predation is also dependent on prey and predator behavior
and distribution. Given the high densities of Utah Chub in the
reservoir and the relatively small size and simple bathymetry,
high spatio-temporal overlap with trout is likely, making foraging for prey fish more favorable. The extremely enriched
nitrogen signatures of large Cutthroat Trout and large tiger
trout, as well as diet composition consisting of a large proportion of Utah Chub, confirm these predators are piscivorous. As
such, they share a top trophic position in the food web, relying
heavily on prey fish. Elsewhere, small Cutthroat Trout also
consistently consume invertebrates and become increasingly
piscivorous as they grow, large adults consuming fish nearly
exclusively (Nowak et al. 2004; McIntyre et al. 2006; Heredia
2014).
In contrast to Cutthroat Trout and tiger trout, Rainbow
Trout were less piscivorous in Scofield Reservoir. In addition,
based on carbon in isotopic signatures (more depleted), Rainbow Trout also occupied a more pelagic habitat than the other
trout species, which were more littoral, as has been observed
elsewhere (Vander Zanden et al. 1999; Layman et al. 2012).
Although our sample size of Rainbow Trout diets was unsurprisingly low, the diet results were corroborated by isotopic
signatures and demonstrated a lower trophic position (nitrogen
signature). In addition, our results were consistent with some
previous studies demonstrating that most strains of Rainbow
Trout rely heavily on aquatic invertebrates, zooplankton, or on
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
CROWDED RESERVOIR FISH ASSEMBLAGE
only a small percentage of prey fish (Tabor et al. 1996; Baldwin et al. 2000; Haddix and Budy 2005). In contrast, however,
there are many studies which depict Rainbow Trout as aggressive piscivores (McDowall 2003; Juncos et al. 2011; Yard
et al. 2011). As such, it appears the current suite of top predators in Scofield Reservoir may have indirect effects on Rainbow Trout through exploitative competition or behavioral
interactions (Duffy et al. 2007; Sih et al. 2010). The presence
of Cutthroat Trout and tiger trout could cause Rainbow Trout
to decrease activity, exhibit predator avoidance, or change
feeding strategies (Romare and Hansson 2003). Diet overlap
between small Rainbow Trout and Utah Chub may also lead to
poor fitness, if food is limiting at that trophic level. Accordingly, catch rates of Rainbow Trout throughout our study were
low (Winters 2014), implying low survival rates and potentially negative effects from food web interactions.
We observed limited significant isotopic niche overlap
between large Rainbow Trout and Utah Chub, contradicting
the expectation that Rainbow Trout were performing poorly
due to shared food resources with a nuisance species. However, the small sample size of Rainbow Trout may have substantially and unrealistically altered calculations of diet
proportions by exaggerating average contributions of diet
items. Diets are a short-term snapshot of feeding patterns and
subject to stochasticity (Wallace and Ramsey 1983). Nonetheless, Marrin and Erman (1982) also observed minimal diet
overlap between Brown Trout and Rainbow Trout with Tui
Chub Siphateles bicolor, demonstrating these trout and nongame fish partition resources and contradicting the common
belief that competition for food resources was the cause for
the decrease in trout performance in that system. Although
apparent survival of Rainbow Trout, a potential indicator of
interaction strength (Keeley 2001), is extremely low in Scofield Reservoir, we may not have captured changes in competitive abilities associated with size and environmental
conditions (Hayes 1989).
The trophic positions of small Cutthroat Trout, Rainbow
Trout, and tiger trout all shifted to higher nitrogen values at
larger sizes, indicating a strong ontogenetic shift in diet
(Werner and Gilliam 1984; Grey 2001). Rainbow Trout also
shifted to a more negative d13C value (more pelagic) as they
became larger. Interestingly, Vander Zanden et al. (1999)
documented a similar carbon shift of Lake Trout Salvelinus
namaycush in black bass-invaded (Micropterus) Canadian
lakes, reflecting a diet shift towards zooplankton and reduced
dependence on littoral prey fish. This diet shift from the littoral
to the pelagia for Rainbow Trout in Scofield Reservoir may
indicate Rainbow Trout are altering their trophic niche in
response to direct or indirect competition with the relatively
newer predators stocked into this system (Correa et al. 2012).
Accordingly, their diet and habitat use appear to be limiting
their overall performance.
Our dietary findings are particularly relevant given the
recent interest in tiger trout and their expanding presence in
1125
stocking regimes. Several lines of evidence suggest that tiger
trout are performing very well and are not limited by strong
food web interactions such as competition or predation. Utah
Chub make up at least 30% of the diet of tiger trout seasonally,
suggesting encounter rates are high and prey fish are energetically favorable (Wallace 1981). Large tiger trout also have a
wide breadth of diet preferences, relying on a diversity of
prey. In addition, this hybrid species feeds more littorally than
other trout, thus potentially allowing them to minimize competition for food and space (Petchey 2000; Helland et al. 2011).
Accordingly, tiger trout hold a top trophic position in the ecosystem, and they are ideal predators to stock into reservoir
sport fisheries because they are sterile hybrids and energy normally allocated for gamete production should be diverted into
growth (Budy et al. 2012). Finally, for connected water bodies, sterile trout are easier to control over the long-term (Scheerer and Thorgaard 1987), a consideration that has important
conservation implications.
In sum, the combination of gut analyses, stable isotopes,
and gape limitation studies applied herein provide a more
complete understanding of potential limitations due to competitive or predatory interactions among a suite of apex predators
in a novel fish community. Competition for food resources
between sport and nongame fishes is commonly cited as a reason for decreased trout angling success, and competition is
often assumed to exist between desired sport fish and undesired invasive prey species (Marking 1992). Nevertheless,
competition does not always occur, and our data suggest two
top trout predators in the system are not currently competing,
probably because food is not limiting. Both tiger trout and Cutthroat Trout monopolized upon the abundant prey fish, Utah
Chub; the growth and survival rates of these predators were
high, and there is considerable evidence indicating they are
able to control the chub population (Winters 2014). However,
reservoirs are dynamic, and for effective management, novel
fish communities may need to be monitored carefully because
large-scale changes in the food web and annual changes in reservoir volume can alter predator–prey interactions.
ACKNOWLEDGMENTS
Funding and support for this project was provided by the
Utah Division of Wildlife Resources (Federal Sport Fish Restoration, Project F-134-R), U.S. Geological Survey, Utah
Cooperative Fish and Wildlife Research Unit, in-kind, and the
Ecology Center at Utah State University. Special thanks to
Craig Walker, Paul Birdsey, Justin Hart, and Calvin Black of
the Utah Division of Wildlife Resources for their assistance
with data collection and project logistics. We are also grateful
to the many other individuals from Utah State University who
assisted in collecting and analyzing fish stomachs for this
study, including Konrad Hafen, Jared Baker, and Hannah
Moore. Additionally, we thank Deanna Strohm for editorial
assistance and Alan Kasprak for assistance with figures. We
1126
WINTERS AND BUDY
are grateful to Bryce Roholt, who helped complete the gape
limitation work as part of an undergraduate research project.
Finally, we thank Gary Thiede for his logistical support,
expertise, and comments that greatly improved this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the
U. S. Government. This research was conducted under the auspices of a State of Utah COR number 1COLL8712 and
IACUC protocol 1544.
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
REFERENCES
Arim, M., and P. A. Marquet. 2004. Intraguild predation: a widespread interaction related to species biology. Ecological Letters 7:557–564.
Bacheler, N. M., J. W. Neal, and R. L. Noble. 2004. Diet overlap between
native Bigmouth Sleepers (Gobiomorus dormitor) and introduced predatory
fishes in a Puerto Rico reservoir. Ecology of Freshwater Fish 13:111–118.
Baldwin, C. M., D. A. Beauchamp, and J. J. Van Tassell. 2000. Bioenergetic
assessment of temporal food supply and consumption demand by salmonids
in the Strawberry Reservoir food web. Transactions of the American Fisheries Society 129:429–450.
Bearhop, S., S. Waldron, S. C. Votier, and R. W. Furness. 2002. Factors that
influence assimilation rates, and fractionation of nitrogen and carbon isotopes in avian blood and feathers. Physiological and Biochemical Zoology
75:451–458.
Beauchamp, D. A., D. L. Parrish, and R. A. Whaley. 2009. Coldwater fish in
large standing waters. Pages 97–118 in S. A. Bonar, W. A. Hubert, and
D. W. Willis, editors. Standard methods for sampling North American freshwater fishes. American Fisheries Society, Bethesda, Maryland.
Brooks, J. L. 1957. Identifications and nomenclature of Cladocera, Calanoida,
and Cyclopoida. Memoirs of the Connecticut Academy of the Arts and Science 13(1).
Budy, P., G. P. Thiede, A. Dean, D. Olsen, and G. Rowley. 2012. A comparative and experimental evaluation of performance of stocked diploid and triploid Brook Trout. North American Journal of Fisheries Management
32:1211–1224.
Bunnell, D. B., C. P. Madenjian, and R. M. Claramunt. 2006. Long-term
changes of the Lake Michigan fish community following the reduction of
exotic Alewife (Alosa pseudoharengus). Canadian Journal of Fisheries and
Aquatic Sciences 63:2434–2446.
Carey, M. P., and D. H. Wahl. 2010. Interactions of multiple predators with
different foraging modes in an aquatic food web. Oecologia 162:443–452.
Chipps, S. R., and J. E. Garvey. 2007. Assessment of diets and feeding patterns. Pages 473–514 in C. S. Guy and M. L. Brown, editors. Analysis and
interpretation of freshwater fisheries data. American Fisheries Society,
Bethesda, Maryland.
Clavero, M., V. Hermoso, E. Aparicio, and F. N. Godinho. 2013. Biodiversity
in heavily modified waterbodies: native and introduced fish in Iberian reservoirs. Freshwater Biology 58:1190–1201.
Correa, C., A. P. Bravo, and A. P. Hendry. 2012. Reciprocal trophic niche
shifts in native and invasive fish: salmonids and galaxiids in Patagonian
lakes. Freshwater Biology 57:1769–1781.
Denlinger, J. C. S., R. S. Hale, and R. A. Stein. 2006. Seasonal consumptive
demand and prey use by stocked saugeyes in Ohio reservoirs. Transactions
of the American Fisheries Society 135:12–27.
Dodds, W. K., and M. R. Whiles. 2010. Freshwater ecology: concepts and environmental applications of limnology, 2nd edition. Elsevier, Burlington, Vermont.
Duffy, J. E., B. J. Cardinale, K. E. France, P. B. McIntyre, E. Thebault, and
M. Loreau. 2007. The functional role of biodiversity in ecosystems: incorporating trophic complexity. Ecology Letters 10:522–538.
Edmonson, W. T. 1959. The rotifera. Pages 420–494 in G. C. Whipple and
H. B. Ward, editors. Freshwater biology, 2nd edition. Wiley, New York.
Eilers, J. M., H. A. Truemper, L. S. Jackson, B. J. Eilers, and D. W. Loomis.
2011. Eradication of an invasive cyprinid (Gila bicolor) to achieve water
quality goals in Diamond Lake, Oregon (USA). Lake and Reservoir Management 27:194–204.
Fritts, A. L., and T. N. Pearsons. 2006. Effects of predation by nonnative
Smallmouth Bass on native salmonid prey: the role of predator and prey
size. Transactions of the American Fisheries Society 135:853–860.
Grey, J. 2001. Ontogeny and dietary specialization in Brown Trout (Salmo
trutta L.) from Loch Ness, Scotland, examined using stable isotopes of carbon and nitrogen. Ecology of Freshwater Fish 10:168–176.
Haddix, T., and P. Budy. 2005. Factors that limit growth and abundance of
Rainbow Trout across ecologically distinct areas of Flaming Gorge Reservoir, Utah–Wyoming. North American Journal of Fisheries Management
25:1082–1094.
Hambright, K. D. 1991. Experimental analysis of prey selection by Largemouth Bass: role of predator mouth width and prey body depth. Transactions of the American Fisheries Society 120:500–508.
Hart, J., and P. W. Birdsey. 2008. Scofield research proposal. Utah Division of
Wildlife Resources, Southeastern Regional Office, Price.
Havel, J. E., C. E. Lee, and J. VanderZanden. 2005. Do reservoirs facilitate
invasions into landscapes? BioScience 55:518–525.
Hayes, J. W. 1989. Social interaction between 0+ Brown and Rainbow trout in
an experimental stream design. New Zealand Journal of Marine Freshwater
Resources 23:163–170.
Hays, G. C. 2003. A review of the adaptive significance and ecosystem consequences of zooplankton diel vertical migrations. Hydrobiologia 503:
163–170.
Helland, I. P., A. G. Finstad, T. Forseth, T. Hesthagen, and O. Ugedal. 2011.
Ice-cover effects on competitive interactions between two fish species. Journal of Animal Ecology 80:539–547.
Hepworth, R. D., M. J. Ottenbacher, and M. J. Hadley. 2011. Angler survey
and monitoring results, Fish Lake Utah 2010. Utah Department of Natural
Resources, Division of Wildlife Resources, Salt Lake City.
Hepworth, R. D., J. Warner, M. J. Ottenbacher, and M. J. Hadley. 2009. Panguitch Lake: an evaluation of the sport fishery management plan. Utah
Department of Natural Resources, Division of Wildlife Resources, Salt
Lake City.
Heredia, N. 2014. Trophic status, energetic demands, and factors affecting
Lahontan Cutthroat Trout distribution in Pyramid Lake, Nevada. Master’s
thesis. Utah State University, Logan.
Hill, J. E., L. G. Nico, C. E. Cichra, and C. R. Gilbert. 2004. Prey vulnerability
to peacock cichlids and Largemouth Bass based on predator gape and prey
body depth. Proceedings of the Annual Conference Southeastern Association for Fish and Wildlife Agencies 58:47–56.
Irwin, B. J., D. R. Devries, and R. A. Wright. 2003. Evaluating the potential
for predatory control of Gizzard Shad by Largemouth Bass in small
impoundments: a bioenergetics approach. Transactions of the American
Fisheries Society 132:913–924.
Jackson, A. L., R. Inger, A. C. Parnell, and S. Bearhop. 2011. Comparing isotopic niche widths among and within communities: SIBER - Stable Isotope
Bayesian Ellipses in R. Journal of Animal Ecology 80:595–602.
Jensen, H., T. Bohn, P. Amundsen, and P. E. Aspholm. 2004. Feeding ecology
of piscivorous Brown Trout (Salmo trutta L.) in a subarctic watercourse.
Annales Zoologici Fennici 41:319–328.
Juanes, F., J. A. Buckel, and F. S. Scharf. 2002. Feeding ecology of piscivorous fishes. Pages 267–283 in P. J. B. Hart and J. D. Reynolds, editors.
Handbook of fish biology and fisheries. Blackwell Scientific Publications,
Oxford, UK.
Juncos, R., D. Milano, P. J. Macchi, M. F. Alonso, and P. H. Vigliano. 2011.
Response of Rainbow Trout to different food web structures in northern
Patagonia: implications for growth, bioenergetics, and invasiveness. Transactions of the American Fisheries Society 140:415–428.
Keeley, E. R. 2001. Demographic responses to food and space competition by
juvenile steelhead trout. Ecology 85:1247–1259.
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
CROWDED RESERVOIR FISH ASSEMBLAGE
Kitchell, J. F., and L. B. Crowder. 1986. Predator–prey interactions in Lake
Michigan: model predictions and recent dynamics. Environmental Biology
of Fishes 16:205–211.
Knapp, R. A., C. P. Hawkins, J. Ladau, and J. G. McClory. 2005. Fauna of
Yosemite National Park lakes has low resistance but high resilience to fish
introductions. Ecological Applications 15:835–847.
Kohler, C. C., and W. R. Courtenay. 1986. Regulating introduced aquatic species: a review of past initiatives. Fisheries 11(2):34–38.
Layman, C. A., M. S. Araujo, R. Boucek, C. M. Hammerschlag-Peyer, E. Harrison,
Z. R. Jud, P. Matich, A. E. Rosenblatt, J. J. Vaudo, L. A. Yeager, D. M. Post, and
S. Bearhop. 2012. Applying stable isotopes to examine food-web structure: an
overview of analytical tools. Biological Reviews 87:545–562.
Lundvall, D., R. Svanback, L. Persson, and P. Bystrom. 1999. Size-dependent
predation in piscivores: interactions between predator foraging and prey
avoidance abilities. Canadian Journal of Fisheries and Aquatic Sciences
56:1285–1292.
Marking, L. L. 1992. Evaluation of the toxicants for the control of carp and
other nuisance species. Fisheries 17(6):6–13.
Marrin, D. L., and D. C. Erman. 1982. Evidence against competition between
trout and nongame fishes in Stampede Reservoir, California. North American Journal of Fisheries Management 2:262–269.
Matthews, W. J., J. Bek, and E. Surat. 1982. Comparative ecology of the darters Etheostoma podostemone, E. flabellare, and Percina roanoka in the
upper Roanoke River drainage, Virginia. Copeia 1982:805–814.
Matthews, W. J., K. B. Gido, and F. P. Gelwick. 2004. Fish assemblages of
reservoirs, illustrated by Lake Texoma (Oklahoma–Texas, USA) as a representative system. Lake and Reservoir Management 20:219–239.
McDowall, R. M. 2003. Impacts of introduced salmonids on native galaxiids in
New Zealand upland streams: a new look at an old problem. Transactions of
the American Fisheries Society 132:229–238.
McDowall, R. M. 2006. Crying wolf, crying foul, or crying shame: alien salmonids and a biodiversity crisis in the southern cool-temperate galaxioid
fishes? Review in Fish Biology and Fisheries 16:233–422.
McIntyre, J. K., D. A. Beuchamp, and M. M. Mazure. 2006. Ontogenetic trophic interactions and benthopelagic coupling in Lake Washington: evidence
from stable isotopes and diet analysis. Transactions of the American Fisheries Society 135:1312–1328.
McMahon, T. E., A. V. Zale, and D. J. Orth. 1996. Aquatic habitat measurements. Pages 83–120 in B. R. Murphy and D. W. Willis, editors. Fisheries
techniques. American Fisheries Society, Bethesda, Maryland.
Merritt, R. W., and K. W. Cummins. 1996. An introduction to the aquatic
insects of North America. Kendall and Hunt, Dubuque, Iowa.
Miller, A. L. 2010. Diet, growth, and age analysis of tiger trout from ten lakes in
eastern Washington. Master’s thesis. Eastern Washington University, Cheney.
Miranda, L. E., and D. R. DeVries, editors. 1996. Multidimensional
approaches to reservoir fisheries management. American Fisheries Society,
Symposium 16, Bethesda, Maryland.
Mittelbach, G. G., and L. Persson. 1998. The ontogeny of piscivory and its
ecological consequences. Canadian Journal of Fisheries and Aquatic Sciences 55:1454–1465.
Newsome, S. D., C. M. del Rio, and S. Bearhop. 2007. A niche for isotopic
ecology. Frontiers in Ecology and the Environment 5:429–436.
Ney, J. J. 1990. Trophic economics in fisheries: assessment of demand–supply relationships between predators and prey. Reviews in Aquatic Sciences 2:55–81.
Ney, J. J. 1996. Oligotrophication and its discontents: effects of reduced nutrient
loading on reservoir fisheries. Pages 285–295 in L. E. Miranda and D. R.
DeVries, editors. Multidimensional approaches to reservoir fisheries management. American Fisheries Society, Symposium 16, Bethesda, Maryland.
Ney, J. J., and D. J. Orth. 1986. Coping with future shock: the challenge of
matching predator stocking programs to prey abundance. Pages 81–92
in R. H. Stoud, editor. Fish culture in fisheries management. American
Fisheries Society, Bethesda, Maryland.
1127
Nilsson, P. A., and C. Bronmark. 2000. Prey vulnerability to a gape-size limited predator: behavioural and morphological impacts on northern pike piscivory. Oikos 88:539–546.
Noble, R. L. 1986. Management of reservoir fish communities by influencing species interactions: predator–prey interactions in reservoir communities. Pages
137–143 in G. E. Hall and M. J. Van Den Avyle, editors. Reservoir fisheries management strategies for the 80s. American Fisheries Society, Bethesda, Maryland.
Nowak, G. M., R. A. Tabor, E. J. Warner, K. L. Fresh, and T. P. Quinn. 2004. Ontogenetic shifts in habitat and diet of Cutthroat Trout in Lake Washington, Washington. North American Journal of Fisheries Management 24:624–635.
Petchey, O. L. 2000. Prey diversity, prey composition, and predator population
dynamics in experimental microcosms. Journal of Animal Ecology 69:874–882.
Polis, G. A., and R. D. Holt. 1992. Intraguild predation: the dynamics of complex trophic interactions. Trends in Ecology and Evolution 7:151–155.
Polis, G. A., and D. R. Strong. 1996. Food web complexity and community
dynamics. American Naturalist 147:813–846.
Post, D. M. 2002. Using stable isotopes to estimate trophic position: models,
methods, and assumptions. Ecology 83:703–718.
Romare, P., and L. Hansson. 2003. A behavioral cascade: top-predator induced
behavioral shifts in planktivorous fish and zooplankton. Limnology and
Oceanography 48:1956–1964.
Ruzycki, J. R., W. A. Wurtsbaugh, and C. Luecke. 2001. Salmonine consumption and competition for endemic prey fishes in Bear Lake, Utah–Idaho.
Transactions of the American Fisheries Society 130:1175–1189.
Scheerer, P. D., and G. H. Thorgaard. 1987. Performance and developmental
stability of triploid tiger trout (Brown Trout £ Brook Trout). Transactions
of the American Fisheries Society 116:92–97.
Schoener, T. W. 1970. Non-synchronous spatial overlap of lizards in patchy
habitats. Ecology 51:408–418.
Sih, A., D. I. Bolnick, B. Luttbeg, J. L. Orrock, S. D. Peacor, L. M. Pintor, E. Preisser, J. S. Rehage, and J. R. Vonesh. 2010. Predator-prey naivete, antipredator
behavior, and the ecology of predator invasions. Oikos 119:610–621.
Stein, R. A., D. R. DeVries, and J. M. Dettmers. 1995. Food-web regulation by
a planktivore: exploring the generality of the trophic cascade hypothesis.
Canadian Journal of Fisheries and Aquatic Sciences 52:2518–2526.
Svanback, R., and D. I. Bolnick. 2007. Intraspecific competition drives
increased resource use diversity within a natural population. Proceedings of
the Royal Society Series B 274:839–844.
Tabor, R. A., C. Luecke, and W. A. Wurtsbaugh. 1996. Effects of Daphnia
availability on growth and food consumption of Rainbow Trout in two Utah
reservoirs. North American Journal of Fisheries Management 16:591–599.
Tabor, R. A., and W. A. Wurtsbaugh. 1991. Predation risk and the importance
of cover for juvenile Rainbow Trout in lentic systems. Transactions of the
American Fisheries Society 120:728–738.
Terra, B. F., and F. G. Araujo. 2011. A preliminary fish assemblage index for a
transitional river-reservoir system in southeastern Brazil. Ecological Indicators 11:874–881.
Teuscher, D., and C. Luecke. 1996. Competition between kokanees and Utah
Chub in Flaming Gorge Reservoir, Utah–Wyoming. Transactions of the
American Fisheries Society 125:505–511.
Truemper, H. A., and T. E. Lauer. 2005. Gape limitation and piscine prey sizeselection by Yellow Perch in the extreme southern area of Lake Michigan, with
emphasis on two exotic prey items. Journal of Fish Biology 66:135–149.
Turek, K. C., M. A. Pegg, and K. L. Pope. 2013. Review of the negative influences of non-native salmonids on native fish species. Great Plains Research
23:39–49.
UDWR (Utah Division of Wildlife Resources). 2015. About the blue ribbon
fisheries program. Utah Department of Natural Resources. Available: http://
wildlife.utah.gov/br-about.html. (July 2015).
U.S. Bureau of Reclamation. 2009. Scofield Dam. U.S. Department of the Interior.
Available: http://www.usbr.gov/projects/Facility.jsp?fac_NameDScofieldC
Dam. (January 2012).
Downloaded by [Utah State University Libraries] at 08:14 13 October 2015
1128
WINTERS AND BUDY
U.S. Bureau of Reclamation. 2011. Scofield project. U.S. Department of the
Interior. Available: http://www.usbr.gov/projects/Project.jsp?proj_NameD
Scofield%20Project. (January 2012).
Utah DEQ (Department of Environmental Quality). 2010. Scofield Reservoir
TMDL. Utah Division of Water Quality, Salt Lake City.
Vander Zanden, M. J., J. M. Casselman, and J. B. Rasmussen. 1999. Stable isotope evidence for the food web consequences of species invasions in lakes.
Nature 401:464–467.
Vinson, M., and P. Budy. 2011. Sources of variability and comparability between
salmonids stomach contents and isotopic analyses: study design lessons and recommendations. Canadian Journal of Fisheries and Aquatic Sciences 68:137–151.
Wahl, D. A. 1999. An ecological context for evaluating the factors influencing
Muskellunge stocking success. North American Journal of Fisheries Management 19:238–248.
Wallace, R. K. 1981. An assessment of diet-overlap indexes. Transactions of
the American Fisheries Society 110:72–76.
Wallace, R. K., and J. S. Ramsey. 1983. Reliability in measuring diet overlap.
Canadian Journal of Fisheries and Aquatic Sciences 40:347–351.
Walsworth, T., P. Budy, and G. P. Thiede. 2013. Longer food chains and
crowded niche space: effects of multiple invaders on desert stream food
web structure. Ecology of Freshwater Fish 22:439–452.
Ward, A., J. Robinson, and R. B. Wilson. 2008. Management of a
Cutthroat Trout predator to control Utah Chub in a high-use sport fishery.
Pages 595–608 in M. S. Allen, S. Sammons, and M. J. Maceina, editors.
Balancing fisheries management and water uses for impounded river systems. American Fisheries Society, Symposium 62, Bethesda, Maryland.
Werner, E. E., and J. F. Gilliam. 1984. The ontogenetic niche and species
interactions in size-structured populations. Annual Review of Ecology and
Systematics 15:393–425.
Werner, E. E., and D. J. Hall. 1974. Optimal foraging and size selection of prey by
the Bluegill sunfish (Lepomis macrochirus). Ecology 55:1042–1052.
Werner, E. E., and D. J. Hall. 1977. Competition and habitat shift in two sunfishes (Centrarchidae). Ecology 58:869–876.
Wetzel, R. G. 1990. Land-water interfaces: metabolic and limnological
regulators. Verhandlungen des Internationalen Verin Limnologie 24:
6–24.
Winters, L. K. 2014. An evaluation of the food web dynamics and predator prey
interactions in Scofield Reservoir. Master’s thesis. Utah State University, Logan.
Wuellner, M. R., S. R. Chipps, D. W. Willis, and W. E. Adams Jr. 2010. Interactions between Walleyes and Smallmouth Bass in a Missouri river reservoir with consideration of the influence of temperature and prey. North
American Journal of Fisheries Management 30:445–463.
Yard, M. D., L. G. Coggins Jr., C. V. Baxter, G. E. Bennett, and J. Korman.
2011. Trout piscivory in the Colorado River, Grand Canyon: effects of turbidity, temperature, and fish prey availability. Transactions of the American
Fisheries Society 140:471–486.