https://orcid.org/0000-0002-0702-7332
Figures
Abstract
Species assemblages composed of non-native and native fishes are found in freshwater systems throughout the world, and interactions such as interspecific competition that may negatively affect native species are expected when non-native species are present. In the Smith River watershed, Montana, rainbow trout were introduced by 1930. Native mountain whitefish and non-native rainbow trout have presumably occurred in sympatry since the introduction of rainbow trout; however, knowledge about how these two species compete with one another for food resources is sparse. We quantified diet compositions of rainbow trout and mountain whitefish in the mainstem Smith River and in a tributary to the Smith River—Sheep Creek—to determine the degree of overlap in the diets of mountain whitefish and rainbow trout in the Smith River and between the mainstem Smith River and a tributary stream. Rainbow trout and mountain whitefish had generalist feeding strategies, which probably contribute to the amicable coexistence of these species. Diet overlap between rainbow trout and mountain whitefish was high (Pianka’s index value = 0.85) in the Smith River and moderate in Sheep Creek (Pianka’s index value = 0.57). Despite overlap in diets, some resource partitioning may alleviate resource competition (e.g., rainbow trout consumed far more Oligochaeta than mountain whitefish but fewer Brachycentridae and Chironomidae). Diet composition of rainbow trout and mountain whitefish did not differ greatly between the Smith River and Sheep Creek. Prey categories most commonly used by mountain whitefish at the population and individual levels (i.e., Ephemeroptera and Trichoptera) are sensitive taxa and many species within these orders have experienced extinctions and population declines. Therefore, future changes in resource availability or competition could be of concern.
Citation: Cox TL, Lance MJ, Albertson LK, Briggs MA, Dutton AJ, Zale AV (2023) Diet composition and resource overlap of sympatric native and introduced salmonids across neighboring streams during a peak discharge event. PLoS ONE 18(1): e0280833. https://doi.org/10.1371/journal.pone.0280833
Editor: Benigno Elvira, Complutense University of Madrid: Universidad Complutense de Madrid, SPAIN
Received: April 28, 2022; Accepted: January 9, 2023; Published: January 24, 2023
This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Data Availability: All relevant data are within the manuscript and its Supporting Information files (S1 File.csv).
Funding: Funding for this project was provided by the Montana State University Undergraduate Scholars Program, Montana Fish, Wildlife & Parks, the Bair Ranch Foundation, and the Smith River Corridor Enhancement Account. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The Montana Cooperative Fishery Research Unit is jointly sponsored by the U.S. Geological Survey, Montana Fish, Wildlife & Parks, Montana State University, and the U.S. Fish and Wildlife Service.
Competing interests: The authors have declared that no competing interests exist.
Introduction
Non-native fishes have been introduced to freshwater systems throughout the globe, often with dramatic consequences for the receiving ecosystems [1]. Non-native fishes can harm native fish populations through predation, competition, and hybridization [2, 3]. Throughout the western United States, the introductions of non-native salmonids such as rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis), and brown trout (Salmo trutta) have displaced or reduced the abundances of native salmonids [4–6]. Rainbow trout have been widely introduced throughout the world because they are a preferred sport fish that can be successfully cultured and acclimate to a wide array of conditions (e.g., lentic and lotic systems). Rainbow trout have been particularly successful in habitats outside of their native range and often become dominant after outcompeting native salmonid species [7]. Understanding how native and introduced salmonids compete for resources such as food and habitat is important for ensuring the persistence of native fishes.
Investigating the feeding ecology of fishes is fundamental for understanding species interactions, population dynamics, trophic structure, and energy transfer both within and between ecosystems [8]. Comparing the feeding ecology of species can shed light on the ways in which two species may compete for or share available food resources. For example, diet overlap can indicate resource sharing; however, the same prey types may be consumed in different portions of the water column or at different times [9]. Quantifying and characterizing fish diets are the first steps in evaluating resource sharing and provide managers with vital information about habitat requirements with implications for species conservation and restoration. Investigating feeding ecology can inform our understanding of how introduced and native salmonids interact within a watershed.
Interactions between native and introduced fishes may vary among locations within a watershed. Tributaries are important components of watersheds and are often subject to fewer anthropogenic influences, such as flow regulation, than mainstem habitats [10]. Tributaries increase habitat heterogeneity, provide spawning and rearing habitat, serve as thermal refugia during warm summer months, and often support abundant food resources for fishes [11–13]. Communities can have stark differences in the abundance and diversity of macroinvertebrate species between a mainstem stream and tributaries of it [14], which may induce changes in patterns of competition between fish species between mainstem and tributary habitats. Therefore, characterizing diet overlap in both mainstem and tributaries—particularly during migrations—may provide additional insight about interactions between competing species.
Mountain whitefish (Prosopium williamsoni) are a salmonid native throughout the northern Rocky Mountains. They persist in much of their native range and often make up a large portion of fish biomass and abundance [15, 16]. Because they are often not considered a desirable game species, mountain whitefish remain relatively understudied, with little information on population trends, diet, and habitat requirements (but see [17, 18]). Although extensive literature exists on the effects of introduced salmonids on native cutthroat trout and bull trout [19–21], studies identifying interactions between mountain whitefish and introduced salmonids, such as rainbow trout, are relatively rare. Understanding the interactions between mountain whitefish and introduced species may provide insights into population dynamics, such as why mountain whitefish remain abundant in certain locations where non-native salmonids are also abundant. In addition, knowledge of species interaction could help managers conserve mountain whitefish populations as introduced species continue to spread throughout the native range of mountain whitefish.
We investigated the feeding ecology of an introduced salmonid, the rainbow trout, and an understudied native salmonid, the mountain whitefish, in the Smith River watershed, a large, unfragmented watershed located in central Montana. Mountain whitefish have been documented entering tributaries during the spring [22, 23]. We hypothesized that they may be moving into tributaries to access seasonally available food resources because these movements occur outside of their annual spawning period and during periods when water temperatures are not stressful. Therefore, during the spring of 2016, we sampled the diets of mountain whitefish and rainbow trout to understand the patterns of food items consumed by both species during the spring and compare diets between the species. Our specific objectives were (1) to determine the degree of overlap in the diets of mountain whitefish and rainbow trout in the Smith River watershed and (2) to determine if diets of these two species vary between the mainstem Smith River and a tributary stream. Our findings provide some of the first evidence of diet overlap and potential competition between native mountain whitefish and non-native rainbow trout.
Materials and methods
Study area
This study took place in two streams in central Montana, USA. The Smith River originates near White Sulphur Springs, Montana, and flows 195 km northwest to its confluence with the Missouri River near Great Falls, Montana (Fig 1). At its headwaters, the Smith River flows through a broad, agricultural valley. The river then flows through a canyon in the Little Belt Mountains, followed by a prairie region as it approaches its confluence with the Missouri River [24]. Sheep Creek is one of the primary tributaries of the Smith River. Sheep Creek flows 59 km west from its headwaters in the Little Belt Mountains to its confluence with the Smith River, at the upstream portion of the canyon region. Both streams follow a typical hydrograph for snowmelt runoff streams, with peak discharge occurring in late spring to early summer and discharge gradually declining to base flows in mid-summer.
The Smith River watershed in central Montana. Sampling reaches are denoted by grey circles. Graphic created using ESRI software (Redlands, CA) and open-access data from the Montana State Library including Montana Major Streams and Lakes (1993) and Grayscale Shaded Relief Image of Montana, 30-meter (2002).
Native salmonids in the Smith River watershed include mountain whitefish and westslope cutthroat trout (Oncorhynchus clarkii lewisi; mainly isolated in small tributaries). Other native fishes include white sucker (Catostomus commersonii), longnose sucker (C. catostomus), mountain sucker (C. platyrhynchus), burbot (Lota lota), stonecat (Noturus flavus), Rocky Mountain sculpin (Cottus bondi), and longnose dace (Rhinichthys cataractae). Three non-native fishes were intentionally introduced in the twentieth century for recreational angling: brown trout, rainbow trout, and brook trout. The most abundant fishes in the watershed are mountain whitefish, rainbow trout, and brown trout [25, 26].
Data collection
Diet sampling was conducted on May 24–25, 2016, during spring runoff. Sampling was performed under the auspices of Montana State University institutional animal care and use protocol 2014–50 and scientific collectors permit 12–2016 issued by the Montana Department of Fish, Wildlife and Parks to Alexander Zale. We sampled one reach (about 2 km long) in each stream. The sampling reach on the Smith River was in the headwater portion of the watershed, upstream of the confluence with Sheep Creek. The sampling reach on Sheep Creek was located about 26 km upstream of the confluence of Sheep Creek and the Smith River. To capture fish, we used raft electrofishing in the Smith River and raft or barge electrofishing in Sheep Creek. A single pass was conducted along each site. We used a Coffelt VVP electrofisher and a smooth DC waveform. Output voltage varied between the sites; it was adjusted to the minimum voltage necessary to achieve good capture efficiency. Output amperage was maintained at five amperes or less. In Sheep Creek, a mobile anode configuration was used, and in the Smith River, the anodes were fixed booms attached to the electrofishing raft. The mobile-anode and boom-anode configurations were both maneuvered to effectively cover the width of the stream being fished. Fish were anaesthetized using Aqui-S 20E (Aqui-S New Zealand Ltd.; 10 to 20 mg/L titrated to efficacy) prior to handling and were allowed to recover in fresh water prior to release. We measured the total length (TL) of each fish and collected diets using a standard gastric-lavage technique with a modified garden sprayer with an attached 6.35-mm polypropylene tube for insertion into the esophagus of the fish. Stomach contents were collected into a tub, rinsed into a 500-μm mesh sieve, and preserved in 95% ethanol. Diets were collected from 11 mountain whitefish (TL range = 306–396 mm) and six rainbow trout (TL range = 285–415 mm) from the Smith River and 20 mountain whitefish (TL range = 244–408) and 20 rainbow trout (TL range = 138–395 mm) from Sheep Creek. In the laboratory, aquatic insects were identified to family, terrestrial insects were grouped and not identified further, and other diet items were identified to order using standard methods at 10×–100× magnification under a dissecting microscope [27]. We quantified the number of individuals and the proportion by count of each diet category in every fish diet. We evaluated diets with diet items categorized at both the family and order levels to understand hierarchical patterns in diet composition.
Data analysis
We used generalized linear models (GLM) with a quasi-binomial distribution and a logit-link function to model the proportion of each of eight prey types in the diets of mountain whitefish and rainbow trout. A quasi-binomial distribution is appropriate for proportional data with overdispersion. Our response variable was the proportion of items from the modelled diet category in the diet, weighted for the total number of items in the diet. Our explanatory variables were species, stream, the interaction between species and stream, and fish length. We constructed nine models for each response variable, ranging from a global model containing all explanatory variables to a null model containing only an intercept term. We used QAICc, which incorporates a dispersion parameter, to select a top model for each prey type [28]. From all model options, we selected the most parsimonious model with ΔQAICc ≤ 2 as the final model for each prey type (S1 Table). We then used model output to estimate the predicted probability of consumption of each prey type by mountain whitefish and rainbow trout in the Smith River and Sheep Creek. For models that included length, we used the median fish length in our dataset to calculate probability estimates (306 mm). All data analysis occurred in Program R version 4.1.0.
We used Pianka’s index of niche overlap to quantify diet overlap between mountain whitefish and rainbow trout in the Smith River and Sheep Creek. Niche overlap Ojk is defined as where pij and pik are the proportions of the ith resource used by the jth and kth species, respectively [29]. Proportions were calculated using quantities of diet items within each category. Values of Ojk range from 0 to 1, with 0 indicating no diet overlap and 1 indicating complete diet overlap. Values of Ojk ≥ 0.60 are commonly used to indicate high diet overlap (e.g., see Novakowski; 2008 [30]).
To investigate patterns in feeding ecology, we calculated both frequency of occurrence (Fi) and prey-specific abundance (Pi) for both fish species in the Smith River and Sheep Creek. Frequency of occurrence is defined as , and Pi is defined as
where Ni is the number of individuals with prey i in their stomach and N is the number of individuals with stomach contents, Si is the number of diet items made up of prey i, and
is the total stomach content (i.e., total quantity of diet items) in only those individuals with prey i in their stomach. Presenting these results in a bivariate plot provides information about feeding strategies, prey importance, and variability in diets [31]. Prey categories that have a high prey-specific abundance indicate that individuals specialize on the prey type, whereas low prey-specific abundance results from more opportunistic, generalist feeding strategies. Prey categories that have high frequency of occurrence indicate that most individuals in the population consume the prey type, whereas low frequency of occurrence indicates that few individuals make use of the prey type.
Results
Rainbow trout and mountain whitefish in the Smith River and Sheep Creek (i.e., across both species and rivers) consumed a wide array of prey types, but mayflies (Ephemeroptera), caddisflies (Trichoptera), segmented worms (Oligochaeta), and midges (Diptera) were found in most diets (Fig 2). The proportion of diets composed of invertebrates from different families varied between species and locations where fish were captured. In the Smith River, mountain whitefish consumed primarily Hydropsychidae, Ephemerellidae, and Brachycentridae; these taxa made up an average of 30.2%, 20.7%, and 10.2% of total diet items, respectively (Fig 3). In Sheep Creek, mountain whitefish diets were generally composed of Brachycentridae (22.4%), Baetidae (21.1%), Chironomidae (16.8%), and Ephemerellidae (16.3%). Diets of rainbow trout in the Smith River were composed of 19.1% Ephemerellidae, 18.4% Oligochaeta, and 11.8% fish eggs, and in Sheep Creek, rainbow trout most commonly consumed Oligochaeta (30.2%), Ephemerellidae (25.0%), and Baetidae (14.8%).
Standard boxplots showing proportions of diet item from the specified category found in mountain whitefish (MW) and rainbow trout (RB) in Sheep Creek and the Smith River. Panels show proportions of (A) Diptera; (B) Ephemeroptera; (C) Trichoptera; (D) Oligochaeta; and (E) any other taxa pooled. Box ends represent the 25th and 75th quantiles, horizontal lines are the medians, the upper whisker extends to the largest observation no further than 1.5 × interquartile range (IQR) from the 75th quantile, and the lower whisker extends to the smallest observation no further than 1.5 × IQR from the 25th quantile.
Boxplots showing proportions of diet items from the specified category (family) found in mountain whitefish (MW) and rainbow trout (RB) diets in Sheep Creek and the Smith River. Panels show proportions of (A) Athericidae; (B) Baetidae; (C) Brachycentridae; (D) Chironomidae; (E) Ephemerellidae; (F) Heptageniidae; (G) Hydropsychidae; and (H) all other taxa pooled. Box ends represent 25th and 75th quantiles, horizontal lines are the medians, the upper whisker extends to the largest observation no further than 1.5 × interquartile range (IQR) from 75th quantile, and the lower whisker extends to the smallest observation no further than 1.5 × IQR from 25th quantile.
The occurrence of diet items from some categories did not vary between the two species but did vary between the two sites. Baetidae and Heptageniidae made up a higher proportion of fish diets in Sheep Creek than in the Smith River (Tables 1 and 2). However, Heptageniidae made up a small portion of fish diets in both streams. Modeling results indicate the probability of a fish consuming Baetidae in Sheep Creek was 0.20 (95% CI 0.17–0.24), compared to 0.04 in the Smith River (95% CI 0.02–0.08). Both Hydropsychidae and Ephemerellidae constituted higher proportions of fish diets in the Smith River than in Sheep Creek (Tables 1 and 2). Hydropsychidae were commonly consumed in the Smith River, where they made up a quarter of the total diet items (95% CI 0.19–0.31) but were rarely consumed in Sheep Creek (proportion = 0.03, 95% CI 0.02–0.05). Ephemerellidae, which were an important diet category in both streams, were more commonly consumed in the Smith River (0.26 of total diet items; 95% CI 0.20–0.33) than in Sheep Creek (0.16 of total diet items; 95% CI 0.13–0.20).
Summary of model results for the eight most commonly consumed diet categories. Dashes indicate parameters that were not included in the final model. Coefficient estimates, in the first column under each explanatory variable, are reported on the log-odds scale. Baseline categories were Sheep Creek for the stream variable and mountain whitefish for the species variable. Adjusted deviance explained for each model was Athericidae = 0.40, Baetidae = 0.40, Brachycentridae = 0.32, Chironomidae = 0.52, Ephemerellidae = 0.08, Heptageniidae = 0.19, Hydropsychidae = 0.57, and Oligochaeta = 0.56.
The estimated proportions of total food items in the diets of mountain whitefish and rainbow trout in the Smith River and Sheep Creek by taxon. Estimates are derived from generalized linear models, and confidence intervals (95%) are shown in parentheses. Only the eight most common taxa in fish diets are listed.
Patterns of consumption of some diet categories varied between both sites and between both species. The proportion of Brachycentridae in fish diets was higher in mountain whitefish and in Sheep Creek (Tables 1 and 2) and Brachycentridae composed small portions of rainbow trout diets in both streams. The proportion of fish diets composed of Oligochaeta differed by species, stream, and fish length. Oligochaeta were larger portions of diets of rainbow trout, fish in the Sheep Creek, and larger fish (Tables 1 and 2). The differences in composition of diets were greater between species than between streams. The predicted proportion of rainbow trout diets composed of Oligochaeta was 0.06 (95% CI 0.01–0.24) in the Smith River and 0.33 (95% CI 0.21–0.48) in Sheep Creek, whereas the predicted proportion of mountain whitefish diets composed of Oligochaete worms was < 0.01 in both streams. The model for the proportion of Chironomidae in fish diets included length and the interaction between species and stream (Tables 1 and 2). The estimated proportion of Chironomidae in fish diets was highest for mountain whitefish in the Sheep Creek at 0.21 (95% CI 0.18–0.26) followed by mountain whitefish in Smith River, rainbow trout in Smith River, and rainbow trout in the Sheep Creek (Table 2). Additionally, smaller fish had a higher proportion of Chironomidae in their diets. The estimated proportion of Athericidae in fish diets was generally low (Table 1), and the proportion of Athericidae in diets varied by stream and species with an interaction between the two predictors (Table 2).
In the Smith River, diet overlap was high between mountain whitefish and rainbow trout (Pianka’s index value = 0.85), but diet overlap between the two species was lower in Sheep Creek (Pianka’s index value = 0.57, Table 3). Whereas a Pianka’s index value of 0.57 does not indicate high overlap, this value indicates some overlap in diets between mountain whitefish and rainbow trout in Sheep Creek. Rainbow trout had high diet overlap between Sheep Creek and the Smith River, whereas mountain whitefish had lower overlap between the two streams.
Pianka’s index of niche overlap comparing diets of mountain whitefish and rainbow trout in Sheep Creek and the Smith River. Values ≥ 0.60 indicate high diet overlap.
Mountain whitefish and rainbow trout in the Smith River and Sheep Creek exhibited generalist feeding strategies with prey-specific abundance of prey items remaining below 0.50 in all cases (Fig 4). However, there were dominant prey types on the population level. For some prey types, frequency of occurrence was high (up to 1.0) and prey-specific abundance was low (< 0.5), indicating these prey types were consumed by most individuals but made up a small proportion of the diet of those individuals. Many prey types had low frequency of occurrence (< 0.5) indicating they were not widely consumed on a population level. Furthermore, diets of all groups of fish included infrequently consumed taxa on the population and individual levels. Rainbow trout in the Smith River had a slightly more specialized feeding strategy than the other groups of fish sampled. Hemiptera (true bugs) and Oligochaeta had a higher prey-specific abundance and a lower frequency of occurrence (Fig 4; panel A), indicating that these diet categories made up a large portion of the diet of relatively few individuals in the population. Individual fish consumed up to 17 different prey items (range 1–17), and the median prey-item richness was eight or more for all groups of fish (Fig 5). Among all fishes, the median prey-item richness was 10.
Feeding strategy diagram. Feeding strategy diagram of the modified Costello method according to Amundsen et al. (1996) [31]. Prey-specific abundances and frequencies of occurrence in the diets of mountain whitefish (MW) and rainbow trout (RB) in the Smith River and Sheep Creek. Numbered diet categories are 1: Fish larvae; 2: Coleoptera; 3: Amphipoda; 4: Scorpaeniformes; 5: Hemiptera; 6: Gastropoda; 7: Diptera pupae; 8: Oligochaeta; 9: Acari; 10: Neuroptera. Panels show (A) rainbow trout in the Smith River; (B) rainbow trout in Sheep Creek; (C) mountain whitefish in the Smith River; and (D) mountain whitefish in Sheep Creek.
Prey-item richness (i.e., number of unique prey-items) in the diets of mountain whitefish (MW) and rainbow trout (RB) in the Smith River and Sheep Creek. Box ends represent 25th and 75th quantiles, horizontal lines are the medians, the upper whisker extends to the largest observation no further than 1.5 × interquartile range (IQR) from 75th quantile, and the lower whisker extends to the smallest observation no further than 1.5 × IQR from 25th quantile.
Discussion
During spring runoff, mountain whitefish and rainbow trout in the Smith River and Sheep Creek fed on a variety of taxa and showed moderate to high overlap in diets interspecifically as well as conspecifically between locations within the watershed. Mountain whitefish most frequently consumed aquatic invertebrates, particularly mayflies, caddisflies, and midges as seen in previous studies, although mountain whitefish in other rivers consumed fewer mayflies [32, 33] and higher quantities of midges and stoneflies (order Plecoptera) than in the Smith River watershed. Previous research indicated that mountain whitefish feed in both the drift and the benthos, and the taxa most frequently consumed by mountain whitefish in our study are found in both the drift and the benthos [32]. Mountain whitefish consumed small amounts of terrestrial insects, which is consistent with previous research showing that these fish only occasionally feed from the surface [33].
Although diet overlap between the two species was high in the Smith River and moderate in Sheep Creek during spring runoff, the generalist feeding strategies—including a large diversity of prey types consumed—exhibited by rainbow trout and mountain whitefish may reduce competition for food resources. Mountain whitefish tend to use similar food resources to other salmonids such as rainbow trout and cutthroat trout, resulting in niche overlap [33, 34]. However, similarity between diets of mountain whitefish and other salmonids does not explicitly indicate competition between species [34, 35]. Competition between species can be alleviated by abundant food resources and by subtle differences in feeding strategy that are not apparent from gut contents (e.g., although both species feed in the drift, mountain whitefish also feed in the benthos, and rainbow trout also feed at the surface). Therefore, different portions of the water column being used more commonly by each species may buffer overlap in diets like previously demonstrated in other species (e.g., in stream-dwelling Arctic charr [Salvelinus alpinus], Atlantic salmon [Salmo salar] and alpine bullhead [Cottus poecilopus]; [9]). Mountain whitefish in the Smith River watershed are adapted to coexist with other Oncorhynchus species since cutthroat trout are native to the Smith River watershed. Therefore, useful resource partitioning strategies and coping mechanisms that pre-date rainbow trout introductions may exist. Minor differences in diet composition by species indicated some partitioning of resources within the Smith River watershed. For example, rainbow trout consumed far more Oligochaeta than mountain whitefish but fewer Brachycentridae and Chironomidae. The ability of mountain whitefish to take advantage of numerous prey types may help the species remain abundant in watersheds where introduced salmonids have displaced other native salmonids, such as cutthroat trout. Nonetheless, fluctuations in availability of important diet categories along with other factors such as low flow and thermal limitations during late summer and early autumn are potential limiting factors that could be evaluated by future research.
Diets did not differ greatly between fish in the Smith River and Sheep Creek during spring runoff, and rainbow trout showed high overlap between the two locations. Although, stream was considered a useful predictor variable of probability of consumption for all prey types that were modeled. Baetidae were consumed more frequently in Sheep Creek, while Ephemerellidae and Hydropsychidae were consumed more frequently in the Smith River. These differences probably parallel differences in macroinvertebrate assemblages. Rainbow trout spawn in tributary habitats in the spring (i.e., when we were sampling), and spawning rainbow trout may have used the food resources available in Sheep Creek during their migration. Some mountain whitefish move into tributary streams in the spring [23, 24, 36], and this movement may be driven in part by a search for food resources. However, given the availability of diet resources in the mainstem Smith River and the generalist feeding strategy of mountain whitefish, a search for food resources may not be driving the movement of mountain whitefish into tributaries in this system. Interestingly, mountain whitefish in Sheep Creek did not feed on fish eggs or embryos despite a common conception that mountain whitefish enter tributaries in the spring specifically to feed on rainbow trout eggs or embryos and historical beliefs that interspecific competition and consumption of sportfish eggs indicated a need for reducing mountain whitefish density [37]. Movements into tributaries in the spring are probably influenced by factors beyond food resources, such as seeking refuge from high mainstem discharges and water velocities during runoff or impending warm mainstem water temperatures in summer [36].
The timing of our sampling, changes in prey availability, and relatively low sample sizes have implications for our results. Fish diet composition and diet overlap between fish species can change seasonally as the availability of various diet categories, such as terrestrial insects, changes [38, 39]. Food resources for fish may be relatively abundant during the spring, when water temperatures are increasing and flows dislodge benthic invertebrates into the drift, but many insect taxa have not yet emerged [40]. Food availability may be limited during the autumn and winter and thereby could alter feeding strategies and increase diet overlap and competition between mountain whitefish and rainbow trout [35]. Furthermore, variation in food types available throughout the year resulting from cyclical or episodic events (e.g., eggs from fish spawning events or hatching of macroinvertebrates) could alleviate or exacerbate competition between rainbow trout and mountain whitefish. We cannot fully address feeding strategies without quantification of prey availability or sampling events stratified by season. Therefore, additional research could more fully characterize the feeding ecology of these two species, feeding strategies of the species, and how competition between the species might vary throughout the course of a year. Low sample sizes—particularly for Smith River rainbow trout (n = 6) and Smith River mountain whitefish (n = 11)—may influence our results, and future research may be useful to validate our findings that include Smith River fishes. Replicating the analysis with prey biomass would allow the results to be interpreted in a bioenergetic-based framework.
Quantifying fish diets can help us predict how these species may respond to increasing anthropogenic effects and can inform conservation efforts for native fishes. Concerning declines in insect biomass and diversity have been detected around the globe including in protected areas [41, 42], and among aquatic insects, sensitive taxa such as Trichoptera and Ephemeroptera are especially imperiled [42]. These imperiled macroinvertebrate orders made up most of the diets of mountain whitefish and rainbow trout in the Smith River watershed, and the continued presence of robust insect populations is vital to the diversity of fish species and the persistence of these fishes in the watershed.
Supporting information
S1 Table. Model selection table.
ΔQAICc for each of nine models for the eight most common diet categories in mountain whitefish and rainbow trout diets in the Smith River and Sheep Creek. We selected the most parsimonious model out of models with ΔQAICc ≤ 2, shown in boldface.
https://doi.org/10.1371/journal.pone.0280833.s001
(PDF)
S1 File. Data collected.
Count data for each prey category by individual fish diet.
https://doi.org/10.1371/journal.pone.0280833.s002
(CSV)
Acknowledgments
We wish to thank all technicians that assisted with field and laboratory work for this project including Keenan Blackbird. Additionally, we thank those that provided constructive feedback for the analysis and manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
References
- 1. Strayer DL. Alien species in fresh waters: ecological effects, interactions with other stressors, and prospects for the future. Freshw Biol. 2010;55:152–74.
- 2. Gozlan RE, Britton JR, Cowx I, Copp GH. Current knowledge on non-native freshwater fish introductions. J Fish Biol. 2010;76(4):751–86.
- 3. Cucherousset J, Olden JD. Ecological impacts of nonnative freshwater fishes. Fisheries. 2011;36(5):215–30.
- 4. Dunham JB, Adams SB, Schroeter RE, Novinger DC. Alien invasions in aquatic ecosystems: Toward an understanding of brook trout invasions and potential impacts on inland cutthroat trout in western North America. Rev Fish Biol Fish. 2002;12(4):373–91.
- 5. McKelvey KS, Young MK, Wilcox TM, Bingham DM, Pilgrim KL, Schwartz MK. Patterns of hybridization among cutthroat trout and rainbow trout in northern Rocky Mountain streams. Ecol Evol. 2016;6(3):688–706. pmid:26865958
- 6. Krueger CC, May B. Ecological and genetic-effects of salmonid introductions in North-America. Can J Fish Aquat Sci. 1991;48:66–77.
- 7. Seiler SM, Keeley ER. Competition between native and introduced salmonid fishes: cutthroat trout have lower growth rate in the presence of cutthroat-rainbow trout hybrids. Can J Fish Aquat Sci. 2009;66(1):133–41.
- 8. Braga RR, Bornatowski H, Vitule JRS. Feeding ecology of fishes: an overview of worldwide publications. Rev Fish Biol Fish. 2012;22(4):915–29.
- 9. Sánchez-Hernández J, Glaber H-M, Amundsen P-A. Food resource partitioning between stream-dwelling Arctic charr Salvelinus alpinus (L.), Atlantic salmon Salmo salar and alpine bullhead Cottus poecilopus Heckel, 1836: an example of water column segregation. Hydrobiologia. 2016;784:105–115.
- 10. Pracheil BM, McIntyre PB, Lyons JD. Enhancing conservation of large-river biodiversity by accounting for tributaries. Front Ecol Environ. 2013;11(3):124–8.
- 11.
Rice SP, Kiffney PM, Greene CM, Pess GR. The ecological importance of tributaries and confluences. In: Rice SP, Roy AG, Rhoads BL, editors. River Confluences, Tributaries, and the Fluvial Network: Wiley; 2008.
- 12. Sutton R, Soto T. Juvenile coho salmon behavioural characteristics in Klamath river summer thermal refugia. River Res Appl. 2012;28(3):338–46.
- 13. Erkinaro H, Erkinaro J. Feeding of Atlantic salmon, Salmo salar L., parr in the subarctic River Teno and three tributaries in northernmost Finland. Ecol Freshw Fish. 1998;7(1):13–24.
- 14. Milner VS, Yarnell SM, Peek RA. The ecological importance of unregulated tributaries to macroinvertebrate diversity and community composition in a regulated river. Hydrobiologia. 2019;829(1):291–305.
- 15. Northcote TG, Ennis GL. Mountain whitefish biology and habitat use in relation to compensation and improvement possibilities. Reviews in Fisheries Science. 1994;2(4):347–71.
- 16. Lance MJ, Baxter CV. Abundance, production, and tissue composition of mountain whitefish (Prosopium williamsoni) in a central Idaho wilderness stream. Northwest Science. 2011;85(3):445–54.
- 17. Meyer KA, Elle FS, Lamansky JA. Environmental factors related to the distribution, abundance, and life history characteristics of mountain whitefish in Idaho. N Am J Fish Manag. 2009;29(3):753–67.
- 18. Boyer JK, Guy CS, Webb MAH, Horton TB, McMahon TE. Reproductive ecology, spawning behavior, and juvenile distribution of mountain whitefish in the Madison River, Montana. Trans Am Fish Soc. 2017;146(5):939–54.
- 19. McHugh P, Budy P. Experimental effects of nonnative brown trout on the individual- and population-level performance of native Bonneville cutthroat trout. Trans Am Fish Soc. 2006;135(6):1441–55.
- 20. Peterson DP, Fausch KD, White GC. Population ecology of an invasion: Effects of brook trout on native cutthroat trout. Ecol Appl. 2004;14(3):754–72.
- 21. Warnock WG, Rasmussen JB. Assessing the effects of fish density, habitat complexity, and current velocity on interference competition between bull trout (Salvelinus confluentus) and brook trout (Salvelinus fontinalis) in an artificial stream. Can J Zool. 2013;91(9):619–25.
- 22.
Ritter TD. Connectivity in a montane river basin: Salmonid use of a major tributary in the Smith River system [PhD]. Bozeman, MT: Montana State University; 2015.
- 23.
Baxter CV. Fish movement and assemblage dynamics in a Pacific Northwest riverscape [PhD]. Corvallis, OR: Oregon State University; 2002.
- 24.
Lance MJ. Spatial and temporal variability in movements and vital rates of sympatric salmonids in an unfragmented, inland watershed[M.S.]. Bozeman, MT: Montana State University; 2019.
- 25.
Montana Department of Fish and Game. 1973. Smith River drainage inventory and planning investigation. Great Falls, Montana.
- 26.
Montana Fish, Wildlife, and Parks. 2013. Statewide fisheries management plan 2013–2018. Helena, Montana.
- 27.
Merritt RW, Cummins KW, Berg MB. An Introduction to the aquatic insects of North America. 4th Edition ed: Kendall Hunt Publishing; 2008.
- 28. Lebreton JD, Burnham KP, Clobert J, Anderson DR. Modeling survival and testing biological hypotheses using marked animals: a unified approach with case studies. Ecol Monogr. 1992;62(11):67–118.
- 29. Pianka ER. The structure of lizard communities. Annu Rev Ecol Syst. 1973;4:53–74.
- 30. Novakowski GC, Hahn NS, Fugi R. Diet seasonality and food overlap of the fish assemblage in a pantanal pond. Neo Icht. 2008;6:567–576.
- 31. Amundsen P-A, Gabler H-M, Staldvik FJ. A new approach to graphical analysis of feeding strategy from stomach contents data—modification of the Costello (1990) method. J Fish Biol. 1992;48:607–14.
- 32. Pontius RW, Parker M. Food habits of the mountain whitefish, Prosopium williamsoni (Girard). Trans Am Fish Soc. 1973;102(4):764–73.
- 33. Laakso M. Food habits of the Yellowstone whitefish Prosopium williamsoni cismontanus (Jordan). Trans Am Fish Soc. 1951;80:99–109.
- 34. Meeuwig MH, Peacock MM. Food web interactions associated with a Lahontan cutthroat trout reintroduction effort in an alpine lake. J Fish Wildl Manag. 2017;8(2):449–64.
- 35.
DosSantos JM. Comparative food habits and habitat selection of mountain whitefish and rainbow trout in the Kootenai River, Montana [M.S.]. Bozeman, MT: Montana State University; 1985.
- 36. Benjamin JR, Wetzel LA, Martens KD, Larsen K, Connolly PJ. Spatio-temporal variability in movement, age, and growth of mountain whitefish (Prosopium williamsoni) in a river network based upon PIT tagging and otolith chemistry. Can J Fish Aquat Sci. 2014;71(1):131–40.
- 37.
Sigler WF. Bulletin No. 347—The Life History and Management of the Mountain Whitefish Prosopium Williamsoni (Girard) in Logan River Utah [Bulletin]. Logan, UT: Utah Agricultural Experiment Station; 1951.
- 38. Hasegawa K, Yamazaki C, Ohta T, Ohkuma K. Food habits of introduced brown trout and native masu salmon are influenced by seasonal and locational prey availability. Fish Sci. 2012;78(6):1163–71.
- 39. Li JL, Gerth WJ, Van Driesche RP, Bateman DS, Herlihy AT. Seasonal and spatial fluctuations in Oncorhynchus trout diet in a temperate mixed-forest watershed. Can J Fish Aquat Sci. 2016;73(11):1642–9.
- 40. Romero N, Gresswell RE, Li JL. Changing patterns in coastal cutthroat trout (Oncorhynchus clarki clarki) diet and prey in a gradient of deciduous canopies. Can J Fish Aquat Sci. 2005;62(8):1797–807.
- 41. Hallmann CA, Sorg M, Jongejans E, Siepel H, Hofland N, Schwan H, et al. More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS One. 2017;12(10):e0185809. pmid:29045418
- 42. Sanchez-Bayo F, Wyckhuys KAG. Worldwide decline of the entomofauna: A review of its drivers. Biol Conserv. 2019;232:8–27.