Feeding strategy of the non‐native rainbow trout,Oncorhynchus mykiss,in low‐order Patagonian streams

The dietary composition and feeding strategy of rainbow trout, Oncorhynchus mykiss (Walbaum), in two low‐order Patagonian streams were studied. Benthic macroinvertebrate availability was estimated in both riffles and pools. Fish stomach contents were examined to determine prey richness and diversity, prey electivity, food‐niche width, and the feeding strategy employed by trout throughout the year. Availability of benthos varied seasonally with Ephemeroptera, Trichoptera, Plecoptera and Diptera species dominating. Rainbow trout diet was composed mainly of benthic macroinvertebrates, followed by terrestrial insects, fish, algae and plants. Different trout size classes segregated the use of food resources to reduce predation pressure. Elected prey included organisms displaying no to high mobility. A high feeding plasticity allows trout to buffer changes in food availability by switching from a specialised to a generalised feeding behaviour. Consequently, trout may exploit abundant but temporary food resources opportunistically, which would explain their marked expansion in Patagonian environments.     

Conserving fluxes of terrestrial invertebrates to trout in streams: A first field experiment on the effects of cattle grazing

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Trophic interactions between introduced lake trout (Salvelinus namaycush) and native Arctic charr (S. alpinus) in a large Fennoscandian subarctic lake

Introduced fishes may have major impacts on community structure and ecosystem function due to competitive and predatory interactions with native species. For example, introduced lake trout (Salvelinus namaycush) has been shown to replace native salmonids and induce major trophic cascades in some North American lakes, but few studies have investigated trophic interactions between lake trout and closely related native Arctic charr (S. alpinus) outside the natural distribution of the former species. We used stomach content and stable isotope analyses to investigate trophic interactions between introduced lake trout and native Arctic charr in large subarctic Lake Inarijärvi in northern Finland. Both salmonids had predominantly piscivorous diets at >280 mm total length and were mainly caught from the deep profundal zone. However, lake trout had a more generalist diet and showed higher reliance on littoral prey fish than Arctic charr, whose diet consisted mainly of pelagic planktivorous coregonids. According to length at age and condition data, lake trout showed slightly faster growth but lower condition than Arctic charr. The results indicate that introduced lake trout may to some extent compete with and prey upon native Arctic charr, but currently have only a minor if any impact on native fishes and food web structure in Inarijärvi. Future monitoring is essential to observe potential changes in trophic interactions between lake trout and Arctic charr in Inarijärvi, as well as in other European lakes where the two salmonids currently coexist.     

Are invasive and native trout functionally equivalent predators? Results and lessons from a field experiment

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Can translocated native fishes retain their trophic niche when confronted with a resident invasive?

Diet interactions between native and non‐native fishes may influence the establishment of native species within their historical range (i.e., reintroduction). Therefore, we illustrated the food web structure of and followed the transition of the federally endangered humpback chub Gila cypha into a novel food web following translocation and determined the potential for a non‐native species, rainbow trout Oncorhynchus mykiss, to influence translocation success. Humpback chub and rainbow trout used resources high in the food web and assimilated similar proportions of native fishes, suggesting non‐native rainbow trout may occupy an ecological role similar to humpback chub. Subsequently, humpback chub may be well suited to colonise tributaries because of their ability to consume resources high in the food web. Additionally, diet partitioning may occur between all members of the fish community as indicated by separation in trophic niche space and little trophic overlap; although all species, particularly bluehead sucker Catostomus discobolus, used a broad range of food resources. Rainbow trout stomach content analysis corroborated stable isotope analysis and suggested rainbow trout diet consisted of aquatic and terrestrial macroinvertebrates, while larger rainbow trout (>120 mm total length) consumed a greater proportion of fish (incidence of piscivory = 5.3%). Trophic interactions may reveal an underutilized niche space or biotic resistance to the establishment of translocated native fishes. Continued translocation of humpback chub into tributaries appears to be one option for conservation. However, successful establishment of humpback chub may depend on continued removal of non‐native trout, increasing availability of diet sources at higher trophic levels.     

Stable isotopes and gut contents indicate differential resource use by coexisting asp (Leuciscus aspius) and pikeperch (Sander lucioperca)

Differential use of habitat and prey resources is an important mechanism that may allow coexistence of sympatric species. Unlike interactions between smaller cyprinid and percid fishes, the resource use by coexisting predatory asp (Leuciscus aspius) and pikeperch (Sander lucioperca) is relatively unknown. Here, gut content and stable isotope analyses were used to study ontogenetic dietary shifts and interspecific trophic niche overlap between asp and pikeperch coexisting in two reservoirs. The hypothesis that both species show an ontogenetic dietary shift from small invertebrates to large fish prey, but at the same time use different prey resources to reduce potential competitive interactions, was validated. The isotopic niches of the two predators showed no, or only a moderate, degree of overlap (0%–65%). The ontogenetic changes in the degree of interspecific isotopic niche overlap were different in the two reservoirs, suggesting that trophic segregation can be dynamic and variable among systems. Gut contents revealed that small (<100 mm standard length) asp consumed mostly terrestrial invertebrates and emerged aquatic insects, whereas small pikeperch foraged on zooplankton, larval and pupal stages of aquatic insects and fish. Larger individuals (>100 mm) of both species were predominantly piscivorous, with asp consuming more cyprinid prey and pikeperch more percid prey. Coexisting asp and pikeperch populations are able to utilise different prey resources, thereby reducing potential negative competitive interactions.     

Episodic predation of mammals by stream fishes in a boreal river basin

Stream‐dwelling fish typically feed on small prey items, such as benthic invertebrates, but maintain the capacity to opportunistically feed on rare, large‐bodied prey when available. However, consumption of particularly large prey is typically viewed as isolated events that are not organised spatially or temporally across watersheds. We assessed the occurrence of small mammals in the stomach contents of rainbow trout (Oncorhynchus mykiss) and Arctic grayling (Thymallus arcticus) over 13 years in the Wood River basin, Alaska (59°34′N, 158°48′W). Shrews (Sorex spp.) were the dominant mammals observed episodically every 2–3 years in the stomach contents of fish. Notably, shrew consumption was correlated in both Arctic grayling and rainbow trout within individual streams, and across the river basin in several subwatersheds. Predators of shrews were usually the largest individuals within each population, suggesting that smaller fish are gape‐limited and that dominant fish monopolised mammal prey. On average, 24% (11–38%) of Arctic grayling >298 mm (fork length) and rainbow trout >290 mm contained mammal prey during peak years. Small mammal populations often cycle every 2–5 years with well‐known functional and numerical effects for terrestrial predators, a dynamic that may be reflected in our 13 years data set of diet contents for aquatic predators. Although numerically infrequent when averaged over time, small mammal subsidies to streams may be episodically important to the energy budgets of long‐lived consumers in freshwater environments.     

Trophic pathways supporting Arctic grayling in a small stream on the Arctic Coastal Plain,Alaska

Beaded streams are prominent across the Arctic Coastal Plain (ACP) of Alaska, yet prey flow and food web dynamics supporting fish inhabiting these streams are poorly understood. Arctic grayling (Thymallus arcticus) are a widely distributed upper‐level consumer on the ACP and migrate into beaded streams to forage during the short 3‐month open‐water season. We investigated energy pathways and key prey resources that support grayling in a representative beaded stream, Crea Creek. We measured terrestrial invertebrates entering the stream from predominant riparian vegetation types, prey types supporting a range of fish size classes, and how riparian plants and fish size influenced foraging habits. We found that riparian plants influenced the quantity of terrestrial invertebrates entering Crea Creek; however, these differences were not reflected in fish diets. Prey type and size ingested varied with grayling size and season. Small grayling (<15 cm fork length (FL)) consumed mostly aquatic invertebrates early in the summer, and terrestrial invertebrates later in summer, while larger fish (>15 cm FL) foraged most heavily on ninespine stickleback (Pungitius pungitius) throughout the summer, indicating that grayling can be insectivorous and piscivorous, depending on size. These findings underscore the potential importance of small streams in Arctic ecosystems as key summer foraging habitats for fish. Understanding trophic pathways supporting stream fishes in these systems will help interpret whether and how petroleum development and climate change may affect energy flow and stream productivity, terrestrial–aquatic linkages and fishes in Arctic ecosystems.     

Stable isotopes and gut content show diet overlap among native and introduced piscivores in a large oligotrophic lake

Abstract – In past dietary studies kokanee Oncorhynchus nerka were prominent in the diet of Pend Oreille Lake's large piscivores: native bull trout Salvelinus confluentus, cutthroat trout O. clarki and northern pikeminnow Ptychocheilus oregonensis, and introduced lake trout S. namaycush and Kamloops rainbow trout O. mykiss gairdneri. However, kokanee have declined to 10–20% of their former abundance. We therefore initiated this study to understand current predation demands on kokanee and diet overlap among piscivores, using gut content samples and analysis of stable nitrogen (δ¹⁵N) and carbon (δ¹³C) isotopes from the lake's fish and invertebrate community. In gut content samples, kokanee were the main prey item of large [i.e., ≥400 mm total length (TL)] bull and lake trout; a conclusion that was affirmed by stable isotope analysis. Rainbow trout >500 mm TL consumed mostly kokanee, thus there was a high degree of diet overlap among large bull, lake and rainbow trout. Small (i.e., <400 mm TL) rainbow and cutthroat trout diets overlapped, and were composed mostly of littoral benthic invertebrates. However, gut content and stable isotope analysis did not accord for 400–500 mm TL rainbow trout, small lake trout, and large cutthroat trout. In these instances, a linear mixing model using stable isotope results predicted kokanee consumption for each species, but no kokanee were identified in rainbow or lake trout gut content. Gut content and stable isotope analysis of native northern pikeminnow indicated a diet of mostly littoral benthic invertebrates at smaller (100–150 mm TL) lengths, with kokanee becoming more prominent in the diet of individuals >300 mm TL. Percent of kokanee in the diet of northern pikeminnow has declined from a prior study; otherwise piscivore diets have apparently remained unchanged. In this study, judgments as to the feeding of some piscvores, based on gut content alone, would be tenuous because of small sample sizes, but stable isotope analysis provided an efficient means for confirming diets.     

Reintroduction of a native galaxiid (Galaxias fasciatus) following piscicide treatment in two streams: response and recovery of the fish population

The rehabilitation of native communities by means of eradicating unwanted fish species using piscicides is an example of employing disturbance to achieve conservation successes. Such projects provide a valuable opportunity to test the efficiency of the tool and the impacts on the receiving aquatic communities, as disturbance occurs at a known time. The piscicide ‘rotenone’ has been widely used to eradicate invasive or unwanted fish species worldwide. However, there is little information regarding the impact on native fish being reintroduced to a stream after rotenone treatment. The mass depletion of aquatic invertebrates due to rotenone dosing is of particular concern, as food‐limitation could negatively impact on fish growth, condition and recruitment, compromising the aims of rehabilitation. For the first time in New Zealand, rotenone was employed to eradicate brown trout (Salmo trutta) from two streams that also supported populations of banded kokopu (Galaxias fasciatus). Impacts on fish and aquatic invertebrates were studied in two treatment and two reference streams in Karori, Wellington. Analysis showed that invertebrate densities declined significantly in the treatment streams in the 2‐week to 2‐month period after dosing. Following reintroduction after rotenone treatment, banded kokopu condition declined significantly and levels of fish mobility were variable. One year after rotenone dosing, there was recruitment of banded kokopu juveniles in the treatment streams indicating successful reproduction, with no equivalent increase in the reference streams. Results are a positive indication for the use of rotenone as an effective conservation tool to remove unwanted fish species where they threaten native populations.     

Prey selectivity and ontogenetic diet shift of the globally invasive western mosquitofish (Gambusia affinis) in agriculturally impacted streams

The dietary breadth of invaders can influence their success, and having a wide dietary niche can facilitate the spread and survival of invaders under a variety of resource scenarios. The western mosquitofish (Gambusia affinis) is a globally distributed freshwater invasive fish. The spread of G. affinis is associated with agricultural land use, although the trophic role it plays in degraded systems is not well understood. We analysed the invertebrate community in 11 stream reaches in the North Island of New Zealand, in catchments spanning a range (45%–90%) of agricultural land use to determine how prey availability changes with land use. We then analysed the gut contents of 400 G. affinis from the 11 sites to determine how diet varied with prey availability and ontogeny. Invertebrate communities varied along the agricultural land‐use gradient, both in regard to taxonomic richness and community composition. G. affinis consumed a wide variety of food items with invertebrates being the most dominant, in particular Culicidae, Copepods and amphipods were the most commonly consumed invertebrates. There was also an ontogenetic diet shift from microinvertebrates (Cladocera, Copepods and diatoms) to larger invertebrates, including Culicidae, amphipods and terrestrial invertebrates. G. affinis are capable of consuming a wide variety of prey in agricultural streams; their preferred prey are generally pollution‐tolerant taxa commonly found in degraded streams. Having a large level of dietary plasticity coupled with preferring prey that are often associated with degraded systems likely facilitates to the spread of one of the most widely distributed freshwater invasive fish.     

Trophic plasticity of invasive juvenile largemouth bass Micropterus salmoides in Iberian streams

Biological invasions are a major factor for biodiversity loss, particularly in freshwater environments. Largemouth bass Micropterus salmoides is native to North America and is invasive on the Iberian Peninsula, primarily to provide angling opportunities in reservoirs. However, this species is a threat to the endemic Iberian fauna via predation and competition. Currently, there is little information on largemouth bass in European streams. Thus, we assessed the trophic plasticity and body condition of young largemouth bass in both invasive (the regulated Bullaque River) and native (Murray Creek) streams. Abundance of juvenile largemouth bass, percentage of full stomachs and body condition were higher in Bullaque River. Largemouth bass preyed on benthic invertebrates much more heavily in the Bullaque River, whereas fishes were the most important prey in Murray Creek. Prey richness, diet diversity and trophic niche breadth were higher in the Bullaque River population. Largemouth bass preferred water-column fishes as prey and avoided consuming benthic fishes in Murray Creek, whereas water-column fishes were avoided in Bullaque River. These results demonstrate that largemouth bass display substantial trophic plasticity which possibly facilitates its success as invasive species. Regulated Iberian streams may provide both suitable food and habitat resources with minimal predation pressure, and hence may serve as recruitment sources for this invasive fish.     

Spatial and temporal variation in prey selection of brown trout in a cold Arkansas tailwater

Abstract –  We examined prey utilisation patterns of brown trout ( Salmo trutta Linnaeus) as related to available benthos in a cold tailwater (Little Red River, Arkansas) having low biodiversity of benthic macroinvertebrates and low availability of fish as prey. Benthic macroinvertebrates were sampled monthly in upstream and downstream sites over a 1-year period. Stomach content analysis was also performed on 30 brown trout (10 each of three size classes) collected monthly from each site. Benthic and diet diversities were low at both sites, as isopods ( Lirceus ) accounted for 67% and 51% of the numerical abundance upstream and downstream, respectively, and 80% and 70% of all prey consumed by upstream and downstream brown trout. Physid snails (upstream) and chironomid larvae (downstream) accounted for most of the remaining prey taxa consumed. Piscivory and consumption of terrestrial invertebrates were rare. There was no clear relation between diet diversity and trout size. Densities of benthic macroinvertebrates and prey consumption were significantly greater upstream than downstream. Benthic macroinvertebrate assemblages and prey consumption were seasonally constant at both sites. Generally, prey consumption was consistent with availability of each prey in the benthos; the lack of benthic diversity and overwhelming abundance of one taxon ( Lirceus) most likely contributed to food selection patterns. Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna because of the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major limitation to brown trout growth.     

Potential population and assemblage influences of non‐native trout on native nongame fish in Nebraska headwater streams

Non‐native trout are currently stocked to support recreational fisheries in headwater streams throughout Nebraska. The influence of non‐native trout introductions on native fish populations and their role in structuring fish assemblages in these systems is unknown. The objectives of this study were to determine (i) if the size structure or relative abundance of native fish differs in the presence and absence of non‐native trout, (ii) if native fish‐assemblage structure differs in the presence and absence of non‐native trout and (iii) if native fish‐assemblage structure differs across a gradient in abundances of non‐native trout. Longnose dace Rhinichthys cataractae were larger in the presence of brown trout Salmo trutta and smaller in the presence of rainbow trout Oncorhynchus mykiss compared to sites without trout. There was also a greater proportion of larger white suckers Catostomus commersonii in the presence of brown trout. Creek chub Semotilus atromaculatus and fathead minnow Pimephales promelas size structures were similar in the presence and absence of trout. Relative abundances of longnose dace, white sucker, creek chub and fathead minnow were similar in the presence and absence of trout, but there was greater distinction in native fish‐assemblage structure between sites with trout compared to sites without trout as trout abundances increased. These results suggest increased risk to native fish assemblages in sites with high abundances of trout. However, more research is needed to determine the role of non‐native trout in structuring native fish assemblages in streams, and the mechanisms through which introduced trout may influence native fish populations.     

Modelling spatial exclusion of a vulnerable native fish by introduced trout in rivers using landscape features: a new tool for conservation management

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Resource use of native and stocked brown trout Salmo trutta L., in a subarctic lake

Habitat use, food composition and growth of stocked and native brown trout, Salmo trutta L., were studied in the subarctic Lake Muddusjärvi in northern Finland. Stocked brown trout and native brown trout preferred littoral and pelagic areas. Trout were stocked in October. In June stocked trout fed primarily on invertebrates while native fish were piscivorous. From July onwards the composition of the diet of both stocked and native trout was similar and consisted almost entirely of small‐sized whitefish. Brown trout were already piscivorous at a length of about 20 cm. The mean length of prey consumed was about 12 cm. Mean length‐at‐age was similar from the second year in the lake despite of the larger size of stocked fish during the first year in the lake.     

Riparian defoliation by the invasive green alder sawfly influences terrestrial prey subsidies to salmon streams

Invasive species in riparian forests are unique as their effects can transcend ecosystem boundaries via stream‐riparian linkages. The green alder sawfly (Monsoma pulveratum) is an invasive wasp whose larvae are defoliating riparian thin‐leaf alder (Alnus tenuifolia) stands across southcentral Alaska. To test the hypothesis that riparian defoliation by this invasive sawfly negatively affects the flow of terrestrial prey resources to stream fishes, we sampled terrestrial invertebrates on riparian alder foliage, their subsidies to streams and their consumption by juvenile coho salmon (Oncorhynchus kisutch). Invasive sawflies altered the composition of terrestrial invertebrates on riparian alder foliage and as terrestrial prey subsidies to streams. Community analyses supported these findings revealing that invasive sawflies shifted the community structure of terrestrial invertebrates between seasons and levels of energy flow (riparian foliage, streams and fish). Invasive sawfly biomass peaked mid‐summer, altering the timing and magnitude of terrestrial prey subsidies to streams. Contrary to our hypothesis, invasive sawflies had no effect on the biomass of native taxa on riparian alder foliage, as terrestrial prey subsidies, or in juvenile coho salmon diets. Juvenile coho salmon consumed invasive sawflies when most abundant, but relied more on other prey types selecting against sawflies relative to their availability. Although we did not find effects of invasive sawflies extending to juvenile coho salmon in this study, these results could change as the distribution of invasive sawflies expands or as defoliation intensifies. Nevertheless, riparian defoliation by these invasive sawflies is likely having other ecological effects that merits further investigation.     

Brook trout, Salvelinus fontinalis, microhabitat selection and diet under low summer stream flows

Abstract  This study investigated the effects of low summer discharge on habitat, prey use and prey availability for age 1 brook trout, Salvelinus fontinalis (Mitchill), in two small streams in Massachusetts, USA. Stream discharge declined substantially from June to August, with corresponding decreases in microhabitat depth and velocity; but fish habitat preferences were consistent throughout the summer, with fish selecting deep, low current velocity locations. Invertebrate drift rate, drift density and trout stomach fullness were significantly greater in June than August samples. Diets were dominated by aquatic-derived prey (chironomid larvae and adult blackflies) in June, but terrestrial invertebrates were the most frequent diet items in August. Consistent occupancy of low-velocity, deep microhabitats with low invertebrate flux rates indicated that, despite variation in habitat and prey conditions, trout adopted a habitat-use strategy of minimising risks and energy costs rather than maximising forage gain. This observation is consistent with, and provides a potential explanation for, the low summer growth rates of brook trout observed in small streams.     

Terrestrial prey inputs to streams bordered by deciduous broadleaved forests,conifer plantations and clear‐cut sites in southwestern Japan: effects on the abundance of red‐spotted masu salmon

Terrestrial invertebrates falling from the riparian canopy are a major energy source for fishes in headwater streams. Because quantity and quality of such allochthonous resources can vary depending on riparian conditions, conversion of riparian forests to conifer plantations may affect stream productivity. We compared falling and drifting invertebrate abundances and the diet of masu salmon (Oncorhynchus masou) among stream reaches bordered by deciduous broadleaved forests, conifer plantations (Cryptomeria japonica), and clear‐cut sites in southwestern Japan. We also examined whether among‐reach variation in salmon abundance was related to the riparian vegetation types. The results indicated that, on an annual basis, falling inputs of terrestrial invertebrates at the broadleaved reaches were 2–4 times higher than those at the plantation and clear‐cut reaches. In nonwinter seasons, terrestrial invertebrates made up 40–60% and 30–90% of drift and masu salmon diets, respectively, and drifting invertebrate abundance was higher in the broadleaved reaches than in the plantation reaches. Furthermore, a multivariate analysis of salmon abundance showed that variation in salmon biomass was explained primarily by riparian vegetation type, with broadleaved and clear‐cut reaches having higher biomass than the plantation reaches. These results indicate that terrestrial invertebrates are an important resource for masu salmon, and suggest that streams bordered by conifer plantations receive lower terrestrial prey inputs, which results in lower salmon abundance. In regions where natural forests have been extensively converted to conifer plantations, forest management that allows and facilitates recovery of natural riparian stands is important.