Diet Composition and Overlap in a Mixed Warm- and Coldwater Fish Community

Line-diffuser hypolimnetic oxygenation (HO) was initiated in North Twin Lake, Washington, in 2009 to mitigate loss of coldwater fishery habitat due to temperature–dissolved oxygen “habitat squeeze” and to reduce internal phosphorus cycling. Active tracking, net-captures, and hydroacoustic analyses demonstrated that trout populations rapidly expanded into increased hypolimnetic habitat within the first few years of oxygenation; however, long-term fishery benefits and many basic ecological aspects of HO have yet to be established. Diet and food web analyses indicate significant changes in feeding ecology of principal coldwater fish species in North Twin in 2012 compared to preoxygenation (2005) and to unoxygenated South Twin in 2012. North Twin rainbow trout (Oncorhynchus mykiss) consumed significantly more large-bodied Daphnia during midsummer 2012 than in South Twin, where rainbow trout fed primarily on littoral amphipods. Additionally, relative gut weight for brook trout (Salvelinus fontinalis) in August 2012 was significantly higher in North Twin compared to South Twin, apparently due to increased access to hypolimnetic zooplankton. Golden shiner (Notemigonus crysoleucas) diets also seem to include more zooplankton in oxygenated North Twin. Littoral-focused largemouth bass (Micropterus salmoides) diets were not altered by HO. Observed changes in feeding ecology following HO have significant implications for future fishery management in the Twin Lakes.

High mortality (84–89%) of stocked rainbow trout (Oncorhynchus mykiss) in Twin Lakes, Washington, has been partially blamed on predation from non-native largemouth bass (Micropterus salmoides). We examined the gut-content of 434 largemouth bass and applied a bioenergetics model to determine the consumption demand on hatchery-released rainbow trout and other prey species in the Twin Lakes. Largemouth bass consumed approximately 4915 (95% CI; 2393–13,452) fall stocked rainbow trout in South Twin. No rainbow trout consumption was observed in North Twin, despite a similar stocking regime. Approximately 6.3% (95% CI; 3–17%) of total fall stocked rainbow trout in South Twin were consumed by largemouth bass. Rainbow trout stocked in the fall ranged from 100 to 160 mm total length and were all subject to predation by large largemouth bass ⩾300 mm. Large largemouth bass consumed rainbow trout that reached up to 210 mm in length and 58% of bass body length. No predation was observed on larger rainbow trout (215–370 mm) stocked during the spring. Smaller largemouth bass ⩽299 mm consumed primarily invertebrates, including crayfish, and did not consume rainbow trout in either lake. During spring and summer in South Twin Lake, large largemouth bass consumed principally golden shiner (Notemigonus crysoleucas) and crayfish. In North Twin Lake, golden shiner constituted most of the larger bass diet for the entire year. Differences in macrophyte distribution, bathymetry, temperature and/or predator-prey demographics likely influenced variability in largemouth bass predation between lakes. Largemouth bass predation did not account for overall rainbow trout mortality.

There are over 40,000 small ranch ponds in Montana. More than threefourths of these were constructed solely for stock-water purposes and most of the others for either irrigation or the control of erosion. Fewer than 50 are known to have been built specifically for fish. (Brown and Thoreson, 1951). While approximately 700 ponds have been stocked with fish of one species or another, less than 100 are known to have produced any appreciable fishing. During 1949 and 1950, the Montana Agricultural Experiment Station and the Montana State Fish and Game Department conducted a cooperative investigation of existing Montana ranch ponds to explore their suitability for fish. About 20 ponds scattered over the eastern (plains) part of the state were examined at regular intervals throughout all seasons and inventories were made on a large number of other ponds as time permitted. The data included certain physical, chemical and biological information considered essential to an understanding of fish production. A summary of the physical-chemical information collected on twelve ponds is given in Table 1. The most striking physical feature is the prevailing low temperature even during the summer period. The water in ponds of this area is generally quite alkaline with the pH as great as 10.0 and total alkalinities (methyl orange) as high as 325 ppm. While aquatic plants of varied species are present in many ponds, they have not been found detrimental in ponds which were deep enough to insure fish production. In one small pond, mortality of trout was definitely attributed to low dissolved oxygen resulting from duck weed (Lemna) which completely covered the surface and which was found in large quantities, decaying on the bottom. Past records show that the following fish species have been planted in Monta a ponds: rainbow trout (Salmo gairdnerii), brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), cutthroat trout (Salmo clarkii lewisii), bl egills (Lepomis machrochirus), largemouth bass (Micropterus salmoides), black crappie (Pomoxis nigro-maculatus), y llow perch (Perca flavescens), walleye (Stizostedion vitreum), northern pike (Esox lucius), black bullheads (Ameiurus melas), golden shiner (Notemigonus crysoleucas), and bluntnose minnow (Hyborhynchus notatus).

Line-diffuser hypolimnetic oxygenation was initiated in North Twin Lake, Washington, in 2009 to mitigate reductions in Rainbow Trout (Oncorhynchus mykiss) and Brook Trout (Salvelinus fontinalis) habitat due to temperature-dissolved oxygen “habitat squeeze”. Previous studies demonstrated that trout populations rapidly expanded into increased hypolimnetic habitat within the first few years of oxygenation and previous short-term diet analyses indicated an effect on fish diet; however, the long-term effects on fish ecology have yet to be established. In this study, stable isotope analysis of fish liver tissue suggests relatively few differences in feeding ecology of principal coldwater fish species in North Twin Lake compared to unoxygenated South Twin Lake. When compared between lakes, Rainbow Trout and Brook Trout diets contained similar proportions of Daphnia, Chironomidae, and Chaoboridae. Littoral and epilimnetic-focused Golden Shiner (Notemigonus crysoleucas) diets were also similar between lakes. Observed similarities between Golden Shiner and trout diets suggest the effects of interspecific competition between salmonids and non-salmonids may be limiting trout growth and survival. Fisheries managers should therefore consider both habitat limitations and interspecific competition when managing for coldwater fish species in mesotrophic, dimictic lakes.

Largemouth bass (Micropterus salmoides) can be top-down regulators in a fish community. It is important for fisheries biologists who manage predator-prey populations to understand when bass become piscivorous. We examined the stomach contents of 622 largemouth bass in watershield (Brasenia schreberi) beds in North and South Twin Lakes, Washington. Bass displayed temporal and ontogenetic diet variation within and between lakes. Bass <100 mm fed principally on zooplankton and scuds in June and September and on benthic invertebrates, especially midge and damselfly larvae, during July and August. Damselflies, midges and scuds were major diet constituents for bass 100–199 mm. Bass 200–299 mm consumed large numbers of macroinvertebrates but also fed on crawfish and golden shiner (Notemigonus crysoleucus). Overall, the importance of large prey items like golden shiners and crawfish increased while macroinvertebrate importance to bass diets decreased as the fish grew larger. Bass piscivory was focused on golden shiners and was only observed in larger fish >300 mm. Golden shiner consumption was the lowest during June and September when cannibalism, crawfish and trout consumption increased. Temporal and ontogenetic variability in bass diets is most likely due to habitat variability, fish size, and prey availability and size. Manipulation of bass piscivory through slot-length-limits and/or macrophyte removal could be examined as a potential method for controlling nuisance forage fish such as golden shiners.

We studied survival, growth, exploitation, and production from five consecutive matched plantings of brook trout (Salvelinus fontinalis) and rainbow trout (Salmo gairdneri) (8.5-9.5 inches long) in East Fish Lake, Montmorency County, Michigan, 1958-1962. Stocked fish were given varying fin clips for later recognition. Population numbers at several intervals between introductions were determined by Petersen-type estimates. Angler exploitation was tabulated from a complete creel census. Rainbow trout survival was about 98 percent from stocking in October to the following fishing season in April, whereas brook trout survival averaged only 49 percent. About one-third of brook trout deaths occurred between 15 October and the date of ice formation (approx. 15 December). Brook trout stayed in shallow water along shore more than did rainbow trout. Anglers caught 86 percent of the stocked rainbow trout, but only 39 percent of the brook trout. For each pound of trout stocked, anglers caught 3.59 lb of rainbow trout, but only 0.76 lb of brook trout. In addition to the better return on a poundage basis, rainbow trout provided a fishery throughout the angling season, whereas nearly all brook trout were caught during the first 10 days. The brook trout grew well (average increment 0.9 lb/yr), but the rainbow trout averaged 1.3 lb increment per year. Possibly a greater poundage return on rainbow trout would accrue if the beginning of the angling season could be delayed to take advantage of the early summer growing season. Fish managers generally maintain trout in lakes by stocking hatchery fish, because trout do not reproduce in most lakes. A distinct advantage in periodic stocking is that the manager has control over fish density. We can manage trout lake-fisheries most efficiently with an understanding of mortality, growth, and production. This report deals with results from comparable plantings of brook and rainbow trout over a 5year period (1958-62) in East Fish Lake, Montmorency County, Michigan. There have been many studies of trout growth, but few studies of population numbers (and weights) throughout the life of a brood year or a given stocking. Natural mortality is difficult to measure, because the investigator usually has information only on angler exploitation, but lacks corresponding total mortality data. Seasonal differences in natural mortality of trout were observed in a general way by 1 Contribution from Dingell-Johnson Projects F-21-R, F-27-R, and F-30-R, Michigan. 682 Alexander and Shetter (1961), Latta (1963), and Eipper (1964), but we need to understand the seasonal pattern more precisely. Our objectives in this study were to determine, from October releases of rainbow and brook trout, matched for size and numbers: (1) the relative survival and growth of the species; (2) the fraction of each species later creeled; (3) the temporal distribution of the catch; and (4) the biomass of fish flesh produced by the two species. We wish to acknowledge the assistance of past and present members of the Hunt Creek Trout Research Station staff in the collection and tabulation of data. The fine cooperation of the trout fishermen in reporting their angling results is appreciated. G. P. Cooper, M. H. Patriarche, and W. E. Schaaf reviewed the manuscript and offered many helpful suggestions.

Poststocking growth, movement, and catch were compared among hatchery brook trout Salvelinus fontinalis, rainbow trout Oncorhynchus mykiss, and brown trout Salmo trutta in a fifth-order river. Associations of species, size, and stocking date with angler catch were also examined. The river is episodically acidified, and during summer it approaches lethal maximum temperatures for trout. Catchable-sized brook and rainbow trout (168–458 mm total length) were stocked in the late spring of 1996 and 1997. Brown trout were stocked only in 1997. Fish were marked with visible implant tags and were recovered through October of each year. All three species had negative daily growth rates in weight over the summer and early fall. Rainbow trout stocked in 1997 tended to move downstream after stocking, whereas the other groups showed no strong movement trend. Recovery rates significantly differed between brook and brown trouts stocked in early June and those fish stocked in late May. Large (&amp;gt;300-mm) rainbow trout were caught at higher rates than small (&amp;lt;260-mm) fish were. Anglers were estimated to have caught 72% of the stocked brook trout, 51% of the rainbow trout, and 18% of the brown trout. High summer water temperatures (&amp;gt;20°C) did not affect angler catch rates because cool refuges within the river concentrated and made the stocked fish—especially brook trout—vulnerable to angling. By stocking more than one species, we were able to create diversified angling opportunities and sustain a fishery in this thermally marginal river over the entire summer season.

Christensen, D.R. and B.C. Moore. 2009. Using stable isotopes and a multiple-source mixing model to evaluate fish dietary niches in a mesotrophic lake. Lake Reserv. Manage. 25:167–175. We used stable isotope analysis (SIA) of δ13C and δ15N and a multiple-source mixing model to evaluate dietary niches within the Twin Lakes, Washington, fish community to identify potential for exploitative competition, resource partitioning, and predation. The SIA revealed distinct spatial feeding niches; pelagic species were δ13C depleted, while littoral organisms were δ13C enriched. Trophic feeding niches were identified from δ15N enrichment with each successive trophic level. We found δ15N progression with increasing largemouth bass size, suggesting ontogenetic diet variability. Model results suggested that the smaller bass (i.e., ⩽299 mm length) fed principally on golden shiner, crayfish, and macroinvertebrates while larger bass (⩾300 mm) primarily consumed brook trout and golden shiner. Isotopic signatures for golden shiner were intermediary, indicating both pelagic and littoral feeding sources, including phantom midges (nocturnal and pelagic) and damselflies (littoral) as principal diet sources, suggesting possible horizontal diel migration in golden shiner. Pelagic zooplankton was indicated as the most important food source for rainbow trout. Food resources appeared to be partitioned among the Twin Lakes fish community, possibly limiting competitive interactions. Piscivory in largemouth bass was focused on golden shiner and brook trout, suggesting possible top-down regulation of lower trophic levels. Stable isotope analysis and multiple-source mixing models are useful tools that can improve lake and fishery management decisions by providing data on food web dynamics such as competition, resource partitioning, and predation in lakes.

Responses of microRNA and predicted mRNA and enzymatic targets in liver of two salmonids (Oncorhynchus mykiss and Salvelinus fontinalis) following air exposure.

ABSTRACTCross BK, Moore BC, Skinner MM. 2017. Hypolimnetic oxygenation effects on trout condition and growth in North Twin Lake, Washington. Lake Reserve Manage. 33:74–83.Summer stratification can restrict coldwater fish habitat, but we did not observe increases in trout growth or body condition when hypolimnetic oxygenation was applied to improve habitat conditions. Our study investigated the effects of hypolimnetic oxygenation in North Twin Lake (oxygenated 2009–2015 from May through Sep) on rainbow trout (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis) preferred habitat availability, abundance, body condition, and growth while using adjacent South Twin Lake as an unoxygenated reference. When assessing rainbow trout preferred habitat (dissolved oxygen ≥5 mg/L and temperature 13–19 C), oxygen levels were not limiting prior to oxygenation, and oxygenation did not increase preferred habitat availability. Trout abundance in both lakes remained comparable throughout oxygenation, but trout were able to move between lakes outside of summer months. Regardless of oxygenation, North Twin Lake had higher brook trout relative weights (i.e., body condition index) compared to South Twin (P = 0.0002), averaging 89.8 and 86.0, respectively. Rainbow trout relative weight differences were not observed between lakes before or during oxygenation (P = 0.1873). Prior to oxygenation, summer growth rates of rainbow trout were similar between North Twin and South Twin lakes (P = 0.9958). In 2012, growth rates of rainbow trout and brook trout in North Twin were not significantly different from South Twin (P = 0.8210 and P = 0.1474, respectively). Our results suggest that hypoxic conditions were not limiting trout growth or condition in North Twin Lake because preferred habitat volume and prey availability were not enhanced by hypolimnetic oxygenation.

Four 0.1-ha ponds were used to evaluate differences in vulnerability to angling between 1- and 2-year-old northern largemouth bass (Micropterus salmoides salmoides) and Florida largemouth bass (M. s. floridanus). In a total of 32 man-hours of fishing in two 3-day periods in June and July 1976, anglers captured 91.5% of the northern bass but only 58.3% of the Florida bass. Only a trivial difference was observed in the vulnerability of age I and age II bass of either subspecies. Of the 37 northern bass marked and released during a June fishing interval, 22% were recaptured during a July fishing interval; none of the 10 marked Florida bass were recaptured. Fishing was with plugs, spinners, and live golden shiners (Notemigonus crysoleucas). Of the Florida bass, 52% were caught with plugs, 43% with shiners, and 5% with spinners; of the northern bass, 20% were caught with plugs, 53% with minnows, and 27% with spinners.

Prey refuges are an important mechanism by which habitat structure affects ecological communities. In freshwater fish communities, most research has focused on aquatic vegetation and neglected alternative habitats. We explored the interactions between predator foraging modes of two common littoral piscivores (Muskellunge Esox masquinongy and Largemouth Bass Micropterus salmoides), and antipredator behaviors of two common prey species (Golden Shiner Notemigonus crysoleucas and Bluegill Lepomis macrochirus) across a gradient of coarse woody habitat (CWH) complexity in a mesocosm setting. We hypothesized that experiments employing a generalist predator (Largemouth Bass) and behaviorally flexible prey (Bluegills) would show a stronger refuging effect of CWH than would a less gbehaviorally flexible prey (Golden Shiners) and an obligate ambush predator (Muskellunge). Predator–prey interactions were observed in laboratory pools containing coniferous deadfalls. A refuging effect of coarse wood was not supported under our experimental conditions. Golden Shiners experienced an increase in mortality rate with increasing coarse wood complexity when preyed upon by Largemouth Bass. Both prey species reduced activity rates with increasing CWH complexity when preyed upon by Largemouth Bass but exhibited different response patterns for changes in shoal size, number of isolated individuals, and proximity to predators, which may explain differences in vulnerability across the habitat gradient. Increasing CWH complexity was associated with changes in Largemouth Bass behaviors, including reductions in activity rates and reduced capture efficiency at intermediate complexities, but the changes depended on the prey species. Habitat complexity did not strongly affect foraging success or behavior of Muskellunge. Our results reinforce the importance of species‐specific behavioral traits in determining influences of physical habitat on predator–prey relationships in freshwater fish communities.

Due to species introductions, brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) occur together in many North American streams. Some have suggested that the two species do not compete because they select different habitats or are adapted to different environmental conditions. We assessed whether native brook trout and introduced rainbow trout selected different microhabitats in a small Pennsylvania stream. Underwater observations of brook and rainbow trout showed adult fish (≥ 90 mm total length) of both species were found significantly more often in deep water microhabitats than would be expected based on habitat availability. Total depth was the most important microhabitat variable in discriminating between the two species, irrespective of fish size. Adult rainbow trout were found in significantly deeper water than adult brook trout. Adult brook trout also were found significantly farther from cover and closer to the stream bottom than adult rainbow trout. Age-0 brook trout were found in significantly deeper water than age-0 rainbow trout. In small streams during low flow, water depth and distance to nearest cover are likely to be major factors in discriminating between brook and rainbow trout.

The top predator (largemouth bass, Micropterus salmoides) in eutrophic Wintergreen Lake, Michigan, was eliminated by successive winterkills in 1978 and 1979. Within 2 yr, the golden shiner (Notemigonus crysoleucas), a facultative planktivore, dominated the fish community. The zooplankton, previously consisting of large Daphnia pulex and D. galeata mendotae, concomitantly shifted to a community consisting of smaller species: Bosmina and small copepods. In situ fish enclosure experiments in 1981 and 1982 revealed that the large Daphnia species would grow in Wintergreen Lake in the absence of the golden shiner. Golden shiners exhibit both a particulate and filtering mode of planktivory on large and small zooplankton, respectively. This flexible feeding behavior suggests that golden shiners are able to prevent the reestablishment of Daphnia, while Bosmina and the other small zooplankton support a high and apparently stable golden shiner density.

Declines in Pacific salmon Oncorhynchus spp. have been blamed on hydropower, overfishing, ocean conditions, and land use practices; however, less is known about the impacts of introduced fish. Most of the hundreds of lakes and ponds in the Pacific Northwest contain introduced fishes, and many of these water bodies are also important for salmon production, especially of coho salmon O. kisutch. Over 2 years, we examined the predation impacts of 10 common introduced fishes (brown bullhead Ameiurus nebulosus, black crappie Pomoxis nigromaculatus, bluegill Lepomis macrochirus, golden shiner Notemigonus crysoleucas, green sunfish L. cyanellus, largemouth bass Micropterus salmoides, pumpkinseed L. gibbosus, rainbow trout O. mykiss, warmouth L. gulosus, and yellow perch Perca flavescens) and two native fishes (cutthroat trout O. clarkii and prickly sculpin Cottus asper) on wild juvenile coho salmon in three shallow Pacific Northwest lakes, all located in different watersheds. Of these species, largemouth bass were responsible for an average of 98% of the predation on coho salmon in all lakes, but the total impact to each run varied among lakes and years. Very few coho salmon were eaten by black crappies, brown bullheads, cutthroat trout, prickly sculpin, or yellow perch, whereas other species were not observed to eat coho salmon. Juvenile coho salmon growth in all lakes was higher than in nearby streams. Therefore, food competition between coho salmon and introduced fishes in lakes was probably not limiting coho salmon populations. Largemouth bass are widespread and are present in 85% of lowland warmwater public‐access lakes in Washington (n = 421), 84% of those in Oregon (n = 179), and 74% of those in the eight northwesternmost counties in California (n = 19). Future research would help to identify the impact of largemouth bass predation across the region and prioritize lakes where impacts are most severe. Nevertheless, attempts to transplant or increase largemouth bass numbers in lakes important to coho salmon would be counterproductive to coho salmon enhancement efforts.