Lake Superior's fish community has undergone dramatic changes since the mid‐1950s, with major shifts in the forage base, invasion of exotics, and decline of lake trout Salvelinus namaycush. Predator species have been introduced, and many of these are maintained by stocking. The impact of these stocked fish on the forage base and the ability of the forage base to sustain projected stocking levels are unknown. Bioenergetics modeling is a particularly useful tool for coordinating data to answer questions about predator–prey dynamics, stocking quotas, and forage requirements. This study focused on the Minnesota waters of Lake Superior to gain perspective on the impact of fish stocked by Minnesota and to serve as a basis for future expansion of the analysis to all parts of the lake. The objectives of this study were to compile data on the major salmonines in Minnesota waters of Lake Superior for use in a bioenergetics model, to estimate salmonine predation on prey populations through modeling simulations, and to prioritize data needed for future input to the bioenergetics model. Estimates of consumption by predator stocks in 1989 totaled 3,047 metric tons of rainbow smelt Osmerus mordax and 401 metric tons of coregonines, which greatly exceeded estimates of biomass plus production of prey species. Underestimation of prey fish biomass probably accounts for most of this discrepancy. Recent shifts in the forage base, heavy predation on prey species by stocked fish, and reduced survival of stocked chinook salmon Oncorhynchus tshawytscha suggest that caution should be exercised in future stocking. Further data acquisition should focus on determining prey fish biomass and production, predator age and growth, predator diets, predator population abundances, and mortality rates.

An osmotic induction method was used to apply calcein marks to Chinook salmon Oncorhynchus tshawytscha and the Kamloops strain of rainbow trout O. mykiss (both migratory strains used for stocking into tributaries of Lake Superior). Preliminary exposure to a saline solution followed by immersion in a 0.5% calcein solution encouraged rapid infusion of calcein into fish tissues. Four‐month‐old Chinook salmon were exposed to a 1.5% or 5.0% salt solution to evaluate tolerance of different salinities and effects on mark intensity and retention. Rainbow trout were marked at three ages (2 weeks, 3.5 months, and 16 months posthatch) to evaluate the effects of age at treatment on mark intensity and long‐term retention (a 5.0% preliminary salt bath was used). External detection of marks in scales, head, and fin rays was accomplished with a hand‐held detector. The marking technique we used was well tolerated by both Chinook salmon and rainbow trout, and marks persisted into adulthood for rainbow trout treated at 3.5 months posthatch or later. The 5% salt concentration resulted in the persistence of more intense marks than did the 1.5% salt concentration. We demonstrated the potential applicability of calcein marks for instream population studies of migratory or anadromous species treated as fry or for studies involving release of post‐fry‐stage migratory or anadromous fish intended for recapture as adults.

Lake Superior's fish community continues to change as a result of the recovery of populations of lake trout Salvelinus namaycush, the naturalization of introduced salmonids Oncorhynchus spp. and Salmo spp., declines in the populations of rainbow smelt Osmerus mordax, and the fluctuating recruitment of cisco Coregonus artedi. This study used bioenergetics modeling of the dominant predator fish in the western arm of Lake Superior in 2004 to provide a comprehensive picture of the relationship between predator demand and prey fish availability. The results, presented for nearshore and offshore areas and three geographically distinct ecoregions, indicate that the western arm is at or near its carrying capacity for predators. Estimated predator demand was about one‐half the annual biomass plus production of coregonines but exceeded the biomass plus production of rainbow smelt, possibly because of underestimates of this species in shallow areas and recent shifts in predator diets. Lean lake trout were responsible for most of the consumption of these prey fish, while the deepwater siscowet form ranked second. The commercial harvest of prey fish biomass was trivial compared with the consumption by predator fish. Chinook salmon Oncorhynchus tshawytscha, along with other nearshore potamodromous species, played minor roles in total consumption. Chinook salmon are an indicator of forage status, as they have responded to declines in the rainbow smelt biomass with a dramatic diet shift to ciscoes and a decline in weight at age since the early 1990s. Slightly reduced lean lake trout growth and density‐dependent survival in some areas also indicate that competition for prey is intense. Because most of the predators in the western arm are wild fish, stocked predators no longer have a great impact on prey populations. Bioenergetics modeling of predator demand combined with hydroacoustic estimates of prey supply can be valuable for managing the fisheries of large lakes, allocating resources to various interest groups, and increasing basic understanding of lake production and community dynamics.

Two strains of anadromous rainbow trout Oncorhynchus mykiss, naturalized steelhead and “kamloops” (not the pure Kamloops strain from British Columbia, hence not capitalized) currently inhabit the Minnesota waters of Lake Superior and may have the potential to hybridize. This could compromise the genetic integrity of the naturalized steelhead population. Both strains are supplemented by annual stocking, despite the fact that the steelhead population reproduces naturally. Egg viability and fry behavior experiments were undertaken to evaluate the potential for hybridization and to provide information for future management of the two strains. The kamloops eggs were slightly smaller, but sizes overlapped substantially with steelhead egg sizes. Mortalilty of kamloops eggs from spawning to hatching was greater than steelhead eggs. Steelhead fry exhibited a greater fright response (wariness) than kamloops fry when startled by movement over their tanks. Hybrid egg survival and wariness traits were intermediate to those of the pure strains, but more closely resembled those of the maternal strain. These traits appeared to be heritable. Reevaluation of steelhead and kamloops management will be necessary in the future, taking into account the popularity of the kamloops fishery and the potential for degradation or elimination of the naturalized steelhead strain.

Studies of interactions among percids are important for understanding the fish community consequences of using stocking as a management tool and for understanding the ecological role of percids in lakes where they are native species. Percid community responses to discontinuing, then resuming, the stocking of walleye Sander vitreus were monitored in Lake Thirteen, Minnesota, a north temperate lake with a long‐term history of walleye fry stocking. Fish population assessments during 1986–2002 tracked changes in relative abundance, size structure, and growth rates of yellow perch Perca flavescens and walleyes in response to discontinued walleye stocking during 1989–1996. Large changes occurred in the size structure and relative abundance of walleyes. The effect of discontinued stocking was to decrease the abundance of younger age‐classes of walleyes in the lake. Reduced predation on small yellow perch allowed their numbers to increase and led to reduced growth rates while stocking was discontinued. When stocking was resumed, predation by stocked walleyes reversed the trend, reducing yellow perch numbers and improving their growth rates. Density‐dependent growth was evident for both walleyes and yellow perch. Bioenergetics modeling confirmed walleye predation as a plausible explanation for most changes in the yellow perch population. Manipulation of stocking, and the resulting shifts in predator sizes, caused structural changes in the percid populations that altered levels of intraspecific competition for both the predators and prey.