Walleye Introduction Eliminates Predation Refuge for Adfluvial Cutthroat Trout and Rainbow Trout

Abstract

AbstractA predation refuge allowed Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and Rainbow Trout O. mykiss to coexist with Lake Trout Salvelinus namaycush in Buffalo Bill Reservoir, Wyoming. Recently, Walleye Sander vitreus appeared prompting concern about the persistence of Oncorhynchus populations. Study objectives were to (1) understand how environmental conditions have contributed to the coexistence of Lake Trout and Oncorhynchus populations, and (2) determine habitat use, diet, and per capita consumptive demand of Walleyes. While Walleye population expansion is difficult to predict, we compared consumptive demand of Lake Trout to biomass and production of Oncorhynchus populations to evaluate if additional predation demand by Walleyes could be sustained by prey populations. We measured limnological variables and sampled fishes monthly during April–October in 2012 and 2013 to determine habitat use and collect growth and diet samples. Low Secchi depth (1.6 m) and warm (19°C) epilimnetic temperatures in summer appear to have inhibited Lake Trout predation and provided Oncorhynchus spp. with a thermal refuge from predation, but this refuge was eliminated with the introduction of Walleyes. Prior to thermal stratification, Lake Trout and Oncorhynchus spp. co‐occurred at depths less than 18 m, but during summer, Oncorhynchus spp. remained in shallow water and Lake Trout retreated to the hypolimnion. Unlike Lake Trout, Walleyes always co‐occurred with Oncorhynchus spp. Only large (≥600 mm TL) Lake Trout consumed Oncorhynchus spp. and only during the unstratified period; in contrast, 64% of Walleye diet was Oncorhynchus spp., regardless of season. Per capita consumption of Oncorhynchus spp. by Walleyes (13.43 kg) was similar to that by Lake Trout (17.46 kg), despite the longer Lake Trout life span. Thus, although Lake Trout consumed only about 18% (462 kg) of the August biomass (2,624 kg) and 9% of annual production (5,049 kg) of young Oncorhynchus spp., expansion of the Walleye population could threaten Oncorhynchus populations in the future.Received July 13, 2016; accepted October 19, 2016 Published online January 31, 2017