Causes and consequences of population changes in Black Bay Steelhead, Lake Superior

Abstract

Understanding species interactions is an important component of fisheries resource management. Like the other Great Lakes, Lake Superior’s fish species composition (and therefore interactions among species) has been changing for many decades. One of the largest changes in fish species composition in Lake Superior has taken place in Black Bay, with an increase in Walleye abundance since 2002 and a decrease in Steelhead abundance since 2008. However, changes in species interactions associated with this new species composition in Black Bay are unknown. I examined the potential interactions between Walleye and Steelhead in Black Bay using stable isotope analysis and life history theory. My results suggest a complex set of interactions both in stream and lake environments that may act to suppress current Steelhead populations. Specifically, I found shifts in Steelhead smolting strategies within Black Bay not observed elsewhere in Lake Superior that are consistent with size-specific increased juvenile mortality, possibly due to predation on smolts in Black Bay. Niche overlap analysis from stable isotope results showed evidence of some competition between Walleye and adult Steelhead as well as possible predation of smolts by mature Walleye in Black Bay. Furthermore, long-term analysis of Steelhead parr stable isotopes suggests significant changes in Steelhead resource use in the stream environment, possibly resulting from competition with Brook Trout