The influence of conditions in Lake Superior and the Bois Brule River, Wisconsin on returns of migratory rainbow trout

Rainbow trout were introduced to Lake Superior in the late 1800’s and exhibit a potamodromous life history and exhibit high variability in reproductive success. We examined reproductive variability in the Bois Brule River, WI (Lake Superior), through analyses of returns of wild first spawning (hereafter “maiden” returning) adults. We used classification and regression tree analyses to identify in-stream and in-lake (western Lake Superior) sources of variability and to identify the environment (stream or lake) that was most influential to the returns to each location. Among in-stream influences, high discharge rates in the spring period (March – May) during a pre-smolt's first stream year were the strongest source of variability and were negatively correlated with returns. High discharge during the fall period from September to November in the pre-smolt first stream year was also negatively correlated with numbers of maiden returning steelhead from that year class. When variables associated with Lake Superior were considered, maiden returns were positively correlated with higher lake surface temperatures in Lake Superior. Returns were negatively correlated with the abundance of adult rainbow smelt and bloater suggesting a possible competitive interaction among those species. Finally, we also observed a conditional (minor) positive effect of age-0 smelt abundance indicating the importance of this prey for juveniles in colder years in western Lake Superior. Taken together, our findings indicate that both stream and lake conditions in their first lake year are important sources of variability and point to spates in the spring and fall as initial controlling variables.