Shifting Diets of Lake Trout (Salvelinus namaycush) in Northeastern Lake Michigan: A Yearly and Five-Year Assessment

Abstract

Prey fish communities in Lake Michigan have been steadily changing, highlighted by declines in both the quantity and quality of Alewife Alosa pseudoharengus. It has been unclear how the continued decline in Alewife abundance has affected the diets of salmonine predators, such as Lake Trout Salvelinus namaycush, in Lake Michigan. Previous studies have shown that Alewife were the dominant prey of Lake Trout in northeastern Lake Michigan since the salmonid stocking program began in the 1960s. To evaluate shifts in diet composition of Lake Trout, I analyzed stomach contents of individual Lake Trout caught in northeastern Lake Michigan during gillnet surveys and fishing tournaments from May through October 2016. I then compared the composition of 2016 diets to those found in the most recent survey conducted in 2011. I also sought to create an updated diet schedule to outline how Lake Trout feeding patterns change throughout the year. Overall, I found that Lake Trout diets consisted almost entirely (94% by weight) of Alewife and Round Goby Neogobius melanstomus. On an annual basis, approximately 61% of the Lake Trout diet composition consisted of Alewife. There was a clear seasonal shift in diet composition from the early time period (May and June) to the late time period (July through October). In the early time period, Round Goby was the dominant prey, accounting for 77% of the diet composition, while Alewife contributed 21%. In contrast, Alewife was the dominant prey in the late time period, accounting for 85% of the diet composition, while Round Goby only contributed 8% over the same time period. This dominance of Round Goby in spring diets has not been observed in previous studies and marks a stark increase from its contribution to the 2011 spring diet composition (21%). Statistical analyses indicated that gear type and total length of Lake Trout were far less important than collection month in predicting diet composition. Regardless of gear type used throughout the year, Round Goby dominated 5 diets in the early sampling period and Alewife dominated diets in the late sampling period. Although Lake Trout show increased flexibility in prey species consumed in spring months compared to 2011, Alewife still appears to be the most important component of their diet on an annual basis, despite reduced Alewife biomass throughout Lake Michigan. This likely results from lack of alternative prey sources, especially following the summer migration of Round Goby to shallow waters.