Diet Complexity of Lake Michigan Salmonids

Documenting trophic relationships in aquatic ecosystems can facilitate understanding of not only system processes, but also the potential responses of food webs to stressors. In Lake Michigan, the introduction of invasive species (e.g., zebra mussel, Dreissena polymorpha; quagga mussel, Dreissena bugensis; round goby, Neogobius melanostomus) and reduced nutrient loading has resulted in changes in nutrient dynamics and community composition over the past two decades. As a result, abundances of many forage fish have declined, including alewife (Alosa pseudoharengus) which have historically supported the five dominant salmonid species of Lake Michigan (brown trout, Salmo trutta; Chinook salmon, Oncorhynchus tshawytscha; Coho salmon, Oncorhynchus kisutch; lake trout, Salvelinus namaycush; rainbow trout, Oncorhynchus mykiss). With these ecosystem changes, there is uncertainty as to the extent of how different species of salmonids will transition to alternative prey items (e.g., round goby). We investigated the diet complexity of Lake Michigan salmonids by evaluating stomach content composition, diet diversity, and lengths of alewife consumed. Stomachs collected in 2015 and 2016 in Lake Michigan revealed that Chinook salmon almost exclusively consumed alewife and had a lower diet diversity compared to the other four species, which consumed round goby (brown trout and lake trout), aquatic invertebrates (Coho salmon), and terrestrial invertebrates (rainbow trout) in addition to alewife. Additionally, salmonid species appeared to consume the entire size range of alewife that were available to them despite year to year changes in alewife length availability. Due to their reliance on alewife, it is likely that Chinook salmon may be more negatively impacted than other salmonid species if patterns of alewife decline continue in Lake Michigan.