Cisco (Coregonus artedi) are a cold-water fish found in many Minnesota lakes, and are threatened by increasing water temperatures from climate change and declining dissolved oxygen levels due to eutrophication. Body size of cisco varies sharply among Minnesota lakes, but relationships between cisco body size, diet, and habitat use is poorly known. δ15N can be used to assess trophic levels of organisms, but it can also be used to assess habitat use and prey selection by aquatic organisms because it is usually higher in deep-water organisms compared to organisms living primarily in the epilimnion. We sampled eleven lakes and assessed whether diet or habitat use was related to cisco body size by examining cisco diet structure, measuring cisco weight and δ15N, and measuring the δ15N of seston and zooplankton in both the epilimnion and hypolimnion of the lakes.
Results/Conclusions
Results showed δ15N of seston was significantly higher in the hypolimnion relative to the epilimnion across all lakes, and that δ15N of hypolimnetic zooplankton and benthic invertebrates was higher than epilimnetic zooplankton. δ15N of cisco showed a positive relationship with cisco body weight in the eleven lakes, which could be explained by either of two mechanisms 1) the larger fish are feeding in deeper habitats where δ15N is enriched or 2) the fish are feeding at higher trophic levels as they increase in body weight. Diet samples indicated primary consumer zooplankton (non-Chaoborus) were the dominant prey for all sizes of cisco, suggesting δ15N changes in larger cisco were driven by mechanism 1-- feeding in a deeper habitat and not by feeding on a different trophic level. Overall our results indicate habitat use and prey selection varies with body size of cisco within individual and across lakes, and it is possible the large difference in cisco maximum size observed among lakes may be related to the ability of cisco to exploit deep-water prey.