Stable isotopes indicate cisco diets shift from shallow to deep-water prey as they increase in body size
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 oxygen levels due to eutrophication. Body size of cisco varies sharply among Minnesota lakes, but relationships between cisco body size 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 seven lakes and assessed whether habitat use was related to cisco body size by measuring weight and δ15N of cisco, δ15N of seston and zooplankton in both the epilimnion and hypolimnion, and δ15N of benthic invertebrates such as chironomids.
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 six of the seven lakes, indicating either the fish feed at higher trophic levels as they get older, or feed in deeper habitats. Diet samples indicated zooplankton (non-Chaoborus) were the primary prey for all sizes of cisco, suggesting δ15N changes in larger cisco were driven by 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 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.