Morphological Variation of Cisco across Gradients of Lake Productivity

Abstract The effects of lake productivity and late‐summer hypolimnetic oxygen on the size and shape of Cisco Coregonus artedi were examined in 27 Minnesota lakes. Geometric morphometry analyses of specimens captured in vertical gill nets indicated that Cisco in more productive lakes with hypoxic hypolimnia were larger, with deeper bodies, shorter snouts and caudal peduncles, and longer fins. Cisco in unproductive lakes with oxygenated hypolimnia were smaller, with slender bodies, longer snouts and caudal peduncles, and shorter fins. The effect of hypolimnetic oxygen on depths utilized by Cisco and the resulting vulnerability to predation was an important driver of shape and fin length. Deeper body depths and longer fin lengths were possibly a response to greater vulnerability to predation when hypoxic hypolimnia limit coldwater refugia for Cisco, which are forced to inhabit shallower depths near warmwater predators such as Walleye Sander vitreus and Northern Pike Esox lucius . Differences in size and shape relationships of Cisco in lakes on and off the Canadian Shield were also detected. The availability of late‐summer habitat, depth‐mediated predation risk, and growth potential appear to be important ecological drivers of morphological variation in Cisco.