Trophic ecologies of double-crested cormorants and native piscivorous fishes in Lake Nipissing, Ontario

Abstract

Fisheries assessments have indicated a decline in Lake Nipissing’s walleye (Sander vitreus) population in recent decades. This has coincided with an increase in doublecrested cormorants (DCCO; Nannopterum auritum) on Lake Nipissing to conspicuous numbers (3,000 nests in 2012), fueling concerns that cormorant predation may be adversely affecting walleye recovery. I used carbon and nitrogen stable isotope ratios (δ13C and δ15N, respectively) to examine the food web structure of Lake Nipissing, and in particular, the role of DCCO predation and competition in relation to four other native piscivores; walleye, northern pike (Esox lucius), smallmouth bass (Micropterus dolomieu) and burbot (Lota lota). Trophic position (TP), estimated from δ15N, and trophic niche size (SEAc), inferred from the dispersion of individuals in δ13C - δ15N isotopic space, were used to characterize piscivore niches. MixSIAR stable isotope mixing models (SIMM) were used to estimate the diet compositions of DCCO and piscivorous fishes. Among the piscivores, DCCO had the highest reliance on pelagic resources, the lowest trophic position, and the largest trophic niche size. Piscivorous fishes had higher levels of trophic niche overlap with each other than with DCCO. SIMMs predicted that DCCO diet was primarily composed of emerald shiner (Notropis atherinoides) and logperch (Percina caprodes) with low proportions of all other prey fishes. Emerald shiner and logperch also dominated the diets of piscivorous fishes indicating dietary overlap with DCCO to some extent. Juvenile walleye were a relatively small proportion of the diets of both DCCO and piscivorous fishes based on SIMM predictions. Currently, there is no indication of limitations in Nipissing’s forage fish prey base so potential for interspecific competition is considered low. The low likelihood of trophic niche overlap between DCCO and piscivorous fishes and the low contribution of juvenile walleye to DCCO diet suggests a low impact of DCCO predation on Nipissing’s recovering walleye population. Stable isotope-based diet inference can complement bioenergetic models and other ecosystem assessment methods to improve our understanding of how DCCO interact with fish populations.