The Dietary Niche of Double-Crested Cormorants and Overlap with Sport Fish: A Stable Isotopes Approach

Double-crested cormorants (Phalacrocorax carbo) have generated concerns regarding conflict with humans over fisheries.   In addition to direct consumption of sport fish, cormorants may negatively impact predatory fish populations through competition.  However, there is little information available on dietary overlap and competition between cormorants and fish. Using stable carbon and nitrogen isotopes analyses, we examined cormorant diet niche and overlap with sport fish in Saskatchewan, Canada.  Cormorant dietary niche overlapped with walleye (Sander vitreus vitreus), but not with northern pike (Esox lucius).  Walleye niche was narrow and overlapped with both other predators, whereas northern pike and cormorant niches were broad and the majority of individuals did not overlap with other species.  Diet of individual cormorants was surprisingly consistent, but varied widely among individuals.  Sport fish were somewhat less individually specialized.  Only some individual cormorants had overlapping diets with a portion of the walleye.  There appears to be niche partitioning among cormorants and sport fish; however, it is unclear whether this is due to competition or species-specific constraints.  Because overlap varies among lakes, impacts to walleye likely vary.  Our research demonstrates the importance of considering trophic interactions between cormorants and sport fish on a case by case basis.