Invasive species sustain double-crested cormorants in southern Lake Michigan

Double-crested cormorants (Phalacrocorax auritus) have long and often been implicated in having detrimental effects on fisheries. Research has shown that the main components of cormorant diets vary significantly among forage, invasive, or economically important fish species. This study builds on previous work in the north basin of Lake Michigan by focusing on cormorant diet composition at their only significant nesting colony in the southern basin at East Chicago, Indiana where cormorant diet has been unstudied. Regurgitated pellets were collected from the colony and diagnostic bones were used to elucidate diet composition. Alewife (Alosa pseudoharengus), round goby (Neogobius melanostomus), Lepomis spp., white perch (Morone americana) and yellow perch (Perca flavescens) were the most frequently found prey items depending on cormorant feeding period. Invasive species, most notably alewife, round goby, and white perch, contributed over 80% and 90% of the diet of cormorants by biomass and number, respectively. No salmonine species were detected suggesting that negative effects on this important fishery would likely occur only via direct competition for prey (e.g., alewife). Predation on yellow perch, which occurred mainly prior to and during the perch spawning season (i.e., of age-1 and older individuals), may warrant further study to quantify the effects on the local yellow perch population. However, because yellow perch abundance is thought to be currently limited by poor recruitment at age-0, these results do not support the active management of the cormorants at this colony to protect or recover local fisheries.