Colonizing Inland Lakes: Consequences of YOY Fish Ingesting the Spiny Cladoceran (Bythotrephes cederstroemi)

Recently the exotic cladoceran Bythotrephes cederstroemi colonized inland lakes in the Upper Peninsula of Michigan, facing warmer temperatures and greater exposure to smaller fish than in coastal waters of Lake Superior. However, the long caudal appendage (spine) of Bythotrephes protects it against young-of-the-year (YOY) fish predation. After experience, small fish (30-60 mm TL) develop aversion behavior, explaining why this prey item rarely occurs in their stomachs, despite ingestion of other prey species with core body lengths between 2-3 mm. Here we compare laboratory and field size-dependent feeding patterns for YOY of two common fish species (yellow perch, Perca flavescens; bluegills, Lepomis macrochirus) with field patterns from a typical bait-fish species (spottail shiner, Notropis hudsonius). Behavioral responses of bluegills indicate that deterrence is not a simple linear function of spine length, but that longer spines offer disproportionate protection, explaining why long spines are present at birth. Stomach contents of small perch (30–60 mm TL) confirm size-dependent avoidance, with only occasional consumption of Bythotrephes and rare puncture of stomach walls. However, examples of ingestion and frequent stomach puncture were found in the thinner-walled stomachs of spottail shiners. Out of 140 small fish collected when this species encountered high densities of Bythotrephes, 77 had Bythotrephes in stomachs or lower digestive tracts, and 50 (65%) had spines protruding through gut walls. Clearly, this bait-fish species is capable not only of ingesting large numbers of Bythotrephes, but of receiving injury in the process. INDEX WORDS: Gut puncture, Bythotrephes, bluegill, spottail shiner, perch, aversion.