J. Great Lakes Res. 30 (Supplement 1):315–326 Internat. Assoc. Great Lakes Res., 2004 Colonizing Inland Lakes: Consequences of YOY Fish Ingesting the Spiny Cladoceran (Bythotrephes cederstroemi) Justin A. Compton † and W. Charles Kerfoot* Lake Superior Ecosystem Research Center and Department of Biological Sciences Michigan Technological University Houghton, Michigan 49931 ABSTRACT. 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 aver- sion 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-depen- dent feeding patterns for YOY of two common fish species (yellow perch, Perca flavescens; bluegills, Lep- omis 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 pre- sent 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. INTRODUCTION Exotic species are defined as successfully repro- ducing, non-indigenous species transported by hu- mans into regions where they did not previously exist (Lodge 1993). The accelerated immigration of exotic species related to recent globalization poses a legitimate concern. Since European settle- ment, the Laurentian Great Lakes have had a long and storied history of disturbance due to the intro- duction of exotic species, as 139 exotic aquatic species have become established since the early 1800s (Mills et al. 1993). Established exotic species include, among others: sea lamprey (Petromyzon marinus), zebra mussels (Dreissena * Corresponding author. E-mail: wkerfoot@mtu.edu †Present address: Wildlife and Fisheries Science, Pennsylvania State University, University Park, PA 16802 315 sp.), spiny cladocerans ( Bythotrephes sp., Cen- tropagis sp.), Eurasian ruffe (Gymnocephalus cer- nuus ), and Eurasian milfoil ( Myriophyllum spicatum). The perturbations created in aquatic communities by exotic species 1) raise concerns about the long-term repercussions of accelerated immigration, 2) test the ecological importance of single-species substitutions in aquatic food webs and 3) prompt questions about the stability of freshwater communities. One example of an exotic species introduction into the species-poor Great Lakes is Bythotrephes cederstroemi (Schoedler), a species perhaps syn- onomous with B. longimanus (MacIsaac et al . 2000). Bythotrephes is a predaceous zooplankter that appears to have arrived via ballast water from the Lake Ladoga-Neva Estuary-Gulf of Finland sys- tem (Bur et al. 1986, Berg et al. 2002). It was first reported in the Great Lakes in the mid-1980s and