Evidence for predation by young-of-the-year alewife and bloater chub on Bythotrephes cederstroemi in Lake Michigan

The recent invasion of Bythotrephes cederstroemi into the Great Lakes has raised speculation as to its potential effects on the resident food webs. Its long tailspine has been implicated as a post-contact, antipredatory adaptation against small fishes but few field data exist on this subject. Here we present results of gut content analyses on 25 young-of-the-year alewife (Alosa pseudoharengus) and 84 young-of-the-year bloater chub (Coregonus hoyi) collected from Lake Michigan by mid-water trawl during August and September of 1988 and 1990. Alewife of 39.4-59.6 mm and bloater chub of 29.4-55.8 mm standard length contained remains of Bythotrephes. Although limited, these data lower the size range at which alewife and bloater chub in Lake Michigan have been found to eat Bythotrephes. Bythotrephes remains were counted in fish guts by numbers of tailspine kinks and mandible pairs. In bloater chub we found 1.2 times more tailspine kinks than mandible pairs but in alewife guts we found 9.4 times more tailspine kinks than mandible pairs suggesting that tailspine kinks were more strongly retained over mandibles in alewife compared to bloater chub.