The Effects of Differential Digestion Rates of Zooplankton by Alewife, Alosa pseudoharengus, on Determinations of Selective Feeding

Abstract

Abstract Ivlev's electivity index (E) was used to assess selective feeding by the alewife (Alosa pseudoharengus) on crustacean zooplankton in southern Green Bay, Lake Michigan. Alewives positively selected larger species of Copepoda, especially Mesocyclops edax. Alewives appeared to negatively select all Cladocera. However, field data from Lake Michigan and other lakes containing abundant populations of alewives indicate positive selection may be occurring on Daphnia. Positive selection of Daphnia by alewives probably took place in southern Green Bay, although this was not indicated in electivity indices. Laboratory studies revealed that Daphnia were digested most rapidly by alewives and, therefore, were under-represented in stomach contents relative to other planktonic species. Consequently, it is suggested that differential digestion rates of prey organisms may also occur in alimentary tracts of other fish species. This could have an effect on interpretation of selective feeding data using Ivlev's elect...