Feeding Ecology and Vertical Migration of Adult Alewives (Alosa pseudoharengus) in Lake Michigan

Abstract

Vertical distributions of adult alewife (135–216 mm long) and Mysis relicta were measured acoustically on a 24 h basis during July, September, and October 1975 and June 1976 at a station 8 km northeast of Milwaukee, Wisconsin in 50 m of water. Both Mysis and adult alewife concentrated on the bottom during day and migrated upwards to the base of the thermocline at night. Extent and timing of vertical migrations of Mysis and alewife coincided. During bright moonlight Mysis concentrated at depths well below the thermocline and larger adult alewives concentrated within this Mysis layer while smaller adult alewives migrated to the base of the thermocline. An examination of stomach contents indicated that vertical migrations were mechanistically linked to feeding behavior. Adult alewives fed extensively on Mysis at night. Total lengths of Mysis in alewife stomachs were significantly longer than total lengths of Mysis captured in vertical plankton tows. Pontoporeia hoyi occurred in alewife stomachs in late afternoon (September) and early evening (June, October). Microcrustacean zooplankton were eaten mostly during day (June) or early morning (October). As Mysis was the most important food, the vertical migration of alewives is interpreted as an adaptation for feeding on vertically migrating Mysis.Key words: alewives, Lake Michigan, Mysis, Pontoporeia hoyi, vertical migration, feeding ecology, light