Spatial Segregation of Adult and Young-of-the-Year Alewives across a Thermocline in Lake Michigan

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Abstract
Distributions of young‐of‐the‐year and adult alewives (Alosa pseudoharengus) in Lake Michigan were determined by bottom trawling along isotherms at the intersection of the thermocline with the bottom near Grand Haven, Michigan over a 1‐week period during September. Few alewives were caught on the bottom at night. During the day, adult alewives were caught primarily at 11–14 C. Young‐of‐the‐year alewives on the bottom occurred only at temperatures greater than 15 C and primarily at 17–20 C. Bathymetric distributions of the two size groups of alewives shifted with oscillations in thermocline location. Young of the year and adults also differed in prey selection. Young‐of‐the‐year alewives ate primarily cyclopoid copepods, calanoid copepods, and Daphnia sp. and these items were most common in stomachs during the day. Adult alewives fed mainly on Daphnia sp. and more were eaten during the day than night. Copepods, Bosmina sp., Pontoporeia hoyi, and Mysis relicta were also present in stomachs of adult alewives. It is suggested that behavioral thermoregulation can reduce the potential for food competition between adult and young‐of‐the‐year alewives.