The Influence of Alewife Year‐Class Strength on Prey Selection and Abundance of Age‐1 Chinook Salmon in Lake Michigan

Abstract

AbstractWe used growth and diet data from a fishery‐independent survey of Chinook salmon Oncorhynchus tshawytscha, acoustic estimates of prey density and biomass, and statistical catch‐at‐age modeling to study the influence of the year‐class strength of alewife Alosa pseudoharengus on the prey selection and abundance of age‐1 Chinook salmon in Lake Michigan during the years 1992‐1996 and 2001‐2005. Alewives age 2 or younger were a large part of age‐1 Chinook salmon diets but were not selectively fed upon by age‐1 Chinook salmon in most years. Feeding by age‐1 Chinook salmon on alewives age 2 or younger became selective as the biomass of alewives in that young age bracket increased, and age‐1 Chinook salmon also fed selectively on young bloaters Coregonus hoyi when bloater density was high. Selection of older alewives decreased at high densities of alewives age 2 or younger and, in some cases, high densities of bloater. The weight and condition of age‐1 Chinook salmon were not related to age‐1 Chinook salmon abundance or prey abundance, but the abundance of age‐1 Chinook salmon in year t was positively related to the density of age‐0 alewives in year t − 1. Our results suggest that alewife year‐class strength exerts a positive bottom‐up influence on age‐1 Chinook salmon abundance, prey switching behavior by young Chinook salmon contributing to the stability of the predator‐prey relationship between Chinook salmon and alewives.