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

Abstract

We 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.