Response of alewife abundance to the bacterial kidney disease outbreak in the Chinook salmon population of Lake Michigan: Importance of predation

Prey fish abundance can be influenced by predation (top-down) and food limitation (bottom-up) effects. I characterized temporal trends in yearling and older (YAO) alewife (Alosa pseudoharengus) biomass density, as estimated by a long-term bottom trawl survey, in Lake Michigan during 1973–2019. Special attention was given to the bacterial kidney disease (BKD) outbreak in the population of Chinook salmon (Oncorhynchus tshawytscha), the predominant predator on alewives in the lake. YAO alewife biomass density exhibited a steep and significant decline during 1973–1985, but then partially rebounded with a significant increase between the 1983–1985 and 1986–2003 (BKD years) periods, followed by a significant decrease between the 1986–2003 and 2004–2006 periods. YAO alewife biomass density showed another significant decline during 2004–2019. The BKD outbreak led to a partial relaxation of predation on the YAO alewife population, and YAO alewife biomass density responded by showing a moderate increase in 1986 and then fluctuating about this moderately higher level for more than 15 years. Overall, temporal trends in YAO alewife biomass density were primarily driven by predation.