Declines in lake whitefish larval densities after dreissenid mussel establishment in Lake Huron

Abstract

Lake whitefish (Coregonus clupeaformis) is an ecologically and commercially significant species across the Laurentian Great Lakes. Over the past 20 years, lake whitefish population abundance has substantially declined across lakes Huron and Michigan, being driven by reduced recruitment of juvenile fish into the population. However, the life stage at which the recruitment bottleneck is occurring and what factors are contributing to the declines remain unknown. One hypothesis is that dreissenid mussels reduced zooplankton availability to larval lake whitefish, leading to poor growth and survival of this critical life stage. Here, we present results of a larval fish survey conducted at the Fishing Islands spawning region in Lake Huron and examine whether declines in juvenile recruitment are linked to reduced larval fish density. Larval fish were collected annually during two time periods: (1) a historical time period before dreissenid mussel establishment (1976–1986); and (2) a contemporary time period after dreissenid mussels became established (2017–2019, 2021). We found significant declines in larval densities and growth between historical and contemporary time periods. Following dreissenid establishment, larval densities and growth were on average only 23% and 55% of historical values, respectively. Moreover, year class strength at the juvenile stage (age 4) was positively related to larval density. Several explanatory variables contributed to annual variation in larval densities, with dreissenid mussels and water levels having the most consistent effect. Our results suggest that juvenile recruitment is being limited at the larval stage, owing to overall lower larval production and potentially exacerbated by slower growth.