Causes and consequences of alewife collapse in New York lakes

Alex Koeberle (PhD student), Sethi, Rudstam, Watkins, Evan Cooch and Brad Hammers (NYSDEC), (Funded by NYSDEC)

Alewife (Alosa pseudoharengus) populations have modified food webs in lakes throughout their introduced range in North America. This study, in collaboration with SUNY Oneonta Biological Field Station, NYSDEC, and Keuka Lake Association, analyzes long-term datasets of water quality, benthos, zooplankton, and fishes from Keuka Lake, New York (45 years, 1979-2024) and from Otsego Lake to present evidence for the causes of alewife population collapse, which was likely the combined effect of low zooplankton prey, high predation by an apex piscivore, and a series of cold winters. While multiple pathways contributed to alewife collapse, we find that the relative influence of contributing factors was lake-specific. In addition, we found common consequences of alewife collapse between lakes. Predation release from alewife collapse led to a rapid response in zooplankton communities, including increased Daphnia spp. biomass in both lakes, and in Keuka Lake, an increase in the mysid population to densities approaching 200 individuals/m2, higher than in most Laurentian Great Lakes. The number of top predator lake trout Salvelinus namaycush declined in both lakes but remained relatively high feeding on mysids in Keuka Lake and other prey fish species in both lakes. Persistent piscivory likely suppressed alewife recovery in both lakes, overwhelming their well-known compensatory response. Our study shows that identifying the causes of the collapse of a dominant planktivore in two lakes is important for understanding the food web consequences.