Lake Ontario spring prey fish bottom trawl survey and Alewife assessment, 2025

Abstract

The multi-agency Lake Ontario spring prey fish survey quantifies changes in pelagic prey fish populations, in particular Alewife Alosa pseudoharengus , which are the primary prey supporting the lake’s sport fishes. The 2025 survey included 230 trawls in the main lake and embayments and sampled depths from 5.5 to 245 m (15 – 810 ft). The survey captured 504,541 fish from 33 species with a total weight of 7,301 kg (16,095 lbs). Alewife were 85% of the total catch numerically, while Yellow Perch Perca flavescens , Round Goby Neogobius melanostomus , Deepwater Sculpin Myoxocephalus thompsonii , and Rainbow Smelt Osmerus mordax , comprised 5%, 4%, 3%, and 1% of the catch, respectively. The Alewife biomass index decreased from 2024 to 2025 (83 to 78 kg·ha -1 ) however due to an abundant 2024 Alewife year class the density index increased from 3,727 to 9,182 fish per ha -1 . The Age-1 biomass (2024 year class) was 27.5 kg·ha -1 , which was the greatest value estimated in the modern time series (since 1997). The abundance estimate for the 2024 Alewife year class (13.8 billion) was more than three times the number of all other Alewife combined (3.6 billion). Adult Alewife abundance decreased in 2025 which was consistent with predictions from 2024. Those predictive models suggested that adult Alewife biomass is likely to increase in 2026 and 2027, as the 2024 year class matures. Alewife condition declined in 2025, which was expected given the relatively high Alewife density. Acoustic-based prey fish densities were greater than previous years acoustic estimates especially at depths from 180 – 220 m (591 – 722 ft), however acoustic based densities continue to be substantially lower than trawl-based densities. The 2025 biomass index was similar to 2024 for Emerald Shiner Notropis atherinoides and Threespine Stickleback Gasterosteus aculeatus , but was lower for Rainbow Smelt, and higher for Cisco Coregonus artedi . Three purported Bloater Coregonus hoyi were caught in the 2025 survey. Analysis of archived tissue identified five Bloater captured in previous surveys which increased the total number caught in Lake Ontario bottom trawl surveys to n = 24, since restoration stocking began in 2012. Whole lake density estimates of Lake Whitefish Coregonus clupeaformis increased in 2025 relative to 2024. Those density increases were due to increased catches in Canadian waters, as density in U.S. waters has remained low. The density index for wild or naturally reproduced juvenile Lake Trout Salvelinus namaycush increased in 2025 relative to 2024, with the most frequent catches occurring in waters around the Niagara River.