Lake Ontario April prey fish survey results and Alewife assessment, 2024

Abstract

The Lake Ontario April bottom trawl survey assesses pelagic prey fish populations, in particular Alewife Alosa pseudoharengus , which are the primary prey supporting the lake’s sport fish populations. The 2024 survey included 234 trawls in the main lake and embayments and sampled depths from 3.9 to 245 m (13 – 809 ft). The survey captured 441,942 fish from 28 species with a total weight of 10,519 kg (23,142 lbs.). Alewife were 89% of the total catch by number while Deepwater Sculpin Myoxocephalus thompsonii , Round Goby Neogobius melanostomus , and Rainbow Smelt Osmerus mordax , comprised 4%, 3%, and 2% of the catch respectively. The estimated Alewife biomass increases slightly from 2023 to 2024 (83.9 to 84.2 kg·ha-1) and was the largest biomass value since whole lake sampling began in 2016. Adult Alewife abundance increased in 2024 as predicted in 2023, and most of the total Alewife biomass was comprised of adult fish (97%), predominantly from the 2020 and 2022 year classes. In contrast, Age-1 Alewife biomass (2.2 kg·ha-1) was the lowest estimated since whole lake sampling began in 2016 (previous range: 2.7 – 26.7 kg·ha-1), indicating reproductive success was poor in 2023. Adult Alewife biomass is predicted to remain relatively high but decline slightly in 2025 and 2026, due to the smaller year classes produced in 2021 and 2023. Alewife condition as measured by the weight of a standard length fish (165 mm; ~6.5 inches), was 32.8 g, which was within of the range of previously observed values (28.0 – 35.9 g, 1997 – 2023). Acoustic-based prey fish densities, in the water above the bottom trawl, were similar to observations from 2021 – 2023 and were orders of magnitude lower than bottom trawl densities. These acoustic results support the seasonal timing of the April survey, when the majority of Alewife and other pelagic prey fishes are near the lake bottom and susceptible to capture with bottom trawls. The trawl survey also provides information on the status of other pelagic prey fishes and native fish restorations. In 2024, biomass indices for Rainbow Smelt, Emerald Shiner Notropis atherinoides , and Threespine Stickleback Gasterosteus aculeatus , were similar to 2023 values while the index for Cisco Coregonus artedi declined. The density index for naturally reproduced, juvenile Lake Trout Salvelinus namaycush declined relative to 2023. Density estimates of Lake Whitefish Coregonus clupeaformis continue to be orders of magnitude lower in U.S. waters relative to Canadian waters. A single purported Bloater Coregonus hoyi (total length = 148 mm, sampling depth = 105 m) was captured near Rochester, NY during the 2024 survey. This is the eighth Bloater recaptured during this survey since restoration stocking began in 2012.