Bottom trawl assessment of Lake Ontario prey fishes, 2019

Multi-agency, collaborative Lake Ontario bottom trawl surveys provide information for decision making related to Fish Community Objectives including predator-prey balance and understanding prey fish community diversity. In 2019, bottom trawl surveys in April (n = 252 tows) and October (n = 160 tows) sampled main lake and embayments at depths from 5–226 m. Combined, the surveys captured 283,383 fish from 39 species. Alewife were 67% of the total catch by number while round goby, deepwater sculpin, and rainbow smelt comprised 13, 10, and 4% of the catch, respectively. In 2019, the lake-wide adult alewife biomass index declined from 2018 and age-1 biomass, a measure of reproductive success the previous year, was low. Year-class catch curve models identified years where estimates from surveys conducted only in U.S. waters were biased, potentially due to a greater portion of the alewife population inhabiting unsampled Canadian waters. Accounting for spatial survey bias, these model estimates indicated the 2019 adult alewife biomass was the lowest value in the 42-year time series. Models also identified the extent to which age-1 alewife biomass was historically underestimated, however lake-wide results from 2016-2019 appear less biased. If below-average year-class estimates from 2017 and 2018 are accurate, adult alewife biomass will continue to decline in 2020. Abundance indices for other pelagic prey fishes such as rainbow smelt, threespine stickleback, emerald shiner, and cisco were low and similar to 2018 values. Pelagic prey fish diversity is low because a single species, alewife, dominates the community. Deepwater sculpin and round goby were the most abundant demersal (bottom-oriented) prey fishes in 2019. Despite declines in slimy sculpin and other nearshore prey fishes, demersal prey fish community diversity has increased as deepwater sculpin and round goby comprise more even portions of the community. New experimental trawl sites in embayment habitats generally captured more species, a higher proportion of native species, and higher densities relative to main lake habitats. In 2019, a western tubenose goby (Proterorhinus semilunaris) was captured for the first time in the trawl surveys.