Evaluating Bloater (Coregonus hoyi) natural reproduction in Lake Ontario

Brian O'Malley (USGS, bomalley@usgs.gov), Krystal Dixon (USGS), Scott Stahl (USGS), Brian Weidel (USGS), Alex Gatch (USGS), Dimitry Gorsky (USFWS), Collin Farrell (USFWS), Nicholas Sard (SUNY-Oswego),

Our goal was to evaluate if Bloater (Coregonus hoyi) natural reproduction occurs in Lake Ontario by applying survey methods and timing consistent with approaches used in the upper Great Lakes. Bloater have been stocked annually in Lake Ontario since 2012 and have been captured in recent lake-wide bottom trawl surveys. We hypothesized that the presence of Bloater larvae would be indicative of natural reproduction because stocking has consisted of juvenile to adult life stages. However, prior to the start of our survey in May 2024, the U.S. Fish and Wildlife Service had a surplus of larval-stage Bloater in their hatchery that were released in Lake Ontario near Sodus Point, NY, which had the potential to impact our study goal. To mitigate this effect, we took a subsample of larvae at the time of stocking and measured length frequency because we assumed any naturally produced Bloater larvae in the lake would be smaller in length than hatchery-reared fish at the time of collection. Additionally, the stocking of Bloater larvae created an opportunity to evaluate our survey methods by sampling immediately after stocking. However, despite sampling in the same area where Bloater larvae were stocked within 1-3 days of release, we were unable to detect any of the stocked larvae (N=23 tows). From May 29 to July 29 in 2024, we completed an additional 266 larval tows from various ports across Lake Ontario using both small and large research vessels at sites that ranged in bottom depth from 4 to 226 m (Figure 1). Tows were completed using a variety of gear (ichthyoplankton nets, neuston nets, and Tucker trawl). Ichthyoplankton and neuston nets were fished at the surface. We used the Tucker trawl at some offshore sites to fish three different sub-surface strata (mid-epilimnion, metalimnion, and upper hypolimnion). In the lab, larval fish were identified using standard keys, imaged, and measured for total length. Because coregonine larvae are difficult to separate by species based on visual characteristics, we used genotyping-in-thousands by sequencing (GT-seq) to confirm our coregonine identifications. A total of 866 larvae were collected. Larval catch was 96.0% Alewife (Alosa pseudoharengus), followed by 2.2% Deepwater Sculpin (Myoxocephalus thompsonii), and 1.8% other species. One Cisco (Coregonus artedi) larva was collected and confirmed with GT-seq. Survey data was published through a public data release (Dixon et al. 2025). No larval Bloater were captured despite covering a range of sites with a considerable amount of effort during months when peak larval Bloater densities were expected to occur based on findings from Lake Michigan.