Defining bloater spawning habitat to inform potential impediments to Lake Ontario bloater reintroduction – Year 2

Alex Gatch (USGS, agatch@usgs.gov), John Janssen (UWM), Dale Hanson (USFWS), Nicholas Sard (SUNY Oswego), Zach Melnick (Inspired Planet), Yvonne Drebert (Inspired Planet), Ryan Lauzon (Chippewas of Nawash), Andrew Muir (GLFC), Nicole Berry (USGS), Brian O’Malley (USGS)

Bloater (Coregonus hoyi) reintroduction remains a priority for Lake Ontario management agencies. Since 2011, over 2 million bloater (age-0 through age-5) have been released into Lake Ontario without any evidence of natural reproduction by hatchery-origin individuals (Brown et al. 2022; Dixon et al. 2025). It is unknown if current ecological conditions in Lake Ontario are impeding natural reproduction or given the number of stocked individuals, detection of wild-origin bloater is improbable. To determine if ecological conditions, specifically those involved with reproduction (i.e., spawning habitats), are impeding natural reproduction, an understanding of bloater spawning requirements is necessary. Bloater spawning habitats are largely undescribed. Early 20th century accounts point to deepwater (>55m) with no substrate preference (Goodyear et al. 1982; Koelz 1929), however, these accounts are based on gillnet catches of adults during the protracted spawning period and not actual egg deposition. No description of bloater spawning sites or habitat exists for Lake Ontario (Stone 1947), therefore proxy ecosystems must be used to characterize bloater spawning habitat, which can then be compared to the habitats available in Lake Ontario. In 2024 and 2025, our research team was funded by the GLRI coregonine template (FY24 cycle) to identify and characterize bloater spawning habitat near the Lake Michigan Mid Lake Reef Complex (MLRC). Focus on the MLRC was guided by expert opinion provided by commercial fishers who utilized this area in the 1980s to catch adult bloater (pers. comm. Dan Anderson). Over the two-years, we accessed and successfully sampled the MLRC (~45 km offshore) eight times during January – March using the University of Wisconsin Milwaukee’s (UWM) icebreaking RV Neeskay and USGS small vessels. Winter sampling was limited by access to Lake Michigan due to freezing temperatures, ice, and winds and waves. Despite the limited access to the MLRC, we collected more than 50 hours of multibeam bathymetric mapping data, 30 hours of high definition ROV footage, and various biological samples (ponars and benthic prey fishes). Portions of our ROV footage were highlighted in the 2024 Great Lakes documentary “All Too Clear” (Inspired Planet, 2024). Multibeam sonar indicated that fish targets were concentrated between 50-55m at the edges of the MLRC. ROV sampling subsequently identified these sonar targets as lake trout (Salvelinus namaycush) and lake whitefish (Coregonus clupeaformis). Although multibeam sonar identified some targets with decibel return values consistent with bloater, we did not collect ROV footage of bloater to ground truth sonar targets nor did ponar samples collect any bloater eggs on the MLRC. In March 2025, we refocused our efforts on an area 15 km south of the MLRC where we observed sonar targets consistent with bloater. We referred to this area as the “Southern Flats” because it was expansive and had limited ground relief. ROV footage illustrated the Southern Flats as comprised of silt/clay substrates with hard clay ridges (Lineback et al. 1979,1972, 1971) at 100-120m bottom depth. Round goby (Neogobius melanostomus), slimy sculpin (Cottus cognatus), and deepwater sculpin (Myoxocephalus thompsonii) were common but no bloater were observed with the ROV in the Southern Flats. Interestingly, using our ROV suction sampler, we collected benthic prey fish and identified and genetically confirmed two bloater eggs in the stomachs of slimy sculpin collected from the hard clay ridges. The detection of bloater embryos in the diets of a sedentary benthic prey fish (slimy sculpin; Gray et al. 2018) suggests bloater may be spawning at or near the Southern Flats. Contrarily, these structures may trap and accumulate widely dispersed and roaming bloater eggs. The extreme conditions in Jan – March coupled with the offshore distance of the MLRC limited active sampling of bloater in Lake Michigan. In year-2 of our research, we are proposing to pair passive sampling (deepwater egg traps and benthic prey fish traps) with active ROV sampling to maximize data collection. Our efforts will focus on the deeper sites within the Southern Flats where we collected bloater eggs in the diets of sculpin. Additionally, we are proposing to add a site in Lake Huron (Bruce Peninsula) that is protected from prevailing winds, has nearshore access to areas with known bloater distributions, and is targeted for bloater harvest by contemporary tribal fishers (Duncan et al. 2023). Incorporating passive sampling in Lake Michigan and adding a Lake Huron site that does not require the use of a large vessel, should increase the chances of identifying bloater spawning habitat characteristics. Our proposed cross-lake approach will also allow for bloater spawning habitats to be compared between lakes, giving researchers a better understanding of the potential variability of bloater spawning habitats.