Evaluating Bloater (Coregonus hoyi) natural reproduction in Lake Ontario

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

Evaluation of fisheries restoration actions such as the reestablishment of coregonine populations requires a life stage approach to evaluate program success and improve understanding on potential recruitment bottlenecks. Prior to their extirpation, Lake Ontario Bloater (Coregonus hoyi) were last detected in 1983, and earlier in the 1960s, in offshore gillnet and bottom trawl surveys that mainly targeted juvenile and adult life stages (Wells 1969; Weidel et al. 2022). More recently, bottom trawl surveys have recaptured 15 Bloater since restoration stocking began in 2012 (Weidel et al. 2022; O’Malley et al. 2023; Lake Ontario Prey Fish working group, unpublished data). Hatchery applied marks and genetic analyses confirmed these recent Bloater captures were products of the stocking program. Capture locations from trawl tows and acoustic telemetry data suggest that stocked Bloater have dispersed throughout the main lake, although most recaptures have occurred in U.S. waters despite similar trawl effort in Canadian waters (Weidel et al. 2022). While bottom trawl surveys have provided some indication of movement and post-stocking survival, they have not yet provided any indication that natural reproduction is occurring. To our knowledge, Bloater larvae have never been sampled in Lake Ontario nor have any surveys targeting eggs or larvae ever been implemented to assess Bloater natural reproduction. In contrast, Lake Huron has been undertaking several years of larval sampling to assess whether natural reproduction has started among reintroduced Cisco (C. artedi) populations in Saginaw Bay (Lake Huron Technical Committee 2023). Larval coregonine surveys in Lake Ontario, to date, have focused on Cisco, Lake Whitefish (C. clupeaformis), or both (Hart 1931; McKenna et al. 2020; Brown et al. 2022, 2023). Results from these surveys suggest that contemporary populations of Cisco and Lake Whitefish exhibit very similar early life histories with high spatiotemporal overlap in nearshore habitat (Brown et al. 2023). In contrast, knowledge of Bloater early life history from the upper Great Lakes indicates a much different strategy. Bloater spawning is thought to be protracted and occurs from February to March at offshore depths (Dryer and Beil 1968; Wells 1966; Goodyear et al. 1982; Eshenroder et al. 2016). Bloater larvae appear in May-July and are more abundant in offshore pelagic habitat; hatching appears to peak around mid-June (Wells 1966; Rice et al. 1987; Bunnell et al. unpublished data). In Lake Ontario, Bloater stocking has been ongoing for more than a decade, therefore it is likely that surviving adults may now be sexually mature capable of natural reproduction. Monitoring populations undergoing restoration at different life stages allows for identifying potential recruitment bottlenecks and impediments to successful restoration that might be occurring within or between life stages (Zimmerman and Krueger 2009). Furthermore, assessments that target early life stages allows us to evaluate a key goal in coregonine reestablishment, that is, to establish natural reproduction. We propose a Lake Ontario-specific survey to evaluate whether Bloater natural reproduction is occurring in Lake Ontario by focusing on the detection of larvae. Our work complements existing long term annual bottom trawl surveys more effective at assessing older life stages. We applied larval sampling knowledge of Bloater from the upper Great Lakes as well as results from our bottom trawl surveys to inform our study design; thus our work has implications for not only Lake Ontario restoration, but also cross-lake comparisons of coregonine early life history and understanding contemporary distributions.