What physical conditions reduce Bloater embryo survival and development?

Nicole Berry (USGS, nberry@usgs.gov), Alex Gatch (USGS), Brian O'Malley (USGS), John Sweka (USFWS), Marc Chalupnicki (USGS), Gregg Mackey (USGS), Adam Fridman (USGS), Jason Haag (USGS), Christopher Osborne (USFWS)

Conservation and restoration of Cisco (Coregonus artedi) and Bloater (C. hoyi) is ongoing across the Great Lakes (Bunnell et al. 2023). In Lake Ontario over 1,000,000 Bloater have been released from 2012 – 2020, with modest bottom trawl recaptures (n=24) and no evidence of wild reproduction (Weidel et al. 2022, Dixon et al. 2025, Weidel et al. 2025). Environmental conditions, specifically those at spawning habitats, may be contributing to the apparent lack in wild recruitment of Bloater in Lake Ontario. Little is known about Bloater spawning habitats, however, closely related coregonine species in Lake Ontario seem to be negatively affected by current spawning ground substrates. For example, Lake Ontario expert solicitation identified degraded spawning habitat, from increased sedimentation and biofouling (i.e., dreissenid mussel hash), as a potential driver of the recruitment bottleneck for the closely related coregonine species, Cisco and Lake Whitefish (C. clupeaformis) (Brown et al. 2024). Direct evidence of habitats effect on coregonine early life survival was also documented by Weidel et al. (2023) who found lower Cisco embryonic survival in degraded rocky substrates compared to newly constructed reefs in Chaumont Bay, Lake Ontario. Another GLRI Coregonine RFP funded project (“Chemical composition of incubation substrates and their effect on survival of Cisco embryos” Gatch et al. in progress) conducted in a laboratory found lower Cisco embryo survivorship in degraded substrate (mussel hash and silt) treatments. We hypothesize that degraded substrate type may reduce the survivability of Bloater embryos in Lake Ontario, as has been observed for other coregonines. Early 20th century accounts of adult Bloater caught throughout the protracted spawning season suggests that Bloater utilize deepwater (>55 m) habitats with no preference for substrate (Goodyear et al. 1982, Koelz 1929). However, these accounts do not observe actual egg deposition nor embryo survivorship across a variety of substrate types, and in Lake Ontario, no description of Bloater spawning habitats exist (Stone 1947). Deepwater spawning habitats are likely to contain rock, silt, and mussel hash substrates (Elgin et al. 2022). In 2025, excess Bloater embryos were opportunistically used for a pilot study incubating the eyed embryos in four different substrate types and found the lowest survivorship in the silt and mussel hash treatments (Fig. 1). Given the consistently low hatches observed so far, these experiments highlight the need to understand not only the historical spawning substrate, but more importantly, the quality of the available spawning substrate for Bloater and other native fish species (e.g., Cisco, Lake Whitefish, and Lake Trout, Salvelinus namaycush) embryos to incubate. There are multiple routes in which substrate may influence embryo survival and development (ESD). All substrates have a direct physical interaction with developing embryos that may be a benefit (i.e., protection from visual predation) or detriment (i.e., physically covering and reducing the surface area of the chorion for gas exchange) to ESD. In addition to physical contact with the egg chorion, both silt and mussel hash can change the biotic and abiotic micro-environment around the egg by altering the microbial community or water chemistry. Mussels can alter the microbial community, including increases in benthic algae and sediment associated bacteria (Higgins and Vander Zanden 2010), which may directly alter the integrity of the chorion (i.e., a fungal infection) or indirectly reduce ESD by altering the water quality (e.g., pH, dissolved oxygen; Philippot et al. 2024). The efficient filtering of dreissenid mussel leads to bioaccumulation of trace amounts of heavy metals (e.g., Immel et al. 2016, Camusso et al. 2001) and micronutrients (e.g., calcium) from the water column that if released, could reduce ESD (Bourrachot et al. 2008, Ketola et al. 1988). To date, the extent by which any of these mechanisms impact Bloater embryo survival or development remains unknown.             We aim to quantify the suitability of contemporary spawning substrates for Bloater embryos and begin understanding the underlying mechanisms by which less suitable substrates impact ESD. Quantifying substrate suitability for Bloater ESD will further our understanding of early life history impediments to the successful restoration of these fish and further emphasize ongoing needs to classify the deepwater substrates of Lake Ontario where these fish are most likely to spawn. Additionally, identifying specific mechanisms leading to reduced ESD may lead to more targeted and innovative habitat remediation efforts for Lake Ontario Bloater restoration.