Supporting cisco rearing science: evaluating the effects of rearing temperature and long-term OTC mark retention

Bo Bunnell (USGS, dbunnell@usgs.gov), Roger Gordan (USFWS), Reinna Blair (USFWS), Brian Layton (USFWS), Doris Mason (USFWS), Andrew Honsey (USGS), Kevin Keeler (USGS), Jose Bonilla-Gomez (USFWS)

Reintroduction of Cisco (Coregonus artedi) from the Les Cheneaux Island region into Saginaw Bay began in 2018, when 1.1 million fall fingerlings (mean size ~ 75 mm total length, TL) were stocked after being reared at Jordan River National Fish Hatchery (JRNFH). In 2019, spring fingerlings (mean size ~ 55 mm TL) were added to the reintroduction effort, such that spring and fall fingerlings were stocked each year. The motivation for stocking fingerlings rather than yearlings, and even stocking fingerlings as early as the spring was to mitigate potential negative effects of artificial rearing and to maximize the probability of natal homing to Saginaw Bay (R. Claramunt, Michigan DNR, personal communication). The number of spring-stocked fish lagged behind that of fall-stocked fish until 2021, and the number stocked per season has remained relatively equal through 2024. In the hatchery, oxytetracycline (OTC) is used to differentially mark spring- (one mark) and fall- (two marks) stocked fish. OTC is widely used to mark fish (Pedersen et al., 1991; Brown et al. 2002) by incorporating it into fish feed. The effectiveness of the unique marking was assessed by FWS Alpena FWCO (Bonilla-Gomez et al., in preparation). Ninety-three percent of fall stocked Ciscoes (N=30) were found to have identifiable and unique double marks, relative to single-marked, spring-stocked Ciscoes (N=30), suggesting effective discrimination at the time of stocking. It remains unknown, however, how well the second OTC mark is detectable at older ages (i.e., 3-4 years) when adults have been recaptured. Through 2024, nearly 7.5 million age-0 Ciscoes have been stocked and 67% have been fall fingerlings. By 2020, the Alpena FWCO had begun conducting annual fishery independent surveys to assess survival and/or return of hatchery-origin ciscoes to Saginaw Bay. By December of 2023, some promising results had emerged, with 65 hatchery-origin Ciscoes collected between 2021 and 2023 (Bonilla-Gomez et al. in preparation). Eighty-five percent of these hatchery-origin Ciscoes (N=55) were sampled as adults during the spawning season. Of these 55 returning adults, several biases were apparent relative to what was originally stocked. First, 89% (N=49) were determined to be spring-stocked fish (i.e., one OTC ring), even though they represented only 33% of the fish that were stocked. Second, 95% (N=52) were male, although we assume the sex ratio to be balanced at the time of stocking. Interestingly, the smaller sample of hatchery-origin fish that were recaptured during the non-spawning period (N=10) were also dominated by males (70%) and spring-stocked fish (70%). Many factors can influence survival of hatchery stocked fish, including conditions in the hatchery, conditions during transport to the lake, or survival after the fish are stocked. This project seeks to focus on whether conditions in the hatchery, especially those that differ between spring- versus fall-stocked fish, are influencing fitness. The main difference in rearing Ciscoes to be stocked in the spring vs. fall occurs during the embryo incubation stage: spring-stocked fish are incubated in the warmest temperatures: at 2-4 °C until eyed stage, at which point they are incubated at around 7-8 °C until hatching (see red in Fig. 1). An abnormally warm incubation after eye-up is necessary to achieve a size (i.e., ~45 mm TL) desired for accurate counting and stocking by mid-late May, before Saginaw Bay potentially becomes sub-optimal (e.g., hypoxic). Fall-stocked fish maintain more natural incubation temperatures (see gray in Fig. 1) for nearly the entire period; at the end of development, temperatures are increased to stimulate synchronized hatching. Research on the effects of embryo incubation at atypically warm temperatures generally shows a negative effect on their fitness (e.g., Galloway et al. 1999; Johnston et al., 2000; Martell et al. 2007; Macqueen et al., 2008). For coregonines, laboratory studies have documented similar patterns: abnormally warm water temperature during egg incubation affects embryonic development (Lim et al., 2017), larval survival (Stewart et al., 2022), and even growth and muscle mass achieved at later life stages (Steinbacher et al., 2017). This project arose from discussions with those with the most direct experience in incubating Cisco embryos (e.g., Roger Gordon and Reinna Blair from JRNFH; Doris Mason and Brian Layton from Allegheny National Fish Hatchery (ANFH)). They raised concerns about potential unknown effects of warmwater incubation on Ciscoes. Discussions also revealed differences in photoperiod regimes used by the two hatcheries that would be of interest to evaluate, given that rearing under excessive light conditions (i.e., all light or <2 hours of darkness) can lead to abnormal larval development for several species (review by Villamizar et al. 2011) but not all (Puvanendran and Brown 2002). Finally, there was strong desire to conduct further quality control on the fall-OTC mark of Ciscoes to confirm that the second mark remains clearly visible after more than 3 years. Discussions with ANFH staff revealed the relevancy of this project for Lake Erie Cisco reintroduction. May 2025 marked the first year of yearling Ciscoes being stocked in New York waters. These Ciscoes are of Lake Huron origin and were initially reared at JRNFH under the “spring-stocked”, abnormally warm rearing conditions.