Is handling and transport stress limiting post-stocking survival of yearling bloater C. hoyi in Lake Ontario?

Alex Gatch (USGS, agatch@usgs.gov), Brian Weidel (USGS), Marc Calupnicki (USGS), Gregg Mackey (USGS), Steve Davis (USFWS), Meredith Bartron (USFWS), John Sweka (USFWS), Owen Gorman (USGS)

Bloater (C. hoyi) have been stocked in Lake Ontario for 11 years with limited success (15 total recaptures; Weidel et al., 2022). Short term (1-12 days) post-stocking survival has been estimated at 42% with 22% of the mortality occurring in the first hour post-release (Klinard et al., 2020). The cause of mass mortality in the hours following release are not well understood. Hypotheses related to stocking depth, compression barotrauma, and post-stocking predation have been tested but thus far, it’s unclear if these mechanisms are responsible for the majority of bloater death (Klinard et al., 2021, 2020). In April 2023, we conducted a pilot study to determine if environmental conditioning via soft release techniques (i.e., net pens) could increase bloater survival in Lake Ontario. Hatchery settings are starkly different from lake environments and soft release techniques have had some success in increasing post-release survival of some fish species (Brown and Day, 2002). In the April pilot study, we set two floating experimental net pens (cylindrical: ~2.7m diameter X ~10m length) stocked with age-1 bloater in Lake Ontario where release depth was controlled, and predators were absent. All handling and transport methods were kept standardized to previous USGS stockings of bloater in Lake Ontario with exception to release method. Typically, bloater stocked by the USGS are netted from raceways onto transport tank trucks (~9°C, 0.5% salt solution), transferred from the tank trucks via pipe to the USGS R/V Kaho on shore, and then transported offshore where they are released from a pipe below the water surface. One experimental net pen received the standard R/V Kaho release, the other net pen received bloater directly from the tank truck transported offshore on a barge, which eliminated the need to move fish from the tank truck to the R/V Kaho (i.e., an additional handling event). One day after stocking, mortality within each net pen was substantial (~50% via ROV photography and physical counting; Fig.1). We hypothesized that stress directly related to transport and handling was responsible for low post-stocking survival one day after stocking bloater into net pens. Our hypothesis was supported after communications with Region-3 USFWS coregonine experts who have experienced success raising and transporting cisco (C. artedi) for Lake Huron release. Age-1 coregonines raised in hatchery settings seem to be more sensitive than other salmonids to handling techniques (e.g., fish pumps vs netting; Roger Gordon R3 USFWS hatchery manager, pers. comm.) and changing water temperatures (Steve Davis, R5 USFWS, pers. comm). Stress that occurs while handling and transporting fish can be quantified by measuring hormone concentrations (e.g., cortisol), which concentrates in the blood plasma of fish during stressful events (Carmichael et al., 1984; Madaro et al., 2023; Sadoula and Geffroy, 2019). We are proposing to directly measure mortality and the stress (cortisol concentration) caused by handling and transport on the day of annual bloater stocking in Lake Ontario in spring 2024. Additionally, to identify factors that limit transport stress and increase post-stocking survival, we will experimentally test different methods of handling (fish pump vs direct netting) and transport (water temperature) in a controlled setting and monitor total bloater mortality and cortisol concentration before, during, and after experimental transport. Our study will provide hatcheries with information necessary to decrease transport and handling stress during stocking and potentially increase post-stocking survival of bloater. Because reintroduction of bloater is a management objective in Lake Ontario and current hatchery capacity is limited, understanding transport stress may increase survival of the stocked bloaters and improve their contribution to restoration goals.