Survival of Brown Trout following atmospheric and snow exposure during winter

Abstract Objective Despite online discourse about handling practices for captured trout (family Salmonidae) during winter, relatively little is known about Brown Trout Salmo trutta survival following the atmospheric and snow exposure. Our objectives were to (1) compare mortality rates following atmospheric exposure during above freezing and below freezing conditions for three durations and (2) describe the effects of direct contact with snow on mortality of Brown Trout following atmospheric exposure. Methods Brown Trout mortality was quantified following capture and periods of atmospheric and snow exposure times during two air temperature trials (i.e., above freezing temperatures; below freezing). For each air temperature trial, 150 Brown Trout were caught using backpack electrofishing and randomly assigned to one of five treatment groups that were developed to simulate postcapture handling and release scenarios, with and without placing the fish on the snow. The five treatment groups for each air temperature trial were (1) ≤5 s of air exposure to simulate a capture and immediate release scenario, (2) 30 s of atmospheric exposure without placement on snow, (3) 30 s of atmospheric exposure while placed on snow, (4) 60 s of atmospheric exposure without placement on snow, and (5) 60 s of atmospheric exposure while placed on snow. Following the treatment, the fish were given a treatment-specific fin clip and released into one of six instream holding pens to monitor survival for 24 h. Each net-pen held five Brown Trout from each treatment group (n = 25 fish per ­net-pen). Results No mortality was observed within 24 h following any of the atmospheric and snow exposure treatments during either the above or below freezing trials. The water, snow, and air temperatures that were measured during the below-freezing trial were lower than those measured during the above-freezing trial, but this had no effect on the survival of Brown Trout. Conclusions Despite strong stakeholder opinions about the risk of Brown Trout mortality when they are briefly exposed to snow and freezing atmospheric temperatures, we found no evidence of short-term mortality (i.e., within 24 h) when these trout were subjected to winter conditions (minimum: −8.4°C at exposure) for up to 1 min in southwestern Wisconsin.