Relating cold tolerance to winterkill for spotted seatrout at its northern latitudinal limits

- Spotted seatrout were subjected to rapid and prolonged exposure to low temperatures.
- Equilibrium was not maintained at temperatures ≤ 4 °C for rapid exposure trials.
- Survival depends on exposure duration but was limited at temperatures ≤ 5 °C.
- Field estimates of natural mortality strongly related to metrics of winter severity.
- Population-level impact of winterkill was predicted for 8 of the last 22 winters.

In the absence of winter thermal refugia, acute cold stress can lead to episodic mass mortality (winterkill) in fishes. Populations existing near the northern extent of a species' latitudinal range, such as spotted seatrout, Cynoscion nebulosus (Cuvier, 1830), in North Carolina, USA, are particularly vulnerable to winterkill. Information on cold tolerance for spotted seatrout is incomplete, which limits understanding of a likely important source of natural mortality for this species. In this study, two laboratory experiments for controlled exposure of spotted seatrout to dynamic decreases in water temperature were conducted in order to determine cold tolerance as affected by either rapid or prolonged exposure to low-temperature extremes across upper- (10) and lower-estuarine (30) salinities. Under rapid exposure, spotted seatrout were unable to maintain equilibrium at temperatures ≤  4°C, with a small but measured mitigating effect of high salinity on the onset of observed physiological stress. No fish survived prolonged exposure (2 d) to 3 °C but spotted seatrout were tolerant of exposures to 5 °C for approximately 5 d, after which survival precipitously declined. Survival after 10-d exposure to 7 °C was high but not absolute. Salinity had no measured effect on mortality rates in the prolonged exposure trials. These empirical estimates of low-temperature thresholds, along with previously determined field estimates of instantaneous winter natural mortality rate (M), were used to develop models for predicting M. Historic daily water temperatures were used to estimate winter M of spotted seatrout from 1994 to 2015. Predictions of M suggest winterkill (≥ 50% population loss) in eight of the last 22 years; these years correspond to anecdotal and fishery-independent observations of winterkill events in North Carolina. The results of this study provide strong evidence for thermally-limited overwinter survival of spotted seatrout at its northern latitudinal limits, where winterkill events can have population-level impacts.