The effect of variable winter severity on size-dependent overwinter mortality caused by acute thermal stress in juvenile red drum (Sciaenops ocellatus)

Abstract

Abstract Mortality during winter can impact the population dynamics of fish at temperate latitudes. The red drum (Sciaenops ocellatus) supports valuable coastal fisheries throughout its range in the southeastern United States. At the northern edge of its distribution, severe winters may cause considerable overwinter loss and size-selective mortality among juveniles. We conducted a series of laboratory experiments to quantify overwinter survivorship of age 0 fish. To determine thermal tolerance, fish were exposed to various minima (1, 3, or 5°C) for up to 14 d. The effect of winter severity on survivorship was then evaluated by exposing fish to simulated cold-front events of varying frequency and duration. Body size was incorporated as a factor into each set of experiments. Age 0 red drum were intolerant of even brief exposure to temperatures ≤3°C and experienced mortality after prolonged exposure to 5°C. Higher frequency of simulated cold-front events impacted survivorship more than longer-event duration, and recovery time between events improved survivorship. Size-dependent mortality was only evident for fish exposed to mild and moderate winter severity conditions, with larger fish surviving longer. For juvenile red drum, severe winters may cause high mortality independent of body size, whereas size-dependent year-class restructuring may occur during milder winters.