Impacts of Winter On Productivity of Yoy Red Drum: Field and Laboratory Evidence for Cohort Modification Toward the Northern Edge of the Range

Mortality during winter can impact the population dynamics of fishes at temperate latitudes.  The red drum (Sciaenops ocellatus) supports valuable coastal fisheries throughout its range in the southeastern US.  At the northern edge of the distribution, severe winters may cause considerable overwinter loss and size-selective mortality among juveniles.  Evidence of cohort restructuring from otolith microstructural analysis was combined with size-dependent winter survivorship patterns generated during controlled laboratory experiments to predict the impacts of variable winter severity on red drum productivity.  A range of hatch times from late summer through mid-fall as well as variable estuarine growth rates produced a broad range of red drum body sizes at the onset of the first winter.  Examination of spring survivors indicated positive selection for individuals that had been hatched earlier and achieved faster fall growth.  Laboratory results found age-0 red drum to be intolerant of even brief exposure to temperatures ≤3°C, with fish experiencing mortality after prolonged exposure to 5°C.  When exposed to varying levels of winter severity, variation in the frequency of cold front events was responsible for the largest deviation in survival.  Importantly, size-dependent mortality was only evident for fish exposed to mild and moderate winter severity conditions, with larger fish surviving longer.  The empirical evidence indicates that severe winters may cause high mortality independent of body size, while size-dependent year class restructuring may occur during milder winters.  These outcomes were then used to estimate the parameters of a matrix projection model used to explore the influence of variable and size-dependent survival during the first winter on red drum population growth rate near the northern edge of the range.  Model analysis revealed the potential for pre-winter size distributions coupled with variable winter severity to have a strong influence on fishery recruitment and overall population growth rates, indicating that timing of estuarine arrival and initial settlement habitat, along with winter conditions, have important implications for red drum productivity.