Externally driven changes in the abundance of summer and winter flounder

Marine organisms that utilize nearshore environments for major components of their life histories are subject to both local-scale forcing such as water quality and estuarine degradation as well as large-scale forcing such as fishing and decadal-scale climate variability. Large-scale forcing has the potential to synchronize the dynamics of subpopulations, while local-level forcing can produce asynchronous subpopulation trends. Summer flounder (Paralichthys dentatus) and winter flounder (Pseudopleuronectes americanus) are important commercial and recreational flatfish along the east coast of North America which spend their first year of life in coastal habitats. We found that the two exhibited significant within-species coherence in commercial landings and fisheries-independent surveys across the northeast shelf of the United States, suggesting large-scale external drivers. In laboratory studies, temperature has been found to be an important factor regulating survival during the egg, larva and settlement phases of both species. We reconstructed a 40-year time-series of coastal water temperature for the major spawning and nursery areas to examine changes in the thermally available habitat. Estimates of winter flounder abundance were negatively correlated with the winter water temperature, but not with fishing mortality. Summer flounder abundance, by contrast, was negatively correlated with fishing mortality, but exhibited no link with temperature. In addition, time-varying stock-recruitment relationships indicated that stock productivity declined for winter flounder over time, while summer flounder productivity has varied without a trend. While both species declined in the 1980s and early 1990s due to heavy fishing pressure, the reduction in fishing over the last two decades has led to rebuilding of the summer flounder stock and an expansion of its age structure. Declining productivity due to warming estuarine conditions has kept the winter flounder stock at low levels despite low fishing pressure. The two stocks illustrate the importance of controlling fishing mortality in the management of natural marine resources while also accounting for changes in productivity due to climate variability and change.