THE ROLE OF CLIMATE CHANGE IN THE DECLINE OF NARRAGANSETT BAY WINTER FLOUNDER

Winter flounder (Pseudopleuronectes americanus) has historically supported productive commercial and recreational fisheries throughout its range in the northwest Atlantic. As a small flatfish that inhabits estuaries, bays, and the coastal zone, this species is vulnerable to shifting environmental conditions in many of the habitats critical to its life cycle. At the southern edge of the species range, the Southern New England/Mid-Atlantic Bight winter flounder stock entered a steep decline during the 1980s following a period of overfishing. Despite repeated reductions in harvest during the proceeding decades, the population has yet to recover. A decreasing trend in recruitment has led to speculation that increased mortality during the early life cycle is preventing this stock from rebuilding. While it is well known that environmental stressors, predation, competition, and anthropogenic disturbances all can impact the survival of juvenile flounder, it remains unclear which of these factors are most important in regulating population productivity. To answer this question for the Narragansett Bay, Rhode Island winter flounder subpopulation, part of the Southern New England/Mid-Atlantic Bight stock, this dissertation aims to model the winter flounder life cycle and its patterns of habitat use to better understand the population dynamics and future recovery potential of this climate-challenged species. The first chapter aimed to model the winter flounder early life cycle to uncover patterns of population regulation and identify trends in juvenile mortality. Using a structural equation model fit to abundance indices of 29 year classes progressing through seven life stages gathered from multiple scientific surveys in Rhode Island waters, it was determined that winter flounder year class strength does not appear to be fully determined until cohorts join the spawning population. However, an increasing trend in mortality was identified during the first summer of life that was linked to rising summer temperatures, hypoxia, and predation. This result supports the hypothesis that juvenile mortality linked to environmental conditions may be inhibiting population productivity. The second chapter used the structural equation model fit in Chapter 1 to project Narragansett Bay winter flounder abundance under potential future environmental and fishery conditions. The projection model was first calibrated using interstage mortality rates from previous research and environmental conditions observed during the 1970s immediately preceding rapid growth in this winter flounder subpopulation. Projections of future abundance were then made under “business as usual” and optimistic environmental conditions. In both cases, however, the results suggest winter flounder are unlikely to recover to historic high abundance levels. Even so, it was determined that population growth to a more moderate abundance level was possible under the right conditions. The third chapter employed spatiotemporal conditional autoregressive models to study patterns of Narragansett Bay winter flounder habitat use throughout the life cycle during the population decline. Two phases of habitat use were identified: 1) a preference for shallow nursery habitats in the upper estuary during the first 15 months of life, and 2) a growing preference for deeper habitats in the lower estuary beginning in the first year, lasting through sexual maturity and the onset of participation in seasonal migrations. Although the studied spatial distributions were highly variable and did not exhibit detectable trends over time as the subpopulation declined, two important patterns were identified. First, an increasing concentration of mature winter flounder in lower Narragansett Bay and coastal waters during spring was indicative of a potential shift in spawning migration timing that could impact the reproductive capacity of the population. Second, an apparent adherence to historic nursery habitats vulnerable to the effects of climate change and anthropogenic disturbance in the shallow upper Bay suggests that juvenile winter flounder will be increasingly exposed to unsuitable conditions for growth and survival in the future. Combined with the results of the first two chapters, this second pattern indicates that juvenile mortality may continue to increase and impact the capacity of this subpopulation to recover. The research in this dissertation aims to provide scientists and fisheries managers with an enhanced understanding of the role of climate change in the decline of winter flounder near the southern edge of their range. The methods and insights developed here are meant to not only inform management of winter flounder, but contribute knowledge to the progression toward ecosystem-based management of climate-challenged fisheries in a rapidly warming ocean