Environmental Protection Agency Narragansett Bay Commission University of Rhode Island Office of Marine Programs

Estuarine Science

Global Environmental Changes
Winter Flounder Case Study

Abundance of winter flounder determined by the Rhode Island Fish and Wildlife service (RIFW) and others in the Niantic River and at Fox Island. (Preparing for a Climate Change, A report of the New England Regional Assessment Group for the US Global Change Research Program.

The following case study looks at winter flounder populations in Narragansett Bay and their relationship with changing climate. The information here is adapted from a case study by Dr. Henry Walker and published in Preparing for a Changing Climate: The Potential Consequences of Climate Variability and Change, New England Regional Overview, 2001, U.S. Global Change Research Program, University of New Hampshire. The entire report can be viewed on-line at http://www.necci.sr.unh.edu/.

Between 1960 and 1990, the winter sea water temperature in Rhode Island's Narragansett Bay warmed by almost 3 °C (5.4 °F). This gave scientists an opportunity to document changes in a marine ecosystem related to a significant temperature shift. In Narragansett Bay, warmer winters are correlated with smaller winter-spring phytoplankton blooms. This observation has been experimentally reproduced by conducting a cooling experiment in a marine mesocosm during a warm winter.

During the past 25 years winter flounder abundances in southern New England have been in decline. One hypothesis is that warmer sea water could result in more of the winter marine phytoplankton being consumed in the water column by pelagic organisms, with reduction in the amount of food available to benthic (bottom dwelling) species such as flounder. Herring stocks (a pelagic food chain member), which feed in the water column have been on the increase. Another hypothesis is that temperature increases could also affect predation and survival of winter flounder during critical early life stages.

The abundance of winter flounder has been independently monitored and documented by Rhode Island Fish and Wildlife (RIFW) service, the University of Rhode Island (URI), and in the vicinity of Niantic River, Waterford Connecticut. There is some debate about how much of the observed decline is due to heavy fishing, and how much may be attributed to warmer winters in southern New England. The physiology and ecology of winter flounder provides some interesting clues. The winter flounder (Pseudopleuronectes americanus) is a former dominant member of the bottom dwelling fish community in southern New England. Most adult fish migrate into inshore waters in the late fall and early winter, and spawn in late winter and early spring when seawater temperatures are quite cold. To help accomplish this feat, winter flounder make use of unique antifreeze proteins found in a number of polar fish which allow them to survive in temperatures as low as -1.9 °C (28.6 °F). In comparison, most fish typically freeze at a temperature of -0.7 ° C (30.7 °F). Winter flounder spawning occurs at night in the upper portions of estuaries. Eggs are attached to the bottom. Hatching rate, larval development rate, and mortality rates due to predation are temperature dependent. It appears that a significant component of the decline in winter flounder abundance in southern New England is associated with a shift from a period with cold winters and seawater temperatures in Southern New England during the 1960s, into a period of relatively warmer winters during the following three decades. A series of warm winters such as recently experienced in southern New England is clearly unfavorable for winter flounder. According to the Intergovernmental Panel of Climate Change, in the Northern Hemisphere the 1900s has been the warmest century in the last 1000 years, and the 1990s have been the warmest decade in the past century. Warming has been greatest over Northern Hemisphere continents during the winter, and the same is true for the New England region. While the New England Region has warmed by an annual average of 0.4 °C (0.7 °F) over the past century (since 1895), due to its coastal location, Rhode Island's annual temperature has warmed by 1.3 °C (2.3 °F) over the same period of time. Warming during the winter months (December, January, February) in Rhode Island has increased a full 1.7 °C (3.0 ° F) since 1895. If this warming trend continues toward milder southern New England winters in the next few decades, even with reductions in fishing, winter flounder stocks could be slow to recover.

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