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Increased water temperature may prevent winter flounder from rebounding in east coast estuaries

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NARRAGANSETT, R.I. -- February 17, 2004 -- Increased levels of water temperature can have critical effects on predator-prey interactions in the marine environment. Increased water temperature, for example, could be beneficial to a predator if the primary effect were to accelerate its level of metabolism, and thus enhance foraging activity. On the other hand, warm temperatures could also enhance the metabolism of the prey, increasing its activity, mobility, and ability to escape from predators.

In a recent issue of the Marine Ecology Progress Series, an article by URI Graduate School of Oceanography fisheries biologists David L. Taylor and Jeremy S. Collie describes the effect of temperature on the feeding behavior of sand shrimp preying on juvenile winter flounder. The study was funded by the National Oceanic and Atmospheric Administration’s Cooperative Marine Education and Research program.

Predation by the sand shrimp has been implicated as one of the most significant sources of mortality for recently hatched metamorphosed winter flounder. Experiments conducted by Taylor and Collie found that shrimp consumption rates on post-settlement flounder significantly increased with increasing flounder density, irrespective of water temperature. At low flounder densities, however, significantly more flounder were consumed at warm temperatures than at cold temperatures (16 °C and 10 °C, respectively, or 61 °F and 50 °F). Model estimates and visual observations of shrimp foraging behavior suggest that the variable feeding rates at different temperatures are the result of cold temperatures decreasing predator activity at low flounder densities, and conversely, shrimp maintaining high predation rates at low flounder densities when exposed to warm temperatures. These findings indicate that shrimp are capable of driving flounder populations to local extinction during warm water conditions.

The population abundance of winter flounder has declined significantly in parts of its geographic range since 1979, and has yet to rebound from the last 24-year decline. The demise of winter flounder has been paralleled by a significant warming trend in many northwest Atlantic estuaries that are used by juvenile flounder as important nursery habitats.

"We believe the increase in water temperature to be a critical factor in determining the survival of juvenile winter flounder in northern-temperate estuaries," said Taylor. "However, the temperature itself is not a direct cause of mortality, but rather it is indirectly affecting mortality by altering the predation pressure in the early life stages of winter flounder."

Water temperature is a key factor that changes the dynamics of the predator-prey relationship. Results from this study suggest that increases in water temperature increase the predator-induced mortality of juvenile flounder and cause the destabilization of the predator-prey interaction with the sand shrimp. Thus, the recent warming trend experienced in northwest Atlantic estuaries may explain the failure of the winter flounder stocks to recover in these areas.

The URI Graduate School of Oceanography is one of the country's largest marine science education programs, and one of the world's foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as acid rain, harmful algal blooms, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Institute for Archaeological Oceanography, and the National Sea Grant Library.