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September 12, 2018
Contact: Shelley Dawicki

Fish Stock Rebuilding Success Influenced by Warming Ocean

The Southern New England/Mid-Atlantic stock of winter flounder has declined over the last 30 years despite reduced fishing pressure. Ocean warming can be an additional factor preventing its recovery to historical levels even if no fishing occurs. The findings were published in the September 2018 issue of the Canadian Journal of Fisheries and Aquatic Sciences.

“Trends in oceanographic conditions due to climate variability and change can impact demographic rates in fish stocks,” said Richard Bell, lead author of the study, who formerly was a research associate at the NOAA Northeast Fisheries Science Center's Narragansett Laboratory and now is with The Nature Conservancy North American Program. “Rebuilding plans developed assuming constant rates for demographic variables such as growth, reproduction, and mortality may not be realistic for stocks like winter flounder.”

The winter flounder is thicker and meatier than most Atlantic flounders, and has supported commercial fisheries as well as being a popular catch for anglers. It is a cold-water coastal fish that spawns in estuaries during the fall and winter when most other flounders head south or offshore. The eggs and larvae develop in the estuaries during the coldest time of the year.

Refuge from Predators

It is believed that the low temperature provides a refuge from predators, as studies have found increased predation in warmer waters. Other work has shown that there is also a relationship between temperature and the number of young winter flounder that survive to enter the population as one-year-olds important to building the stock and replacing fish that have died that year.

Bell and his NOAA Fisheries colleagues modified the stock assessment model used to understand the current winter flounder population, and to estimate its future growth. They included likely future temperatures as a factor when estimating stock size.

Projections: Rebuilding Targets a Challenge

The authors found that future projections showed that under low fishing pressure, stocks would rebuild. However, when projections include warming ocean temperature, meeting rebuilding targets would be challenging even under reduced or no fishing pressure.

“For species with strong environmental influences, the coupling of population models with climate model projections and fishing pressure can provide a greater understanding of different management decisions and possibly more realistic predictions of rebuilding expectations,” said Bell. “While studies like this highlighting the impacts of warming waters on productivity are important, the really important work is the next step, the development of robust management strategies that account for changes in productivity across all species.”

For this study, researchers tested three fishing scenarios: no fishing, moderate fishing representing a small fishery or incidental catch, and fishing at the expected long-term sustainable level if the stock were rebuilt to historic levels.

Five Estuaries in Northeast, Mid-Atlantic Studied

Historical weekly estuary water temperatures were compiled for five estuaries covering the range of this stock: Woods Hole, Mass.; Narragansett Bay, RI; Long Island Sound; Delaware Bay; and Chesapeake Bay. Forecasted temperatures were derived from global climate models downscaled to the region to produce annual estimates of winter temperatures in the selected estuaries, projected to 2050.

A recent NOAA Fisheries climate vulnerability assessment found that many species in the region are likely negatively affected by changing climate while others will be affected positively. NOAA Fisheries also developed a fisheries climate action plan for the Northeast, which includes incorporating climate change affects into stock assessments. This paper develops a method for quantifying climate effects in terms of rebuilding depleted fishery stocks, providing a more realistic picture of future stock status.