Fish trawl data points to global warming

Three University of Rhode Island oceanographers conclude from four decades of fishery data that rising temperatures are the primary cause for significant turnover in Narragansett Bay and Rhode Island Sound fish populations.

Weekly trawl surveys from two stations indicate that over 46 years these communities witnessed a transformation from vertebrates to invertebrates and from bottom-feeding fish to species that make a living higher in the water column. Collie et al posit several hypotheses: the effects of fishing, the abundance of chlorophyll, temperature change, and other climate factors, including the North Atlantic Oscillation. “Mounting evidence has revealed that even small increases in water temperature over extended periods of time can directly influence the species composition, distribution, and abundances of surrounding fish communities,” the authors write, citing observations from the English and Bristol Channels and similar studies in the northwest Atlantic. Temperature increases in these previously studied areas are consistent with the changes documented off Rhode Island: increasing numbers of squids, pelagic fish, bottom dwelling invertebrates.

If temperatures and other environmental factors have indeed driven these changes, the authors predict that the population may begin to more closely resemble warmer water estuaries, such as Delaware Bay and Chesapeake Bay.

The research rests on the valuable set of trawl-survey data. All but one month of the 564 studied had more than two surveys, and in 91 percent of the months three or more surveys were recorded. Twenty-five species made up 96 percent of the total haul (1.8 million animals over the 46 years).

The authors looked carefully at the fishing record to limit the potential influence human commercial activity has had on the transformation. They found “no strong correlations” between the population and the fishing activity. Fishing activity was overwhelmed by the climate signal: Sea surface temperature increased by 2 degrees C since 1959; fish species caught today prefer to swim in waters about 2 degrees C warmer than the water was in 1959. “That seems to be direct evidence of global warming,” Jeremy Collie said. “It’s hard to explain any other way.

Collie, Jeremy S., Anthony D. Wood, and H. Perry Jeffries. “Long-term shifts in species composition of a coastal fish community.” Canadian Journal of Fisheries and Aquatic Sciences. 65: 1352-1365: 2008.