Global warming causing significant shift in composition of coastal fish communities

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Washington, June 28 : A new analysis has revealed that global warming has led to a long-term shift in composition of coastal fish communities.

This finding is a result of a detailed analysis of data from nearly 50 years of weekly fish-trawl surveys in Narragansett Bay and adjacent Rhode Island Sound.

According to Jeremy Collie, professor of oceanography at the University of Rhode Island's (URI) Graduate School of Oceanography, the fish community has shifted progressively from vertebrate species (fish) to invertebrates (lobsters, crabs and squid) and from benthic or demersal species, those that feed on the bottom, to pelagic species that feed higher in the water column.

In addition, smaller, warm-water species have increased while larger, cool-water species have declined.

"This is a pretty dramatic change, and it's a pattern that is being seen in other ecosystems, including offshore on Georges Bank and other continental shelf ecosystems, but we're in the relatively unique position of being able to document it," said Collie.

"These patterns are likely being seen in estuaries around the world, but nowhere else has similar data," he added.

The weekly trawl survey by URI scientists has recorded 130 species, though the analysis focused only on the top 25 species, which accounted for 96 percent of the total number of animals collected.

According to Collie, while most of the changes occurred slowly, an abrupt change appeared to take place in 1980 and 1981 when benthic species like winter flounder and silver hake declined and pelagic species including butterfish and bluefish increased.

"We think there has been a shift in the food web resulting in more of the productivity being consumed in the water column," Collie explained.

"Phytoplankton are increasingly being grazed by zooplankton, which are then eaten by planktivorous fish, rather than the phytoplankton sinking to the bottom and being consumed by bottom fish. It's a rerouting of that production from the bottom to the top," he added.

Overall, the survey analysis found huge changes in the abundance of some species.

Butterfish and bluefish, for instance, have increased in abundance by a factor of about 100 times while cunner has decreased by almost 1,000 times.

The analysis also found that while the total number of fish caught in each trawl increased over time, peaking in the 1990s, the size of those fish decreased.

According to Collie, climate is "the dominant signal."

Sea surface temperature in the area of the trawls has increased by 2 degrees Centigrade since 1959, and the preferred temperature of the fish caught in the trawls has also increased by 2 degrees C.

"That seems to be direct evidence of global warming. It's hard to explain any other way," said Collie.

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