Squid, Lobster Numbers Rise as Fish Fall Due to Warming

Anne Minard
for National Geographic News
June 30, 2008
Lobsters, crabs, squid, and other invertebrates are becoming more common while populations of bottom-feeding fish are plummeting, according to a long-term trawling study of Rhode Island's Narragansett Bay.

The study also shows that small, warm-water species have increased while larger, cool-water species have declined.

Scientists from the University of Rhode Island (URI) say rising sea temperatures linked to global warming is the primary cause of shifts in the abundance and types of species living in the bay and adjacent Rhode Island Sound (see map).

And "these types of changes are probably widespread," said lead author Jeremy Collie.

Ray Hilborn, a fisheries expert at the University of Washington in Seattle, said the results of the new study—and similar trends elsewhere—call for a retooling of fisheries management practices.

"These major changes in marine ecosystems are being recognized to be reasonably common, and the shift from groundfish to invertebrates such as crabs, prawns, and scallops has been seen in quite a few places," he said.

But so far, most fisheries management programs aren't keeping pace with the changes, Hilborn said.

"If the system has changed and stocks no longer can be maintained at the same population sizes—in effect replaced by other species—then we need to rethink what our targets are for these stocks."

Decades of Data

A founder of URI's Graduate School of Oceanography began the trawling surveys of the bay in 1959.

Several generations of researchers and students have kept up the work since then, and the survey now represents what may be the world's longest-running record of its kind.

"Many of the things you hear about the effects of warming are sometimes anecdotal, because people don't have the records of what was there before," study author Collie said. "We have the record."

The survey has tracked 130 species in the bay, although the new analysis focuses on the 25 most common ones.

The data show that most of the changes occurred slowly.

But an abrupt shift showed up in the early 1980s, when bottom-swimming species such as winter flounder and silver hake declined and shallow-water species such as butterfish and bluefish increased.

Microscopic plant-like organisms known as phytoplankton have also taken a nosedive in the deeper waters of the bay, resulting in a decline in the system's chlorophyll levels, the survey shows.

The study authors suggest that zooplankton—free-floating invertebrates that range in size from tiny protozoa to jellyfish—might be gobbling up phytoplankton in shallower waters.

Less of the tiny plants therefore reach deeper levels to sustain bottom-dwelling fish, they propose.

"It's a rerouting of [productivity] from the bottom to the top," Collie said.

Of the animals that still live at the bottom of the bay, a much higher proportion is made up of invertebrates, which Collie believes moved in to fill the void left by fish.

Although the shifts in species composition correlate with the winter North Atlantic Oscillation index—a natural cycle of climate variability—the authors say that global warming is the major driver.

The survey shows that sea-surface temperature in the area of the trawls has increased by 3.6 degrees Fahrenheit (2 degrees Celsius) since 1959.

The URI team presents its findings in the July issue of the Canadian Journal of Fisheries and Aquatic Sciences.

"More Calamari"

The results of the study agree with what fishers have already been seeing and what diners in the region have been tasting, the study authors say.

Fisheries have switched target catches, so "we are eating more calamari and less flounder," Collie said.

His team predicts species composition in Narragansett Bay will soon resemble estuaries to the south such as Delaware Bay and Chesapeake Bay.

But Nils Stolpe, communications manager for the Garden State Seafood Association in New Jersey, argues that people's seafood diets change for reasons apart from availability.

"The reason we're getting more calamari is because we're getting more sophisticated as seafood eaters," he said.

"Ten, fifteen years ago nobody ate salmon, because we weren't in tune with eating salmon. Now everyone's growing it, and we're a lot more familiar with it."

Stolpe hesitated to apply the study results out of Narragansett Bay to the oceans at large, but, he said, "by the same token, the fisheries are changing. We're seeing more warm-water species farther north than we were 10 years ago, 15 years ago.

"They're catching Atlanta croaker off of New Jersey now, which they hadn't done for 20, 30 years."

According to study author Collie, the results add to evidence suggesting that distributions of marine fish will migrate toward the Poles if they can.

"Obviously, fish in lakes can't migrate," he said. "But there are also subpopulations of marine fish that are adapted to particular spawning locations, such as winter flounder in Narragansett Bay, which may die out."

Still, Collie thinks the new research isn't signaling a death knell for ocean systems.

"The productivity of the ecosystem will continue," he said.

"The worst-case scenario is the whole functioning of the system is collapsing. We haven't seen that yet, but we're looking."

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