Global Warming’s Impact on Lobsters Has Scientists in a Stew

ELLSWORTH — Despite the glaze of ice coating much of Downeast Maine and the skepticism of some political leaders, global warming is accepted as an indisputable fact by virtually all responsible scientific opinion.

In a report published in the journal Science recently, a group of scientists, including University of Maine oceanography professor Robert Steneck, offered some dire predictions about the effect of global warming on the world’s coral reefs. According to Steneck and his colleagues, as the Earth’s atmosphere grows warmer and the concentration of carbon dioxide (CO2) continues its increase, global sea temperatures will also rise, and the oceans will become more acidic.

One consequence of that increase is that marine organisms such as coral are finding less of the calcium carbonate that they need to continue to grow in the sea. If the reefs can’t grow they will die, and the scientists predict that there could be a mass die off of coral reefs within the next 50 years. The destruction of the coral reefs will have serious impacts on many of the world’s fisheries, tourism (and the people dependent on those resources and industries) and on coastal protection in many parts of the world.

Less clear is what impact global warming and the resulting ocean acidification will have closer to home.

Like coral reefs, lobsters (and bivalves such as clams) depend on drawing calcium carbonate from the water to build their shells. In theory, if the waters of the northeast Atlantic ocean grow more acidic, crustaceans and bivalves will have to work harder to build their shells and will have less energy available to forage for food, but scientists are unsure of how lobsters in the Gulf of Maine would be affected.

Scientists have known about ocean acidity for a few years, according to Lewis Incze, a senior scientist at the University of Southern Maine Bioscience Research Institute, although the issue “caught quite a few scientists off guard,” he said. Most of the study of acidification, though, has focused on ecosystems in tropical and semitropical waters.

“There are no known predictions regarding the change in ocean acidity at this latitude,” Incze said.

Lobsters precipitate calcium out of the ocean through their blood system. To predict what impact acidification might have on the lobster population over the next 50 to 100 years, Incze said, it is necessary to have “some idea of how much ocean acidity will increase” during that period.

“It’s a complicated process,” Incze said.

Equally difficult to predict is the impact of rising sea temperatures on the lobster population of the Gulf of Maine.

“There are lots of issues to be raised, but we don’t know much about them,” Incze said.

Over the past 20 to 25 years, scientists have seen a “pretty steady increase” in water temperatures in the Gulf of Maine, according to Department of Marine Resources (DMR) biologist Carl Wilson. Over the same period, “the lobster population has seen an exponential increase favorable to Maine,” he said.

As global warming increases water temperatures, though, that could change. According to Incze, many factors could have at least as great an effect on lobsters as a decrease in the ocean’s pH.

Warmer water “could have just as large an impact on lobsters by changing the seasonal blooms of phytoplankton and zooplankton,” Incze said. Those microscopic organisms serve as food for larval lobsters.

In the Gulf of Maine, there are usually two blooms of plankton yearly. One is in the spring, the other in fall. Warmer water could mean the spring bloom came earlier and the fall bloom later, so there “would be less in summer when the lobster larvae are around,” Incze said.

How would that change affect the lobster resource? That’s hard to say.

“We can make predictions about fundamental changes, but not about the linked effects,” Incze said. “We don’t know much about the linkages.”

While warmer water temperatures have coincided with increased lobster landings in Maine, things have been different in southern New England. The lobster population “crashed” in the waters south of Cape Cod about 10 years ago and they haven’t returned.

“People are seriously debating range contraction,” Wilson said. “What is the chance that lobsters will even come back?”

Shell disease is one problem that has had a serious impact on the southern New England lobster fishery, and has recently appeared in southern Maine. Caused by bacteria that invade through pores in the outer layer of the shell, the disease doesn’t affect the lobster meat, but makes the shell too unsightly for sale. More seriously, by preventing molting it can kill the lobster.

As yet, no one knows what causes shell disease, but some scientists believe its prevalence may be related to warmer water. Wilson won’t make that connection directly but, he said, “some pathogens become increasingly active and virulent as the temperature increases.”

Incze is also keeping an open mind on the relationship between increased water temperatures and shell disease.

“I’m not sure the temperature is gonna change in a way to affect shell disease, but nature has her way of surprising us,” he said.

Whether an increase in water temperature will benefit the lobster population or decimate it is literally a matter of degrees, according to Wilson. An increase of a few degrees can produce a “dramatic increase” in lobster growth and development. It can also result in an increase in the amount of microorganisms in the water on which larval lobsters feed.

Warmer water can, however, be too much of a good thing. Water temperatures above 21 degrees Centigrade (about 70 degrees Fahrenheit) “really stress lobsters out,” Wilson said. In southern New England, the water can be that warm for two or three months a year, even on the sea floor where lobsters live.

Those temperatures are extremely uncommon off the Maine coast. Water temperatures in the Gulf of Maine would have to increase “dramatically,” Wilson said, to have the impact on lobsters experienced in southern New England.

“We’re a long way off,” Wilson said. “If we see 21 degrees in the Gulf of Maine three months a year, you’re gonna see Miami Beach in Ogunquit.”

Stephen Rappaport, The Ellsworth American, 27 December 2007. Article.

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