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August 10, 2011

Loaves and Lobsters

by Dr. Heather Deese and Catherine Schmitt

Maine lobsters—they just keep a’ coming. And no one can quite point to the reason why the lobster fishery over the last two decades has brought an unexplained bounty to the Maine coast.

For the 40 years between 1950 and 1990, the commercial lobster landings in the state hovered between 16 and 24 million pounds, according to the Maine Department of Marine Resources. Over the same time period, the amount of money paid directly to lobstermen for their product steadily increased from $6 million in 1950 to $60 million in 1990. Then, the pattern changed as Maine lobster landings skyrocketed, surpassing 40 million pounds by 1997, 60 million pounds by 2002, and a record-setting 90 million pounds in 2010. This dramatic increase may be related in part to increased effort—more people fishing more traps—but is more likely due to a boom in the number of lobsters crawling around on the ocean floor.

But why?

Scientists and fishermen offer a few possible explanations. One is the protection of the broodstock by fishermen’s practice of returning female breeders to the water with a v-notch in the tail and a maximum size limit. Another is that perhaps fishermen are artificially supporting the population, or “farming” lobster, with more traps and more bait in the water. Or the lobsters could be benefiting from fewer predators. Populations of Atlantic cod and other groundfish have plunged over the last 20 years, and adult groundfish feed on juvenile lobsters. Others point to changing water temperatures, currents or other unknown environmental factors that could be contributing to successful reproduction and “recruitment” of juvenile lobsters into the fishery.

A regional perspective lends support to the latter explanation. While the Maine lobster population has boomed, lobster landings to the south of Cape Cod have plummeted. According to Dr. Rick Wahle, a marine biologist at the University of Maine, the numbers of post-larval and juvenile lobsters on the seafloor reflect a similar contrast between the northern Gulf of Maine and Rhode Island (see image). In 2005, the density of post-larval lobsters that had “settled” onto suitable seafloor habitat in New Brunswick increased, while the settlement numbers in Rhode Island declined since monitoring started in the late 1980s. Wahle and his fellow researchers think a number of environmental factors in southern New England, including warm temperatures, low oxygen and freshwater run-off events that contributed to shell disease, triggered a mass movement of adult lobsters into deeper, cooler waters, decreasing the amount of larvae retained locally (see “Compicated and Scary” July, 2010).

The lobsters off Maine’s coast have thus far avoided shell disease, and some combination of the factors above appear to be working together to keep the breeders breeding and the settlers settling, supporting incredibly strong population growth.

This population growth has not been evenly distributed along the coast, however. Comparison of the settlement index in Midcoast Maine and New Brunswick in the accompanying graph shows a consistent trend: there are many more baby lobsters Downeast now than there are in western and southern Maine. This difference is reflected in landings data—Hancock County accounted for roughly 30 percent of the entire landings for the state last year, with more than 31 million pounds.

Dr. Huijie Xue, a physical oceanographer at the University of Maine and Dr. Lew Incze, a biological oceanographer now at the National Science Foundation, studied the potential role of offshore currents in distributing adult and larval lobster by “seeding” virtual larval lobster into a computer model of Gulf of Maine circulation.

Their results showed that the broad-scale offshore Gulf of Maine Coastal Current likely carries lobster larvae in a counter-clockwise pattern, making regional fisheries dependent on adult broodstock living to the east.

Other research has focused on lobster growth rates along the Maine coast. A recent analysis by Dr. Wahle, comparing the number and size distribution of lobsters caught in Department of Marine Resources’s trawl survey with the settlement index, found that not only do Maine lobsters grow relatively slowly, compared to populations further south, but their growth rates are variable, so that lobsters of a specific size can span a range of ages. Newly “recruited” lobsters, those just over the legal size limit sold as “chix” might be anywhere from five to nine years old, with most in Midcoast Maine around seven years old and most farther east averaging eight years.

Environmental conditions appear to be favorable, for now, with settlement and population numbers seeming to shift Downeast. No one knows yet what happens when groundfish stocks rebound, or what happens if water quality degrades. None of this really explains why Maine’s lobster harvest is at a record high, what someday might be referred to as the lobster miracle of the first decade of the 21st century.

This article is made possible in part by funds from Maine Sea Grant.

Heather Deese holds a doctorate in oceanography and is the Island Institute’s vice-president of programs. Catherine Schmitt is communications coordinator for Maine Sea Grant.

click to enlargeLobster settlement index annual survey.
Lobster settlement index annual survey. Figure: Rick Wahle, University of Maine

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