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Commercial Fisheries News
Volume 35 Number 6
February 2008
Collector trays reveal tiny lobsters at new depths
WEST BOOTHBAY HARBOR, ME – Postlarval lobsters maintain a relatively secretive lifestyle, but a 2007 cooperative research project to test a new method for evaluating the deepwater settlement of young lobsters is shedding new light on lobster nursery activities at depths never seen before.
“Early indications are that our lobster settlement collector project is producing some very interesting results regarding the depth-wise patterns of settlement,” said Rick Wahle, senior research scientist at the Bigelow Laboratory for Ocean Sciences.
Through the project, researchers have been able to document settlement at a record 75 meters, or 41 fathoms, below the surface.
Wahle began monitoring lobster settlement in a limited way in 1989 along Midcoast Maine. Largely with support of the key lobster-producing states, the survey has since expanded to become the annual New England Lobster Settlement Index, spanning some 70 ocean-bottom sites from Rhode Island to New Brunswick, Canada.
Until last year, this sampling was carried out exclusively by divers, who were only able go down to a depth of about 20 meters at most to track the settlement of young lobsters in known nearshore nursery areas. That left deeper, offshore waters and other inaccessible ocean bottom largely unexplored.
Tray development
In partnership with lobsterman Matt Parkhurst of Boothbay Harbor, Wahle and his colleague Carl Wilson of the Maine Department of Marine Resources developed what they termed a “passive postlarval collector” to access locations out of divers’ reach.
In 2005, the team received funding from the Northeast Consortium to test the idea in the waters off Boothbay. The collector itself is weighty but effective. It consists of a trap-wire mesh tray, roughly the width and length of a lobster trap, lined with fine screening on the bottom and sides and filled with rocks.
The tray is meant to mimic the juvenile lobsters’ natural nursery habitat, attracting them as they abandon their free-floating planktonic state to settle in among the rocks. The design also meant the tray could be set and retrieved using a standard lobster pot hauler.
By the end of the settlement season in mid-October of 2005, the team determined that young lobsters settled as readily in the collector trays as they did in their natural nursery habitat.
How deep?
Building on the collector tray’s success, Parkhurst, Wahle, and Wilson won support from the consortium for a two-year project they began in 2007 to expand the team and the project’s geographic scope.
The focus was on the original question: How deep do young lobsters settle?
Last summer, with the added help of lobstermen Norbert Liemeux of Cutler and Skip O’Leary of Wakefield, RI, the team deployed 300 collectors to push the project into new territory.
The collector trays were set in three contrasting ocean environments along the New England coast – eastern Maine, Midcoast Maine, and Rhode Island – representing the central and eastern Gulf of Maine and the Southern New England shelf.
The eastern Gulf of Maine is strongly influenced by tidal mixing in the Bay of Fundy, so the water column is uniformly cool from surface to bottom.
The central Gulf of Maine and the Southern New England shelf stratify during the summer with surface waters warming while bottom temperatures remain very cold. Rhode Island’s surface temperatures can warm dramatically during the summer, enough to reach near tropical conditions.
Early results
At the end of the settlement season in mid-October, the trays were hauled to the surface.
“In places that are thermally stratified during the summer, we had much higher settlement in shallower water, although we still detected some settlement in over 70 meters of water,” Wahle said.
“But while young-of-the year densities were dramatically higher in shallow zones on Maine’s Midcoast, they were uniform from shallow to deep in eastern Maine,” he added.
The emerging findings, which suggest that lobster settlement mimics the thermal structure of the water column, are the first of their kind.
Growing interest
The project has generated considerable interest among researchers from New York to Norway, motivating a number of groups across the Northeast US and eastern Canada to participate in the study in 2007 and 2008.
The expanded interest effectively more than doubled the efforts of Wahle and his colleagues, as other researchers deployed some 400 additional collector trays in waters from Buzzards Bay, MA to Bonavista Bay, Newfoundland.
While the official numbers were still coming in early in January, Wahle’s group anticipated obtaining an unprecedented record of young-of-the-year lobster settlement.
The broadened geographic coverage is “an exciting opportunity to gather critical data and collaborate with the industry at the same time,” said Wahle.
While the diver-based method will continue to provide “dependable data on the pulse of baby lobsters entering the population each year,” Wahle said, the postlarval collector trays have “opened a window on a new aspect of this elusive segment of the lobster life cycle.”
Lauren Simmons
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