Peripatetic lobstas

From south to north, young lobsters find cool refuge in deep water

University of Maine School of Marine Sciences

Maine fishermen hauled in 110.8 million pounds of lobsters in 2017 with a value of more than $400 million. While still incredibly large, this volume represented a 16 percent decline and $100 million loss compared to previous years of record-setting landings.

Since the late 1980s Maine’s lobster landings have multiplied six fold, while the area of highest landings has shifted Down East to Hancock and Washington counties. 

The U.S. lobster fishery is now the nation’s most valuable single-species fishery. But last year’s decline was the largest in more than 50 years, leading the industry and scientists to wonder whether the boom has come to an end.  

The patterns are consistent with forecasts based on juvenile lobster population surveys founded and overseen by Richard Wahle in the University of Maine School of Marine Sciences.

In 1989, Wahle initiated data collection for the American Lobster Settlement Index, a program that monitors the number of baby lobsters that “settle” to the sea floor every year. Counts are made at some 100 sites from Rhode Island to Atlantic Canada.

“These lobsters will reach harvestable size in about six to nine years, and so the index can provide a useful bellwether for things to come,” says Wahle. While the monitoring is now conducted by participating marine resource agencies in the U.S. and Canada, Wahle’s lab hosts the collective database, developing and testing the index as a forecasting tool.

“Between 2005 and 2008 were years of peak settlement which we think drove the upsurge in landings. Since then settlement has fallen off considerably across the Gulf of Maine, and 2017 was also well below average,” Wahle says.

There are two prevailing explanations for such little settlement, he says. One is that more larval lobsters are dying before they reach the settlement stage. The other is that they are not so much dying as spreading to new deep-water nursery grounds not covered by current monitoring efforts.

Support for the idea that lobster larvae are dying faster comes from a recent study published in the July issue of the Bulletin of Marine Science, linking declines in lobster settlement to changes in the marine food web. Joshua Carloni of New Hampshire Department of Fish and Game, Wahle, and two other co-authors report findings suggesting larval mortality may be up because the supply of their favorite food is down.

They observed strong correlations in the abundance of tiny planktonic crustaceans called copepods with the abundance of the lobster’s final planktonic larval stage, as well as the Settlement Index. These findings were first presented at the 11th International Conference and Workshop on Lobster in Portland in June 2017 and hosted by UMaine.

“The other possible reason for the decline is that larval settlement has spread out across a larger range of depths, effectively reducing settlement densities in the routine shallow-water monitoring locations,” says Wahle.

Warmer temperatures in the Gulf of Maine have expanded the area of optimal habitat for young lobsters, which transition out of the larval stage and settle to the bottom at temperatures of 12 C (53 F) or higher.

“Under this scenario the observed declines in settlement could be misleading if settlers are spreading over a larger area of suitable habitat not included in the monitoring program,” says Wahle.

To understand settlement in deep water out of reach of standard diver-based sampling, Wahle received funding from Maine Sea Grant in 2016 to expand the settlement survey to deeper water. His specific aim was to examine links between temperature gradients and lobster settlement, both depth-wise and along the coast.

Working with research partners and lobstermen Curt Brown of Portland and Norbert Lemieux of Cutler, Wahle deployed collectors (wire mesh trays full of rocks) over a range of depths in two oceanographically contrasting segments of Maine’s coast: off Casco Bay in the west and Machias Bay in the east. 

The team also collaborated with the ventless trap monitoring program run by the Maine Department of Marine Resources to follow the movements of young lobsters as they grow.

Their results to date confirm newly settled lobsters as deep as 80 meters, but they saw consistent differences in patterns from east to west. 

Taken together with DMR’s ventless trap data, their results suggest that in the west where the shallows warm while the deeps stay cold, larvae settle shallow and then spread to the depths as they grow. 

In the east, where temperatures are more uniform surface to bottom, settlers spread more evenly over all depths and that pattern is mirrored in the catch of older lobsters.

“We know now they are able to settle deep, especially in northern areas, but we’re not sure how typical it is, because we only have a two-year snapshot of deep-water settlement,” says Wahle. “We know there was an eastward expansion of settlement starting around 2005, likely related to warming conditions, but the question is whether an expansion of settlement into deep water also contributed to the boom in landings.”

Wahle is now looking to see if accounting for expanded settlement habitat in the Settlement Index, which is now based only on the shallow monitoring sites, will give a more optimistic forecast of future lobster populations.

Ready Seafood Co. has contributed financial resources to continue the deep-water settlement monitoring for another two years.

“At Ready Seafood, we are excited to be a part of a research project that is improving our understanding of Maine’s lobster resource,” says Brown, who also is the staff scientist at Ready Seafood. “Lobster is the lifeblood of Maine’s marine economy and we see this project as an investment in not only the future of our company, but the future of our industry.

“Working with Dr. Wahle and the UMaine crew has benefited our business at all levels,” Brown says. This project has really resonated with our entire team, to the point where every October our entire staff and all our customers gets excited to see what will come up in our collectors.”

Wahle says it’s very gratifying to know the industry finds this information useful — useful enough to want to invest in it. “This is filling a critical data gap,” he says.

The deep-water settlement monitoring will continue through 2019. For more information about the American Lobster Settlement Index is online.