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Commercial Fisheries News
Volume 35 Number 8
April 2008
2007 lobster settlement index looks strong
WEST BOOTHBAY HARBOR, ME – According to the 2007 New England Lobster Settlement Index, lobster larvae are comfortably settling in among the Gulf of Maine substrate at a rate that is comparable to, if not better than, favorable years past.
The recently released index indicates that lobster larvae maintained the trend of strong settlement that began in 2001. Except for sites in Buzzard’s Bay and Rhode Island, settlement improved from 2006, though in eastern Maine, it still was not as high as the record 2005 figures.
In 2001, settlement in Midcoast Maine rebounded from a six-year low and has been relatively high ever since. In eastern Maine, settlement has generally followed an upward trend since 2001. Cape Cod Bay and Salem, MA, which both experienced decreases in the past two-to-three years, saw a substantial upswing in settlement last year.
In summary, Rick Wahle, senior research scientist at the Bigelow Laboratory for Ocean Sciences, described 2007 as a “darn good year” for settlement in the Gulf of Maine.
He also observed a curious trend.
“When it’s a good year in one place it’s a good year in several places. This is not a local phenomenon,” he said. “Something is happening on a Gulf-of-Maine scale.”
Jet stream effect
The geographic similarities in the settlement data led Wahle and his colleagues to suspect that lobster larvae are literally riding the winds of change. Across the board, the trends amount to what scientists term “large-scale spatial coherence.”
Wahle and his colleague Andy Pershing at the University of Maine were able to clearly correlate the settlement trends with an indicator of regional climate patterns – the geopotential height anomaly.
“That,” said Wahle, “is basically an indicator of the strength and direction of the jet stream – the upper air winds.
“The jet stream comes from the west and is always flowing over our heads,” he explained. “In some years it is stronger and in some years it is weaker.”
Those deviations from long-term average wind speed and direction are called anomalies. As the data indicated, a good settlement year for lobster larvae tends to match a jet stream that’s relaxed a little, favoring onshore transport more than usual.
“In bad (settlement) years, we’re favoring offshore transport. The larvae are being carried offshore to places that are unfavorable for settlement,” said Wahle.
Data collection
The data for the lobster settlement index tracks patterns in known nearshore nursery areas. State divers suction up and count the newly settled lobsters from more than 65 sites that extend as far south as Rhode Island and as far north as Lobster Bay, Nova Scotia and Beaver Harbor, New Brunswick.
The project, now with data tracing back more than 20 years, involves several major participants – Wahle at the Bigelow lab, the Maine Department of Marine Resources, the Massachusetts Division of Marine Fisheries, the Rhode Island Division of Fish and Wildlife, and Canada’s Department of Fisheries and Oceans.
Safety in numbers
As Wahle discovered in a related postlarval collector project that followed lobster larvae into unprecedented depths (see CFN February 2008 for details), lobster settlement dramatically decreases with depth.
“In deep places, they’re in colder temperatures where they’re unlikely to grow,” he said, explaining that this is a natural fact of life for water-born animals that have a larval stage of life.
“They’re at the mercy of the currents and, therefore, hedge their bets by producing as many offspring as possible,” Wahle said.
The lobsters’ strength in numbers provides a cushion for crisis and a significant number are often able to survive from one year to the next no matter which way the winds may blow.
Relationship to adults
Furthering their study, Wahle and his colleagues investigated whether fluctuations in settlement numbers were proportional to the number of adult lobsters entering the fishery.
“Will the annual fluctuations translate and be seen as you follow year classes through time?” Wahle asked rhetorically.
“The only problem is there’s no way to know the age of a lobster for sure because they don’t have growth rings like trees or even fish bones,” he said. “With an improving understanding of how fast lobsters grow, we can estimate how large they will be at a certain age within a margin of error.”
Wahle said he discovered that the number of larvae able to settle had a fairly close relationship, though not directly related, to the number of one-year-olds counted the following year. However, a third factor came into play.
“There is a crowding effect that kicks in,” he said.
He described this more simply as being like a housing shortage. With low settlement numbers, lobsters are easily able to move in amongst their favorite cobble substrate, left to develop out of the predator’s eye.
“When there are low numbers, there’s no housing shortage,” Wahle said. “But when there are high numbers, they get elbowed out – there’s a housing shortage.”
In this case, the leftover lobsters pushed out of the cobble substrate have to find other, less desirable real estate.
However, the good news for the outcasts is that they seem to be surviving despite the conditions.
“We took a giant step to look at settlement numbers compared to fishery recruits nine years later (in eastern waters),” said Wahle.
The proportional numbers suggested that “lobster larvae are outgrowing their predators before they are outgrowing their houses,” he explained.
Catch rate
With all of the above numbers working in their favor, it seems that the Gulf of Maine should be packed full of lobsters. A more difficult factor to reconcile is the high settlement numbers compared to the downward catch rate lobstermen have seen in recent years.
According to Wahle, there are a number of factors in addition to lobster abundance that influence catch.
“Trends in the catch are dependent on a host of different factors from sea conditions to economics,” he said, adding that weather, too, can affect the fishery.
“We also have to monitor factors that may affect the survival of lobsters as they grow,” Wahle continued.
One of those factors – shell disease – dramatically affected Rhode Island’s catch rates in the late 1990s.
“We can’t assume that natural mortality rates will remain constant through time,” he said.
Wahle said he plans to keep a watchful eye on how predators and other contending players will affect lobster settlement and survival in order to account for those factors in future settlement indexes.
But for now, the combination of the right ocean temperature, a fair weather year, and a bit of luck will keep the future catch coming in.
Lauren Simmons
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