Researchers Track Lobster Migrations to Improve Population Estimates

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December 2008

Lobster tagged to track migration patterns in the Great Bay Estuary UNH Ph.D. candidate Jason Goldstein holds a lobster with a temperature logger, an ultrasonic transmitter and a return tag. These items are secured to the lobster like a lightweight backpack to help UNH researchers learn about their migration patterns.

Icy air hits Jason Goldstein's face around his sunglasses and knitted hat as he drives a small boat in New Hampshire's Great Bay estuary. He motors towards Fox Point, the site of a lobster trap he checks regularly. Lobsters in the trap are not sold at market; instead, they are fitted with transmitters to reveal their movements. And because lobsters are not stationary creatures, Goldstein is out tracking their migrations at least once a week, regardless of the season.

"At least it's not sleeting today," he says with a wry smile.

Goldstein, a Ph.D. candidate in zoology at UNH, has tracked lobsters along the New Hampshire coastline and into Great Bay throughout the past two years. This research, funded by N.H. Sea Grant, will provide more accurate information about the sources of juvenile lobsters and interactions between the population stocks in New England. The information could improve the management of this economically valuable fishery, thus allowing lobster to remain front-and-center among New England menu choices.

Goldstein and UNH professor of zoology Win Watson are particularly interested in the movements of "berried" females, those carrying eggs. This year, they are comparing the berried females' movements with those of the large- and small-sized males and females without eggs. Where the berried females go, so go their eggs, and those movements likely have implications for New England lobster populations.

"We often go diving in early October and there are a lot of lobsters around, and two weeks later, they're all gone and have moved offshore," Watson says. "There's a dramatic shift that takes place in late autumn and causes them to move out into deeper water."

Watson and other researchers believe this drive to migrate is primarily due to seasonal changed in water temperatures. The deep offshore waters are consistently warmer in the winter and cooler in the summer than shallower water close to the shore, Watson explains. Initiation of lobster migrations might also be driven by turbulence in the water, particularly during autumnal storms.

"They are very mobile animals and move around to avoid bad conditions," Watson says. He notes that the average lobster can walk from the Isles of Shoals to the New Hampshire shore — up to 10 miles — in just a few days.

Along with other students and technicians, Goldstein has spent countless hours tracking the lobsters using ultrasonic and handheld hydrophone tracking equipment to locate them and determine their movements. Commercial lobstermen are assisting Goldstein by helping to tag some of the creatures they find in their traps. In many cases, lobstermen who capture tagged lobsters will call Goldstein to report the tag number and location.

In addition, 15 "lobster listening stations" moored throughout Great Bay and the coast are used to identify and record tagged lobsters moving within a 400 m radius of the equipment.

The various tracking efforts have led to a wealth of knowledge. Watson originally hypothesized that berried females would be more likely to move offshore to protect the eggs during the winter months. However, he was surprised to find out that most lobsters, regardless of size, gender or maturity, followed the same migration patterns.

The researchers also learned that very few berried females are located in the estuary, while male lobsters are often found there. Goldstein and Watson theorize that the estuary is too extreme an environment for most females carrying eggs.

In addition to studying the migrations of the adult lobsters, Watson and Goldstein are learning how the movements of berried females could impact egg development and location of larvae upon hatching out.

"Just because you find a lobster with eggs in one location doesn't mean that's where the eggs will drop," Watson says. "This is a really important issue. It speaks to the source of recruits to the fishery, where they come from and where they are going. If New Hampshire berried female lobsters move 10 miles offshore and release their larvae next spring, where will those larvae go? Will they stay in New Hampshire or move south to Massachusetts? Will they survive at all?"

Watson and Goldstein began using oceanic drifters this year to learn more about larval movements. The drifters are submerged box-like structures that mimic the movements of lobster larvae, which typically float within two meters of the surface. The larvae are strongly influenced by the ocean currents until they settle to the bottom in about three weeks after hatch.

The drifters were deployed in locations where the berried females that were tracked had resided, and at the times of the year when the larvae were known to hatch. Every 30 minutes, a satellite transmitter fitted to the drifter reports its location and the coordinates are recorded and displayed on a web site.

Watson originally thought the drifters released about three miles offshore would be carried south with currents parallel to the shore. Instead, several of them crashed into the coast at Hampton Beach and the mouth of the Portsmouth Harbor near New Castle within one week of deployment.

"I found that very surprising," Watson said. "It makes you wonder if larvae would survive if they were carried so close to shore within a week of hatch before they were ready to settle on their own."

In contrast, the drifters released near or beyond the Isles of Shoals ended up near Cape Cod and Georges Bank, and those placed in the Great Bay Estuary stayed there, with some going up the Swampscott River toward Exeter.

"To me, that illustrated that if lobster eggs were hatched out in Great Bay, they would probably be retained in the systems, which might not be optimal," Watson said. "The estuary is not the best place for lobster larvae because of the warmer temperatures, higher turbulence and lower salinity."

This finding fits well with the hypothesis that mature female lobsters migrate out of the estuary, leaving the sex ratio skewed toward males.

Putting all these data into perspective will be Watson's next step. He hopes to apply what he has learned to improving management practices for the species. In particular, Watson wants to know if New Hampshire lobsters can be managed in isolation or if the management unit needs to be larger.

"If New Hampshire lobsters are recruiting to Massachusetts or elsewhere, then we need to work together to better manage the population," Watson says.

Rebecca Zeiber, N.H. Sea Grant Science Writer