A novel lobster study is underway in Muscongus Bay that focuses on the location and movements of egg-bearing female lobsters. The two-year project is funded by the Northeast Consortium, and depends on collaboration between The Lobster Conservancy (TLC) of Friendship and a group of Friendship lobstermen.
Lobsters generally mate in the summer after the female has molted. After mating, a female lobster waits until her shell hardens, and then extrudes the fertilized eggs out onto her tail in the late summer or early fall. Female lobsters carry and care for the eggs for 9-12 months, but little is known of their movements and habitat preferences during this time.
The goal of the study is to determine if and where female lobsters congregate when they are spawning (extruding eggs onto their tail), where they go during the nine to twelve months that they brood their eggs, and where they end up hatching their eggs (releasing larvae). TLC’s founder and Senior Scientist, Diane Cowan, leads a team of scientists that includes Andrew Solow of the Woods Hole Oceanographic Institution and Win Watson of the University of New Hampshire.
After hatching, lobster larvae spend from two to seven weeks living as plankton near the surface of the ocean, and then metamorphose into postlarvae and seek shelter on the bottom. During this planktonic larval phase, lobsters are at the mercy of ocean currents, and it is believed that major currents such as the Eastern Maine Coastal Current carry larvae long distances from where they hatched to populate other areas.
“We are testing the hypothesis that local broodstock may make major contributions to local lobster stocks by exploring the relationship between the movements of egg-bearing female lobsters and the location of larval release,” said Cowan.
Lobstermen are an integral part of the project, allowing their boats to be used as research platforms and actively gathering data for the study. TLC researchers went out on lobster boats during September and October of 2002. Each “egger” caught was outfitted with three pieces of equipment; a sonar transmitter, an identification tag and a temperature data logger. The tagged lobsters were released near where they were caught, and their positions were recorded on a global positioning system.
The sonar transmitter emits a unique series of beeps that can be heard by an underwater listening device called a hydrophone. Each transmitter was fastened to the back of the lobster with Superglue and duct tape. Using a hydrophone, a lobster can be individually identified from a boat up to 800 meters away.
Nine lobster boats were outfitted with hydrophones, and the captain and crew were trained to identify lobsters and record the necessary data. The lobstermen “listen” for the females and report on their current location.
The participating lobstermen have a wealth of knowledge on lobster populations and movements in the areas they fish, and feel this is an opportunity to get that across to scientists in a form both can relate to.
“We’re out there six days a week vs. some scientists that are only out a few times a year,” said Mark Wallace of F/V PAMELA B.
The fishermen also have a sense of ownership in the project, and the results that will come out of it.
“We’re actually doing the science,” said Philip Bramhall of F/V AMANDA KATE. “We know how the data is being collected and that it’s real.”
The identification tag contains TLC’s name and phone number and the lobster’s identification number. This allows any lobstermen who hauls up a sonar-tagged lobster in one of his traps to call in and report its location, depth and condition. Of the 193 lobsters tagged and released during September and October, 30 have been recaptured once, and five have been recaptured twice.
The temperature data logger is cable-tied to one of the mother lobsters’ claws, and will record the water temperature every hour for over a year. This information will be useful in determining the range of temperatures the mother and her eggs were exposed to during the brooding period if the lobster is recaptured during or after hatching.
Lobster eggs develop faster in warmer water. During the summer, shallow water is warmer than deep water, but in the winter the reverse is true. Some scientists believe lobsters, and especially egg-bearing females, move in response to temperature change. Large females are believed to move out to deep water more readily than smaller females. The size range of females tagged for this study may allow this hypothesis to be tested.
The participants in this study anticipate it will provide baseline mapping data on habitat use by egg-bearing female lobsters and answer questions about egg production by local broodstock. If female lobsters that mate and spawn in Muscongus Bay remain there, or return there to release their young, this provides evidence for local production. If these females leave the bay during brooding and release their eggs offshore, then this provides evidence for remote production and larval transport. If both scenarios occur (the most likely outcome) then both hypotheses are valid and could help explain the health of the lobster fishery – if there is depletion in a local stock, that area could be replenished by larvae from far away. If the large females are the ones moving offshore and sending their larvae far and wide on coastal currents, this evidence could justify Maine’s practice of protecting oversized lobsters.
Dan O’Grady is an Island Institute Fellow working with the Lobster Conservancy in Friendship.