Commercial Fisheries News - January 2002
The best-studied reproductively active female lobsters are those that spend their time migrating between Grand Manan Island and the Grand Manan Basin.
Biologists at Canada s Department of Fisheries and Oceans (DF0) have been observing these lobsters for about two decades.Their findings are instructive and should inspire investigation in other, perhaps similar, locations.
DFO scientists decided they would learn more about lobster migrations if they were able to recapture tagged individuals more than once. They wanted to know whether lobsters captured at the same location a year after being tagged had traveled round-trip or simply stayed in one place.
The researchers boarded fishing vessels and chartered boats to tag lobsters. They asked harvesters not to remove the tags upon recapture and offered a monetary reward to those who provided information on tagged lobsters, such as tag identification, location, and depth of capture.An additional reward was offered to those harvesters who allowed scientists to measure the lobsters.
Studying lobsters of a marketable size was a problem because the tagged animals likely would be sold upon recapture. Therefore, scientists decided to tag egg-bearing females,which are protected from capture during the nine to 12 months between spawning and hatching of their eggs. Those females captured repeatedly within the same 12-month period provided valuable information on travel trajectories at specific times of year.
The researchers also gathered temperature data. Their findings reconfirmed the well-known idea that deep water is warmer than shallow water in the winter and shallow water is warmer than deep water in the summer.
Female lobsters followed the warm-water pattern, migrating to the deep water of Grand Manan Basin in the winter and returning to shallow waters near the island in the summer.
The migratory behavior of the egg-bearing females at Grand Manan appears to be related to their reproductive cycle: the females spawn in late summer and early fall; they then migrate to deeper water where their eggs continue to develop; and, the following summer, they return to the spawning grounds where the eggs hatch.
Divers observed that reproductively active females gather at a shallow cove where they extrude their eggs (spawn) in late summer/early fall.At Flag Cove, the spawning females congregate in dense aggregations on a sand bottom where they dig depressions that buttress each other so that the females are often less than a body length apart.
The female lobster spawning ground is all but devoid of males, fish, crabs, and other marine life.
Clearly it is a curious place.The question remains just how curious or common Flag Cove is.
Are there other migratory routes like the one between Flag Cove and Grand Manan Basin? Do female lobsters typically congregate in shallow coves to spawn? Do they choose coves adjacent to deep basins so they won t have to travel far to their overwintering grounds? And, do they return to their spawning grounds to hatch their eggs?
The Lobster Conservancy is developing a new research program to answer these and related questions.