Spawning patterns may hold key to rebuilding cod stocks
SouthCoastToday.com January 09, 2010 12:00 AM
Scientists and fishermen have known for a long time that cod prefer certain areas of the ocean over others.
But what scientists are beginning to document is the extent to which the same cod return to the same area to spawn year after year, the way salmon and river herring return to specific water bodies, and how critical protecting those areas may be to the success of rebuilding those fish stocks to healthy levels.
"The whole field is now moving towards that (theory)," said Mike Armstrong, assistant director of the state Division of Marine Fisheries. "The sad part is we probably have lost 50 percent (of the distinct spawning populations).
Good "cod bottom" doesn't have to be as massive as Georges Bank. It can be as small as a slightly raised plateau, covered in pebbles and small rocks, that offers food and shelter from predators.
Successful fishing is all about finding the fish, and good fishermen know where fish are at certain times of the year. With modern technology such as the Global Positioning System, which uses satellites to pinpoint fishing spots, it's possible to home in when fish are most vulnerable as they group together to reproduce.
"If these are distinct spawning groups ... an unfettered fishery can wipe the whole thing out," Armstrong said. And once a group has been lost, there's a much slimmer chance of recolonizing it with random, roving fish who don't have the fidelity to the area, he said.
If that is the case, it could mean that bringing back cod stocks to healthy levels is a lot more complicated than placing limits on the catch. The operating theory in New England fishery management has been to build a big mass of fish, hoping they will spread out in search of food and repopulate the areas that were wiped out by overfishing.
But the place where fish are caught could be just as critical. Cod in the Gulf of Maine, for instance, might be made up of dozens of spawning subgroups that are genetically distinct from one another and would not tend to repopulate each others' territories.
Edward P. Ames, a fisherman/researcher from Maine, won a MacArthur fellowship in 2005 for documenting through historical research and interviews with longtime fishermen, a dozen or so cod spawning grounds along the Maine coast that have vanished.
Last spring, and again this year, DMF researchers will look at a small patch of ocean bottom off Marblehead where about 1,000 large, 40-to-50 pound cod gather from May to the end of June, to spawn. For the past couple of years, the state closed an 8.8-square-mile area during that period to protect that population, just like it does in winter to protect spawning cod in a much larger area nearby.
The hub of spawning activity in the smaller area, Armstrong said, is probably not more than 100 yards across. But, once it was discovered, fishermen easily returned to that same spot and kept catching the big fish. Without protection, Armstrong said, that population could have been wiped out in a few years.
This spring, the state will use a number of different research techniques to unravel the reasons this small area is so attractive and where the fish go after they spawn.
DNA analysis will help pinpoint whether the fish are actually genetically different than other populations. Listening devices will also track tagged fish. A special type of sonar will be used to estimate population size, and a fine-scale map of the bottom will be made to investigate any unique topographical features.
Different spawning times might be timed to take advantage of favorable currents and plentiful food, Armstrong said. But variety in genetics and in spawning, also means resiliency for the species. Unfavorable environmental or weather conditions, for instance, might only wipe out a portion of cod larvae, not all the larvae produced by spawning cod in the Gulf of Maine that year.
"A couple of million years of evolution have clued these animals in to put their eggs out in a certain spot at a certain time," said Armstrong