Oceanographers Survey Sea To See If Fish Will Recover
WOODS HOLE, MASS.
Georges Bank may be off limits to fishermen, but scientists are having a ball.
Already this year, teams of Canadian and US oceanographers have made 25 two-week cruises over the fish-rich bank off Massachusetts.
''That's a lot of ship time and a lot of people working on the sea,'' observes Peter Wiebe, senior scientist in the biology department at the Woods Hole Oceanographic Institution. He notes that it is indicative of the intensive research needed to answer the fishermen's most important question: Will depleted commercial species - cod, haddock, and yellow-tail flounder - ever recover?
Dr. Wiebe's colleague Andrew Solow in Woods Hole's Marine Policy Center notes that Georges Bank still has a healthy ecosystem teeming with life. It's just that ''less desirable'' species such as spiny dogfish sharks and skates have replaced the fished-out commercial species. This ''appears to represent an ecological shift,'' Dr. Solow says. He adds that it is not at all clear that the system will shift back to favor commercial species as fishing pressure is reduced.
The bank has a distinctive ecosystem, Wiebe says. Surrounding currents tend to isolate it. ''It has a physical circulation pattern in which distinct populations develop and persist,'' Wiebe says. ''We believe the [fish] populations ... stay on the bank. If you change the conditions on the bank, they don't go some place else. There's no place else for them really to go.''
Without the pressure of commercial fishing, Solow says, the intricate interplay of ''other physical and biological processes will become more important in determining the abundance of fish.'' Understanding these processes ''is critical'' to answering the fishermen's question, he says.
Gaining that understanding is what the five-year research program now under way on Georges Bank is all about. It's a happy coincidence of the fishermen's need and a massive ocean-spanning international program called Global Ocean Ecosystems Dynamics, or GLOBEC. Its aim is to try to find out how a changing world climate will affect animals in the sea. Although the research will span the North Atlantic, the first GLOBEC site is Georges Bank. This was not planned with the bank's fishing crisis in mind. Nevertheless, Wiebe notes that ''we're here at the right time and place to help figure out what the problem is.''
That means getting into the intimate details of fish life literally at the microscopic level. It means tracking the lifestyles of minute fish larvae and the even smaller organisms that they eat. It also means learning how conditions in the water and air affect the bank's life. That involves mapping the water's stratification into regions with different temperature and salinity to see how these regions favor or discourage the growth of fish larvae and their food.
What Wiebe calls the ''incredibly hard'' job of mapping the distribution of fish and their food in the bank's environment now engages the attention of 70 scientists at 22 institutions in Canada and the US. They're targeting cod and haddock and the shrimplike copepods that young fish eat. Moreover, they're focusing on the fishes' larval stages, because that's where the action is.
A female fish may produce 1 million to 3 million eggs. But only one in a million of her progeny typically makes it through to adulthood. With odds like that, tiny variations in infant mortality make big differences in the annual additions to the adult fish stock - the so-called year classes.
David Mountain, deputy division chief with the National Marine Fisheries Service in Woods Hole, points out that a difference of 0.0009 percent in mortality rate makes a tenfold difference in the yield of adult fish. Therefore, any influence, however small, that affects fish larvae mortality is of big interest to fishermen.
The more GLOBEC scientists look into such influences, the more they discover they haven't really known what is going on. They knew, for example, that fish larvae right out of the egg still have a yolk sac to nourish them. They now find that, if the larvae also have copepod larvae to feed on, they grow more vigorously. Thus the abundance of copepods and their larvae is important right from the start of fish development. Anything affecting copepods may have a big effect on fish year classes.
In fact, something mysterious was cleaning out copepods in parts of the bank where they should have been abundant.
An unpleasant goo that unexpectedly fouled sampling nets soon identified the culprit for Woods Hole scientists. The mess turned out to be remains of a tiny creature called Obelia - a type of animal called a hydroid. It eats copepods and their larvae voraciously. But it wasn't supposed to be on the bank. Here was an unsuspected microscopic predator that now must be taken into account in assessing the prospects for the fishery's recovery.
For Wiebe, this is ''an exciting story.'' It points up the relevance to fisheries management of the kind of basic research now being done on the bank. ''We go out expecting one thing, we trip on to something we never expected to see,'' Wiebe says. He adds that this must happen over and over again if scientists are to get the knowledge needed to assess the potential for recovery of the once famous cod, haddock, and yellow-tail flounder on Georges Bank.