Fish stew: species interplay makes fisheries management tricky in the long run.

Twice in the past 20 years, the capelin population in the Barents Sea north of Norway and Russia abruptly crashed and then took years to recover. It has again fallen sharply since 2001, in spite of restrictions on how many fish may be harvested there.

According to a new statistical analysis, the varying fortunes of capelin can be best understood and managed by accounting for not only fishing of that plankton-consuming species but also factors that affect cod and herring. Cod prey heavily on capelin, while herring eat some young capelin and compete with older ones for food.

"To manage this system, we have to understand the dynamic interaction of the various species as well as man's harvesting," says ecologist Nils Christian Stenseth of the University of Oslo.

On the basis of more than 2 decades of data from marine-research vessels, Stenseth and his colleagues developed equations that use factors such as cod and herring populations, fishing activity, and weather to predict capelin abundance from one year to the next. The equations enable the researchers to infer the relative contributions of various factors to past changes in capelin populations.

Overfishing was largely to blame in the first capelin collapse, which began in 1984. On the other hand, in the crash of the early 1990s, and possibly also during the current one, growth in the herring population was a major factor, the researchers report in an upcoming Proceedings of the National Academy of Sciences. Predation by cod, the scientists add, substantially slowed the recovery of capelin following the first two collapses, and it could presumably slow the capelins' recovery from the current crash.

Taking dynamic ecological variables into account is also critical to maximizing catches for the long term, Stenseth says. In the Barents Sea, for example, warm years tend to boost numbers of both herring and cod, which then eat into capelin stocks more than they do during cooler years, he says.

Given the importance of species interactions in the Barents Sea ecosystem, fishing quotas there should be coordinated to maximize the sea's overall output, Stenseth says. To increase the sustainable harvest of highly valued cod, he suggests, fisheries managers might consider stricter fishing limits for capelin and, to further support the capelin population, they should allow larger herring catches.

A growing number of scientists--and two recent reports on U.S. ocean policy (SN: 4/24/04, p. 259)--advocate widespread implementation of such an approach, which is called ecosystem-based management.

The new research provides "a great case study that demonstrates the need for ecosystem-based fisheries management," says Ellen K. Pikitch of the Pew Institute for Ocean Science at the University of Miami in New York. It also makes "a very convincing ease" that capelin recovery depends on the abundance of cod, she says.