OREGON STATE UNIVERSITY

Natural controls on fish populations analyzed

08/14/1997

CORVALLIS, Ore. - Researchers at Oregon State University have demonstrated for the first time one of the natural mechanisms that helps regulate populations of marine fish species, so that the fish neither go extinct or expand their numbers to unsupportable levels.

The study, to be reported Friday in the journal Science, documents how the population of a tropical fish species called damselfish is controlled by combined attacks from predators in the reefs below and the waters above.

The research also confirms a theory about "density dependent" mortality, which suggests that as populations of a fish species grow large, the levels of predation increase; but as populations fall, the "roving" predators lose interest and move away to better hunting grounds, allowing populations to rebound.

"Among other findings, this research suggests that it's important to not overfish these predatory species, since they may play a key role in regulating the populations of other fish," said Mark Hixon, an OSU professor of zoology. "If you were to remove the predatory species, you could seriously destabilize the population and make it far more erratic than it already is."

The findings, Hixon said, help explain on a basic ecological level how the population of at least one fish species may be regulated on a short-term basis, and it probably has applications to a number of other species with related mechanisms. However, these small-scale oscillations in fish population are a different issue than large-scale population changes - such as the salmon crisis in the Pacific Northwest - that may be influenced by overfishing, habitat degradation and marine climate change.

It's commonly known, Hixon said, that many fish populations can vary dramatically, sometimes on a scale of 10-100 times over a period of decades or even a few years. It's been far less clear what prevents this natural variation from getting out of control, although it is widely theorized that some natural mechanism causes a high level of natural mortality when fish populations are too high, and the mortality lessens when the populations are low.

In field research done on tropical reefs in the Bahamas Islands, scientists from OSU and the University of California at Santa Cruz showed that the damselfish - during their particularly vulnerable juvenile stage - were preyed upon by two distinct types of predators, both of which were necessary for this type of "density dependent" population control.

Groupers, and other "resident" fish species living within the same coral reefs the damselfish favored, preyed on them if they ventured too close to the reef. Jacks, another tropical fish species, were "roving" predators that attacked the damselfish from above.

"When damselfish populations became high, the jacks would show up and attack the juvenile fish from above and the groupers would eat them if they ventured into the reefs below," Hixon said. "They were literally caught between the devil and the deep blue sea."

But when damselfish populations dwindled, Hixon said, the jacks would lose interest and move on to other feeding grounds, giving the damselfish time and opportunity to rebuild their populations.

These types of natural population control mechanisms are complex, quite variable, and no doubt operate differently for different fish species, Hixon said. Cod fisheries in the North Atlantic - which have practically collapsed due to overfishing - may have predatory fish similar to the damselfish involved in their population control. But there is speculation that marine bird species and other predators may regulate juvenile salmon in the ocean.

Basic ecological research such as this may help fisheries managers better understand the life cycle and ecology of marine fish, Hixon said, and take steps to protect the natural interaction of different species. For instance, the research suggests that overfishing of important marine predators could seriously disrupt natural population controls on other fish species.

"What we basically need to do is understand how natural systems work and then make sure they are not seriously disrupted by human activities," Hixon said.