If you kill more fish, the total population of the species declines. However, kill only small, young fish, the total number of small, young fish increases. This seemingly paradoxical conclusion has far reaching implications for the sustainable management of oceans, and is the result of a theoretical study conducted by a research team led by Prof. André de Roos of the Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam.

A follow up study conducted by the same team now provides experimental evidence to support their initial findings.

When fish are killed, by fishing or predation, a first obvious effect is that the total number of fish of the species in question declines. However, the removal of fish also results in less competition for food among the remaining individuals. For example, young, small fish, one can imagine that when a certain percentage of young small fish is selectively removed, the remaining ones have more food available to grow into bigger and stronger adults. These adults in turn, are able to produce more offspring, i.e. more young small fish, to compensate for the ones that were killed. In fact, the increased reproduction more than compensates for the number of small fish that were selectively removed in the first place.

This is a simplified version of the type of relationships that Prof. De Roos and his team revealed in a theoretical study that was published in the American Naturalist in 2007. The modeling study indeed predicted that selectively removing specimens of certain size classes not only results in compensation induced by changes in competition for resources, but in fact results in over-compensation.

The seemingly paradoxical implication is that selectively fishing away small, young fish results in an increase in the number of small, young fish. In a similar manner, the modeling study also predicted that the selective removal of large, adult individuals can lead to an increase in small, young fish densities. To test their predictions, Prof. De Roos and his team performed an experimental follow up study in Sweden that was recently concluded and supports their theoretical conclusions in all respects. This includes the specific example of selectively culling a certain percentage of small, young fish, which indeed was found to lead to an increase in their total numbers.

The results of the experimental study were published in the Proceedings of the National Academy of Science in February 2009.

Together, both studies show once more that marine food webs are very complicated and that interfering with them may lead to unanticipated results. However, they also show that by carefully studying the relationships, they can be understood and modeled. Both these conclusions have far reaching implications for the sustainable management of oceans. For example, in a forthcoming publication in the Canadian Journal for Fisheries and Aquatic Sciences it is shown that this type of overcompensation may prevent a marine reserve from functioning as a source of young fish. This latter is one way in which marine reserves were believed to contribute to a sustainable exploitation of marine fish stocks.