Predator fish populations recover with fishing away its prey

This can be more effective than releasing large numbers of artificially cultivated predator fish. This is the conclusion of research from the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the Universiteit van Amsterdam (UvA), the Swedish Umeå University, and the University of Tromsø (Norway). A joint publication appeared in the 22 June issue of Science.

The majority of populations of large predator fish such as cod and salmon have been strongly diminished over the last decades because of overfishing, bringing these species to the brink of extinction. The problem is so severe that even a complete embargo on the fishing for these species would appear to provide little remedy. In 2002 Prof. dr. André de Roos from the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the UvA published a theoretical study proposing an explanation for the lack of recovery of the cod population in the northwestern part of the Atlantic Ocean.

This population suffered a dramatic collapse at the end of the 1980’s from which it has failed to recover to date, in spite of the fact that the fishing for this species has been suspended. In the study a controversial solution to the lack of recovery was proposed: by diminishing the capelin population, the main preyfish of cod in the area, the size distribution of capelin could change in such a way that it would promote the recovery of the cod population. In other words, to stimulate the recovery of the cod population, we should fish away its main prey. Using experimental data from the Norwegian lake Takvatn, the current publication in Science shows that this is not just a wild theory.

Better growth possibilities

Since the 1930’s the brown trout had almost completely disappeared from lake Takvatn, leaving its preyfish, the arctic char, as its sole inhabitant. This arctic char population consisted of large numbers of individuals of intermediate size (15-20 cm) and hardly any truly large fish (35-50 cm). In an attempt to increase the number of large individuals, which are a favorite game for recreational fishermen, the University of Tromsø removed a large proportion of the arctic char from the lake at the end of the 1980’s. Thinning the population indeed increased the chances for the remaining individuals to grow to above average proportions, and resulted in an increased number of larger arctic chars in the lake. As a side effect, the brown trout population recovered as well to the point where this species of fish now makes up 15% of the total fish population in the lake.

In the current publication in Science, it is shown that the increased number of large arctic char individuals in the lake led to a significant increase in the production of offspring, even though the total number of arctic char that is reproducing has decreased. As a result, the availability of young char that serve as prey for the brown trout has grown as well. Even more important is that the brown trout is now maintaining the favorable situation itself: by their own preying on small arctic char they are themselves producing a stable population of large arctic char that in turn produce more offspring for the brown trout to eat. As a result the system has remained stable even though the human fishing program for arctic char was stopped in 1989. The strange paradox is that apparently a predator fish can increase the availability of its favorite preyfish by preying on it.

The experimental results from the Norwegian lake together with the model calculations indicate that fishing for preyfish of large predator fish may well be the most effective way of restoring the predator fish population. More effective than releasing large numbers of artificially cultivated predator fish.