Harvesting prey to boost predator fish

“For example, we ought to be able to save the Baltic cod if we fished herring and sprat. Likewise, the salmon trout in depleted lakes could be revived if we culled the char, which is what the salmon trout preys on,” says Lennart Persson, professor of aquatic ecology at Umeå University. The latter is precisely what has been done in a 26-year experiment in Lake Takvatn in Norway.

Thanks to this experiment and a mathematic model, Lennart Persson and his collaborative partners at the universities of Amsterdam and Tromsö have been able to show that it is actually possible to favor predators by fishing their prey.

The more prey, the more predators, says a time-worn ecological theory. However, this is not necessarily true. When there are few prey, the remaining prey grow more rapidly. This, in turn, can lead to more sexually mature individuals, which leads to more small prey, which the predator fish prefer. Paradoxically, a predator fish can therefore increase the amount of small prey fish by eating them. If there aren’t enough predatory fish, owing to increased harvesting, for instance, the reverse situation ensues. The number of prey that the predators live on will decline.

“We will then see a downward spiral leading to the collapse of the predator stock. Since the growth of the prey fish is impaired, it can become impossible for the predatory fish to recover if we introduce a total embargo on fishing. Instead, we should harvest the prey fish in order to bring about more individual prey of the size the predators feed on,” explains Lennart Persson.

In this way the predatory fish stock can recover and thereafter improve their own situation by eating their prey.

By harvesting char in the lake in Norway, Lennart Persson and his colleagues have managed to increase the number of small char individuals that predator fish prefer. The number of salmon trout, in turn, has burgeoned. Before restocking started, the salmon trout had largely disappeared from the lake; the char were not growing properly, and there was too little food for the salmon trout. The restocking was completed 16 years ago, and since then the salmon trout have been able to maintain the entire ecosystem of the lake in a state with rapidly growing char and thereby plenty of prey for the trout.

Both the model prediction and the large-scale lake experiment indicate that harvesting prey fish can be an effective way to restore collapsed stocks of predatory fish. The article is titled “Culling prey promotes predator recovery-­alternative states in a whole lake experiment” and is published this week in the journal Science, see http://sciencemag.org.

The findings are based on collaboration with researchers at Tromsö University and Amsterdam University.