Fish migrate to safer environments

Every year, millions of animals migrate worldwide. In most cases, this is due to a shortage of food or other environmental factors. However, few research studies have focused on migration as a strategy to avoid predators. It is not easy to measure and quantify the risk of an animal being eaten.

"Our findings are therefore quite unique," says Ben Chapman, a researcher from the Department of Biology at Lund University.

In collaboration with Danish colleagues, the researchers at Lund University have published the results of their study. These show that fish, in this case roach, flee from a lake to surrounding streams and wetlands when there are a large number of cormorants hunting in the lake. Ben Chapman and his colleagues note that their findings are among the first evidence that the threat of predators can be a reason for seasonal migration in animals.

The researchers used an inventive method to track the fate of individual roach. They individually marked thousands of fish with a little chip resembling a barcode, and then went to the cormorants' resting places and scanned the earth for chips in the birds' excrement -- i.e. the remains of the fish that have passed through the birds' digestive systems. In this way, the researchers have been able to obtain large quantities of data on which fish were eaten. It emerged that it was mostly larger roach that fell victim to the cormorants.

The fieldwork has been carried out in the Danish lakes of Viborg and Loldrup on Jutland. In the next fieldwork season, the researchers plan to expand their work to include Krankesjön lake in southern Sweden and to investigate whether fish can change their migration patterns in response to increasing numbers of predators.

The study has been published in the scientific journal Biology Letters and will also be featured in Nature.