Fish migrate to escape predators: study

Fish use migration as an effective defence against being eaten, scientists say.
Food and climate are classic explanations for animal migration, but the idea that animals migrate to escape predators is less well studied.
By individually tagging fish in a lake and following their movements, senior scientist Christian Skov, Technical University of Denmark (DTU) Aqua and colleagues from Lund University, Sweden and Eawag, Switzerland found that migrants benefit by evading predators.
The biologists tagged more than 2000 individual fish (roach, Rutilus rutilus) in two Danish lakes over 4 years and monitored migratory behaviour using passive telemetry.
Next, they calculated the predation vulnerability of fish with differing migration strategies, by recovering data from passive integrated transponder tags of fish eaten by cormorants at communal roosts close to the lakes.

The study found that migration in a freshwater fish like roach that commonly migrates from lakes to streams during winter, confers a significant survival benefit with respect to cormorant predation.

"Our study shows that fish can reduce their predation risk from cormorants by migrating into streams, and that the probability of being preyed upon by cormorants is positively related to the time individuals spend in the lake during winter," said lead author, senior research scientist Christian Skov, DTU Aqua, National Institute of Aquatic Resources, Denmark.
"Our data add to the growing body of evidence that highlights the importance of predation for migratory dynamics, and is one of the first studies to directly quantify a predator avoidance benefit to migrants in the field," Skov said in a statement.

The study was published in journal Biology Letters.