Fish migrate to escape predation from fish

Roach. Photo J Brodersen

There has been a lot of discussion about the importance of predation in animal migration, but data is scarce and difficult to collect. We recently collaborated with scientists at the Danish Technical University to quantify predation from cormorants upon migratory and resident roach fish in a 4 year study of two Danish lakes. By tagging >2,000 roach with individual PIT-tags, we were able to show that migration confers profound survival benefits for these fish. We did this by collecting data from tags found at the cormorant roosting sites from fish which had been eaten, and the tags excreted by these predatory birds. We found that 92.5% of the fish eaten by cormorants had been eaten in the lake, and that the longer a fish spent in the streams during winter, the lower the probability that it would be eaten. So we have some nice direct evidence for a mortality cost to residency in these fish.

Cormorant. Photo Michael Gerber

We have also been monitoring the cormorants at Lake Krankesjön, and will study how roach respond to the recent increase in cormorant numbers at the lake. Do we see any shifts in the migratory patterns at the lake in response to the predatory birds?

Here is the paper, which is also featured in this week’s Nature as a research highlight.

/Ben, Kaj, Anders, Christer & Lasse