The odd reason some fish migrate

In the winter, thousands upon thousands of the common roach fish migrate to nearby streams throughout Europe. Credit: Niklas Liljeback

From birds to crustaceans, numerous animal species migrate each year to find more food, pleasant weather or the perfect place to breed. But at least one species -- a freshwater fish called the common roach -- migrates to avoid getting eaten, new research suggests.

Researchers used identifier tags to track the seasonal movements of the common roach (Rutilus rutilus) for four years. These fish live in freshwater lakes throughout Europe and are partially migratory -- that is, during winter, up to 80 percent of the fish migrate to nearby streams, while the rest stay put.

"It's a bit of an overwhelming sight to come across these streams in wintertime, and you suddenly realize that there are thousands upon thousands of fish in this restricting area," said lead researcher Christian Skov, a fish ecologist at the Technical University of Denmark.

Cormorant birds (Phalacrocorax carbo) are known to prey on roach living in both lakes and streams. However, 92.5 percent of the tags retrieved from cormorant pellets came from roach that last swam in lakes specifically, the researchers found. Moreover, the longer the fish stayed out of the lakes, the more likely they were to escape becoming bird food, suggesting that the fish reduce their predation risk from the birds by migrating into streams. [The 10 Most Incredible Animal Journeys]

Lakes vs. streams

The idea that some animals migrate to escape predation isn't new, but evidence for this behavior is hard to come by. In 2010, scientists discovered predation was partially involved in the breeding migrations of Arctic shorebirds: birds that traveled farther north were less likely to have their nests ravaged by foxes and other predators. But little other evidence for this kind of phenomena exists.

In the new study, Skov, along with colleagues from Sweden and Switzerland, attached passive integrated transponder (PIT) tags to more than 2,200 roach living in two lakes in Denmark. Every tag had a unique identifier code, allowing the team to distinguish each individual fish. At the inlets and outlets of each stream, the researchers set up antennas, which picked up signals from the PIT tags. "We knew the position of all of these tagged fish, allowing us to calculate how many days they spent in the stream and how much time they spent in the lake," Skov told LiveScience.

The researchers also knew cormorant birds roosted and bred near the lakes, while preying on roach, among other fish. With a device not so different from a minesweeper, they combed the cormorants' habitat, searching for tags that were pooped out. [The 10 Weirdest Animal Discoveries]

Analyzing the data from the tags, they saw a large variation in the amount of time the fish spent out of the lakes. Some fish spent five months in the streams, while others would visit the streams four or five times in a single winter. "But there was a consistent pattern," Skov said. "The longer [the fish] spent in the stream, the smaller their probability of predation."

For example, in 2008, small fish that stayed in Lake Loldrup the entire winter had about a 20 percent chance of getting eaten, but this probability dropped to about 10 percent if they stayed out of the lake for 100 days. Large roach (around 10 inches, or 25 centimeters, in length or more) showed the same trend, but overall were more likely to get eaten than small fish -- they had a 40 percent chance of being preyed upon if they stayed in Lake Loldrup for the full 2008 winter.

Interestingly, the choice to migrate wasn't risk free. During winter food becomes scarce in the lakes, but there's practically nothing to eat in the streams, Skov said.

Why some, but not all?

Given how much safer the streams appear to be during the chilly months, one can't help but wonder why some roach stay behind in the lakes. The researchers don't have the full picture yet, but their previous research has uncovered clues.

In one study they found that physically fit fish were more likely to migrate than those in poor physical condition. The researchers also found that the fish's personality matters, as bold fish are more likely to migrate than shy fish. "So it's pretty complicated why some fish stay behind," Skov said.

Skov thinks the research will help scientists understand what drives migration and how different migrations evolve. "Here's support for one of the most underexplored ideas as to why animals migrate," he said, adding that he's interested in seeing how adaptive the roach's behavior is and if the fish would continue to migrate when there's no cormorant threat.

Future research may even show that other animals migrate to escape predators. "I could easily imagine that's the case," Skov said.

The research is detailed online today (Feb. 26) in the journal Biology Letters.

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