Common bream (Abramis brama)Animal migration can take many different forms in nature. Partial migration, where not all of the animals in a population migrate, is extremely widespread and occurs in many species of birds, mammals and fish. Yet despite its prevalence in nature, what drives differences in migratory behaviour between individuals is a mystery that continues to puzzle biologists. Why do some individuals stay and others go?
A collaborative team of researchers from the Danish Technical University and CAnMove at Lund University, led by Dr. Christian Skov, recently investigated this question in a species of partially migratory freshwater fish, the common bream (reported in Proc. R. Soc. B.). Common bream stay in lake habitats over the summer and part of the population migration into streams over the winter. In the lakes during the summer the risk of predation is high, but the rewards are also high (high food availablity). During the winter predation risk decreases as predators become less active and eat less, but food availability also decreases, reducing the benefit of staying in the lake. If the risk/reward ratio for residency in the lake differs over the winter between individuals (when the rewards for staying are relatively low), this may lead to partial migration. One way in which individuals differ that would affect this risk/reward ratio is in their vulnerability to predation. Hence, all else being equal, one would predict that fish that are more susceptible to predators would be more likely to migrate, and that differences in vulnerability between individuals might explain why some migrate and some stay resident.
To test this prediction Christian Skov and his team were able to assign >450 individual bream from 2 lakes in Denmark with a 'predation vulnerability' score, which they calculated by sampling large numbers of piscivorous predators (pike) from each lake. The vulnerability score was calculated by estimating what proportion of predatory pike in a lake could eat each specific bream (as pike are gape limited predators). The bream were individually tagged and released and their migratory behaviour monitored over the winter. As predicted, fish with a high vulnerability score showed a much stronger propensity to migrate, migrated earlier and stayed for longer in the streams. This research is important as it suggests a powerful role for predation in shaping patterns of migration.
So it seems for bream the question of 'should I stay or should I go?' depends upon an individual's vulnerability to predation. For smaller, more vulnerable fish, if they go there will be trouble (after all migration is a costly and risky business), but if they stay there may be double...
A collaborative team of researchers from the Danish Technical University and CAnMove at Lund University, led by Dr. Christian Skov, recently investigated this question in a species of partially migratory freshwater fish, the common bream (reported in Proc. R. Soc. B.). Common bream stay in lake habitats over the summer and part of the population migration into streams over the winter. In the lakes during the summer the risk of predation is high, but the rewards are also high (high food availablity). During the winter predation risk decreases as predators become less active and eat less, but food availability also decreases, reducing the benefit of staying in the lake. If the risk/reward ratio for residency in the lake differs over the winter between individuals (when the rewards for staying are relatively low), this may lead to partial migration. One way in which individuals differ that would affect this risk/reward ratio is in their vulnerability to predation. Hence, all else being equal, one would predict that fish that are more susceptible to predators would be more likely to migrate, and that differences in vulnerability between individuals might explain why some migrate and some stay resident.
To test this prediction Christian Skov and his team were able to assign >450 individual bream from 2 lakes in Denmark with a 'predation vulnerability' score, which they calculated by sampling large numbers of piscivorous predators (pike) from each lake. The vulnerability score was calculated by estimating what proportion of predatory pike in a lake could eat each specific bream (as pike are gape limited predators). The bream were individually tagged and released and their migratory behaviour monitored over the winter. As predicted, fish with a high vulnerability score showed a much stronger propensity to migrate, migrated earlier and stayed for longer in the streams. This research is important as it suggests a powerful role for predation in shaping patterns of migration.
So it seems for bream the question of 'should I stay or should I go?' depends upon an individual's vulnerability to predation. For smaller, more vulnerable fish, if they go there will be trouble (after all migration is a costly and risky business), but if they stay there may be double...
//Ben
Read the full article here: http://rspb.royalsocietypublishing.org/content/early/2010/10/26/rspb.2010.2035.full
Here is the full reference:
Skov C, Baktoft H, Brodersen J, Bronmark C, Chapman BB, Hanson L-A, Nilsson PA. 2010. Sizing up your enemy: Individual predation vulnerability predicts migratory probability. Proc. R. Soc. B. (in press)
Read the full article here: http://rspb.royalsocietypublishing.org/content/early/2010/10/26/rspb.2010.2035.full
Here is the full reference:
Skov C, Baktoft H, Brodersen J, Bronmark C, Chapman BB, Hanson L-A, Nilsson PA. 2010. Sizing up your enemy: Individual predation vulnerability predicts migratory probability. Proc. R. Soc. B. (in press)
Thanks to Henrik Baktoft for the bream photograph.
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