Migration is a phenomenally widespread behaviour in the animal kingdom. Yet even for migratory species it is very common that not all individuals migrate within a population (“partial migration”). What maintains these two very different strategies (migrant and resident) in animal populations is still largely a mystery, although recent work into partially migratory freshwater fish (the roach) has shed some light on this fascinating problem.
Roach migrate out of lakes in the winter into streams, returning in the spring. Or at least some of them do, whilst others remain resident the whole year round in the lake. A recent study showed that this migration may function as an antipredator behaviour, with predation from voracious piscivorous birds (cormorants) being significantly lower in the streams compared to the lake during winter (see here). So if the benefits of migration are so great, why don’t they all do it?
One reason might be that migration can be costly. Roach migrate into streams that are most likely food poor environments compared with the lake. In our study, published in PLoS One (see here), we test this idea, and quantify a foraging cost to migration in the roach. By assaying the gut contents of both migratory and resident roach in Lake Søgård in Denmark throughout the migratory season (in collaboration with our buddies at the Danish Technical University), we found that migrants had less food in their guts and also lower quality food items. Hence our data supports the idea that migration involves ecological trade-offs between predation and energy acquisition (the p/g model).
The next puzzle is to ask why certain individuals adopt a migratory or resident strategy. In fitness terms, are these strategies equivalent? Is the behaviour genetically fixed or phenotypically plastic? We will keep you posted…
//Ben et al.