Dam evolution: Rapid evolution of fish migration in response to novel river barriers

Background/Question/Methods

Animal migrations are amazing phenomena that connect disparate habitats, exerting large influence over recipient ecosystem dynamics. However, migrations may also make populations more vulnerable to anthropogenic impacts such as blockage of migration corridors. A population’s fate in the face of a novel threat depends on the adaptive potential of the underlying traits that control migration. Salmonid populations express a range of migration tendencies, including partial migration. Dams are threats to migratory salmonids across their range; where dams bisect partially migratory populations their persistence upstream requires adoption of the completely resident form. Whether fish in these populations undergo migrations is dependent on the genetic and or environmental conditions. Here we examine the evolutionary response and demographic consequences of selection against migration in partially migratory populations. 

Results/Conclusions

To test the potential for adaptive evolution in response to migration barriers we performed a common garden experiment with a pair of partially migratory rainbow trout populations recently isolated by a barrier waterfall. We found that above barrier offspring expressed the migration phenotype at a significantly lower rate. Furthermore, expression of the migratory phenotype was conditional on exceeding a threshold body size, and this threshold was larger in the above barrier population. We propose that this is an evolutionary response to directional selection in favor of threshold sizes larger than can be attained in a normal growing year. As a result, the above barrier population persists due to selection for thresholds that favor the resident form, yet likely maintains some cryptic genetic variation for migration should conditions change. Thus, rapid evolution can provide population resilience to barrier creation. We use these results to parameterize a model to simulate demographic and evolutionary consequences of dam creation and removal. These simulations predicted that population size decreases following dam construction due to the loss of individuals that migrate out of the system and due to the removal of the more fecund migrant adults. We find that, assuming heritability of the threshold size trait, populations can persist--albeit at a lower size--via rapid evolution to the resident state. Following dam removal, re-evolution of the more productive, migratory populations can occur. However, depending on the parameters, this re-evolution of anadromy occurs over longer timescales--if at all. These results not only have important management implications for salmonid recolonization efforts, but also inform understanding of how migratory species in general may respond to changes in landscape connectivity.