Dispersal without Gene Flow: Can Arctic Char Have Their Cake and Eat It Too?

Dispersal has important consequences for the persistence of populations, and rates of dispersal tend to evolve as a balance between the costs and benefits of dispersing.   For instance, most anadromous salmonids home to their natal habitats to spawn, thus maintaining advantageous local adaptation. However, rates of dispersal are expected to be higher in species facing temporally unpredictable environments because it would allow them to hedge their bets spatially. Arctic char (Salvelinus alpinus) is a facultatively anadromous salmonid that inhabits the unpredictable environment of the Arctic. However, they also appear to exhibit high levels of local adaptation, which would be more consistent with a homing strategy. We use data from 15 microsatellite loci to quantify dispersal in Arctic char from 16 rivers around Cumberland Sound, Nunavut. We use comparisons between samples of juveniles (which cannot disperse because restricted to freshwater) and adults from the same rivers to test whether dispersal is higher than gene flow in this system. We find that juveniles are more genetically differentiated than adults from the same river. Further, we use assignment tests to show that fish that are going to spawn that year are slightly less likely to disperse than resting fish. This suggest that char home to their natal rivers for the purpose of spawning, but are less discriminatory in years where they forego reproduction. This pattern, whereby dispersal is higher than gene flow, could explain how Arctic char persist in such an unpredictable environment while remaining locally adapted to their river of origin.