Credit: ISTOCKPHOTO.COM/ DON MACE

The annual migration of salmon upstream is a spectacular sight to tourists and hungry bears alike (see image). Their arduous swim is largely seen as the response to an internal drive to reproduce, but the effects of the journey go beyond the procreation of this predatory fish.

Once the salmon have arrived at their upstream spawning areas, hard-working female fish use their fins and bodies to mobilize sediments lining the streambed, digging small holes, or redds, of up to 50 cm deep in which they lay their eggs. In particularly popular mating areas, the redds can disturb the entire channel bed.

Although the hummocky surface of the nests is readily visible from August to May, the impact of the redds on overall sediment transport within salmon-filled streams has been unclear. Marwan Hassan at the University of British Columbia, Canada, and colleagues therefore used bed-load traps and magnetically tagged particles to analyse the effects of salmon activity in the Fraser River basin, Canada (Geophys. Res. Lett. 35, L04405; 2008).

In individual watersheds, salmon were responsible for up to 60% of sediments mobilized each year, mainly in the form of clays, silts and sands, which are easily resuspended by the digging fish. On average, salmon moved over half as much of these sediments as flood events did during the five years of the study. In years with minimal flooding, salmon were actually the primary drivers of sediment movement.

In their quest to dig a home for their offspring, salmon become a first-order control on sediment transport in their home streams. The relationship between habitat and biology is clearly not a one-way street.