When salmon struggle upstream for hundreds of kilometres from the ocean to their freshwater spawning sites they face a serious drop in salinity on the way. Fish biologists have long suspected that salmon `prepare' physiologically for their freshwater spawning trip. To test this idea, Mark Shrimpton and his Canadian colleagues caught wild sockeye salmon at various points along their migratory route and found that salmon do seem to prepare for freshwater entry(p. 4069).

Fish swimming in saltwater need to excrete unwanted ions, so they increase the activity of the sodium pump Na+/K+-ATPase in their gills. There are freshwater-specific and saltwater-specific isoforms of this enzyme, and Shrimpton's team wondered how the expression levels of these isoforms change as the fish move from seawater to freshwater and during the salmon's upstream migration. To find out, the team collected gill tissue samples from wild sockeye salmon at various points on their 1600 km migratory route from the ocean to spawning sites along the Fraser River in British Columbia. They measured gill Na+/K+-ATPase activity and also analysed the expression of the freshwater- and saltwater-specific Na+/K+-ATPase isoforms using real-time PCR. The team saw that gill Na+/K+-ATPase activity decreased as fish moved between three saltwater locations towards the river, which suggests that salmon do `prepare' before they enter freshwater. But once the fish are in freshwater, expression of the freshwater-specific isoform keeps increasing, so the team concluded that sockeye salmon's freshwater acclimation is not entirely complete before the fish enter the river.

Shrimpton, J. M., Patterson, D. A., Richards, J. G., Cooke, S. J., Schulte, P. M., Hinch, S. G. and Farrell, A. P. (
). Ionoregulatory changes in different populations of maturing sockeye salmon Oncorhynchus nerka during ocean and river migration.
J. Exp. Biol.