Bon Voyage Coho!
There’s a lot of activity in the streams right now as millions of wild Coho salmon smolts (Oncorhynchus kisutch) head out on their seaward migration. This peaks in late May during the period of maximum low tides when the moon is full. An interesting fact is that seaward movement of coho smolts does not appear to be related to either stream water levels or temperature. Streamkeepers and scientists have confirmed that movement is actually connected to the lunar cycle!
Coho and other Pacific salmon undergo transformation from stream-dwelling fish with vertically oval blobs of dark brown pigment (parr marks) to silver seaward-migrating smolts.
Smolting is a physiological and bio-chemical change that enables smolts to prepare for marine life. Once in tidal waters, the smolts move between the top freshwater layer and lower saltwater layer to stimulate three main changes that they will need to adapt to a marine life in the ocean.
The first and most amazing change is osmoreregulation, meaning changes in gill, kidney, gut, and bladder function to help increase the salmon’s salt tolerance. Moving from freshwater to salt water is very hard to do and few other species of animals can do this.
As the salmon smolt, they lose their parr marks and become silver in colour to help camouflage them in the brighter ocean water.
Salmon smolts learning to school
In addition salmon learn to school, that is staying in large groups for protection in a more open environment. In the stream when startled,they all swim indifferent directions looking for cover- which is the best defense from predators. In the ocean they school, moving together, as their defense against predators.
Follow @psf