Smolt 101

Juvenile Chinook salmon. Credit: Rodger Tabor/USFWS

I was talking with Ray Jones, a biologist from the Idaho Fisheries Resource Office this morning about our upcoming Spring Chinook Salmon release and the conversation took a very interesting turn towards the process of smolting. When an anadromous fish smolts it goes through a series of physiological, morphological and behavioral changes that readies its body for the transition from freshwater to saltwater habitats. The most noticeable change we see in smolting fish is body coloration. Pre-smolts, or parr show distinct pigment spots or bars. Smolting fish show a distinct silvering of their scales and underlying skin. This silvering is caused by the synthesis of guaine and hypoxanthine, two purines that are the result of protein catabolism (the breakdown of complex molecules into simpler ones).  If you think about the habitat a salmon smolt is transitioning to you can realize the importance of this adaptation. A slivery colored fish will blend into a pelagic (open ocean) environment much better than a brownish fish with spots and bars.

Chinook salmon smolt are silver in color and lose their parr marks.
Credit: Dr. Billy Conner/USFWS

There is a lot going on inside the fish too. There are a host of hormonal and cellular transformations that allow the fish to survive in salt water. These changes are very complex and are only seen in anadromous fish. It is amazing to know that these animals make the transformation from freshwater fish to saltwater fish and then one to four years later do the reverse!

Schooling Chinook and Coho smolts. Credit: USFWS courtesy of Yakama Nation Fisheries

So next time you see our friend the salmon, whether it be at the Hatchery, in the grocery store, or on the end of your fishing line, remember the journey these amazing fish undergo for their survival, and to benefit of all the animals (and people) that depend on them!

By Angela Feldmann

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