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  By Harvey Thommasen, with Photography by Mike Wigle
During smoltification juvenile fish lose their cryptic parr markings and become silvery in appearance. The keen observer will notice that these smolts are also slimmer and more streamlined than their parr counterparts. These changes are of adaptive significance, slimmer more streamlined fish can move more quickly in the ocean environment. A silver bodied fish is much less conspicuous against the silvery-white ocean surface than is a spotted fish. Less obvious physiological and behavioral changes also take place during smoltification. These smolts develop an increased ability to deal with salinity changes, useful when considering the marked change in salinity the young fish will have to cope with once it moves from fresh water to salty waters of the Pacific Ocean. These smolts become restless, lose their territoriality instincts, and become semipelagic as they begin to increasingly feed in the upper portions of the water column. They also show a marked tendency to school with other smolting juveniles of the same species. These changes only occur in juvenile fish that have achieved a critical size during the appropriate time in early spring.
The pink and chum salmon are an exception to the rule. For these salmon species, the fry that emerge from the eggs can be considered smolts because they head downstream almost immediately after emerging from the gravels. The chinook salmon covers all bases; some of the emerging fry head downstream to the estuary almost immediately after emerging from the gravels, some fry stay in the river for three months (the so-called 90-day smolts) prior to moving downstream, and the remaining fry stay in their stream for a year before smoltification takes place and the smolts head downstream. The different species of salmon smolt in a predictable sequence through spring. In a typical river on the British Columbia coast, for example, chum fry are the first smolts to migrate out of the river system. Typically chum smolt migration begins the last week of February, shortly after emergence from gravels, and peaks (March 19-26) by the last week of March and is over by the last week of April.
In general, the biggest smolts have the greatest chance of surviving the rigors of downstream migration and ocean life, and returning to spawn as mature salmon or sea-run trout. The relationship between steelhead smolt size and survival has been studied for over ten years in the Koeye River on Vancouver Island. In this river, freshwater age 2 smolts average 153 mm in length, 33 gms in weight; age 3 smolts average 177 mm in length, and 48 gms in weight; and age 4 smolts average 218 mm in length and 83 gm in weight. The average survival for age 2 smolts is 4%, survival for age 3 smolts is 15%, age 4 smolts is 25%. So size does make a difference. Once out in the ocean, smolts begin to ravenously dine on euphausids, baitfish, crab larvae and squid. They could never find such a rich diet in the freshwater streams where they hatched - and that is no doubt why they make the perilous journey to the sea, returning only when they are fully grown, and ready to spawn. |
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The longer, warmer days, and increasing water temperatures of late winter and early spring, triggers a physiological response in juvenile salmon, trout, and char residing in coastal streams. This physiological response is called smoltification, and it results in fry transforming into smolts.
For steelhead these changes appear to occur most frequently when the fish are in the 14 to 16 cm range. These fish may be 1 to 3 years of age. 
Then comes the pink fry emergence, the coho, the chinook, the cutthroat, sockeye, and then the steelhead smolts. By mid-July all smolting fry have moved out of the system.
