Variation in the degree of silvering of wild coho salmon, Oncorhynchus kisutch, smolts migrating seaward from Carnation Creek, British Columbia

Degree of silvering (colouration) is often used as an index of the degree of srnoltification in salmonids. To judge the reliability of silvering a s an indicator of migratory readiness, we examined the silvering of wild coho salmon, Oncorhynchus kisutch, smolts upon entry into the Carnation Creek, British Columbia, estuary. Silvering of migrants was greater in larger fish and increased over the course of the migratory period. Photoperiod appeared to account for most of the observed increase in silvering over time; increased silvering was not correlated with temperature or lunar phase. Overall, 50% of seaward migrants were completely silvered. 45% were in a transitional phase, and 5% still retained colouration characteristics of parr. Colouration thus appears to have limited utility as a reliable indicator of migratory readiness. Our findings suggest that estuarine residence is important for completion of parr-smolt transformation and that acclimation of hatchery smolts in brackish water prior to sea water entry may enhance their survival.