Seasonal cycle in vitamin A1/A2-based visual pigment composition during the life history of coho salmon (Oncorhynchus kisutch).

Microspectrophotometry of rod photoreceptors was used to follow variations in visual pigment vitamin A1/A2 ratio at various life history stages in coho salmon. Coho parr shifted their A1/A2 ratio seasonally with A2 increasing during winter and decreasing in summer. The cyclical pattern was statistically examined by a least-squares cosine model, fit to the 12-month data sets collected from different populations. A1/A2 ratio varied with temperature and day length. In 1+ (>12 month old) parr the A2 to A1 shift in spring coincided with smoltification, a metamorphic transition preceding seaward migration in salmonids. The coincidence of the shift from A2 to A1 with both the spring increase in temperature and day length, and with the timing of seaward migration presented a challenge for interpretation. Our data show a shift in A1/A2 ratio correlated with season, in both 0+ (<12 months old) coho parr that remained in fresh water for another year and in oceanic juvenile coho. These findings support the hypothesis that the A1/A2 pigment pair system in coho is an adaptation to seasonal variations in environmental variables rather than to a change associated with migration or metamorphosis.