Some Morphological and Biochemical Changes in Coho Salmon,Oncorhynchus kisutch, During Parr–Smolt Transformation

Abstract

Growth rates and changes in some chemical constituents were investigated in laboratory-reared and wild coho salmon in the pre-smolt, smolting, and early post-smolt stages of their life cycles.Laboratory-reared pre-smolts of coho salmon continued to grow during fall and early winter but at an ever diminishing rate. During late winter, spring, and early summer, growth in both length and weight of these fish was exponential with time. In wild coho pre-smolts there was no apparent winter growth but, with the onset of warmer waters and longer day lengths in the spring, growth in both length and weight became exponential with time. Exponential growth continued in wild post-smolts held in seawater pens but at a slower rate than during smolting.It is postulated that the power b in the weight–length formula W = aL^bis approximately 3.2 and that during smolting, when the fish are becoming more streamlined, the parameter a decreases from approximately 7.5 to about 6.2.Guanine levels in belly skin, which appeared to be related to lighting conditions and physiological state of the fish rather than to length or weight, increased during smolting to about 6 μg/mm²skin and remained at these levels.In pre-smolt laboratory-reared fish total lipid increased with increasing growth. With further weight increases during exponential growth, the weight of lipid remained relatively constant and the amount of lipid per unit weight decreased. A decrease in percent lipid did not occur in wild fish.Total moisture, dry material, and nitrogen were linearly related to weight in the size-range investigated, but a sharp inflection in each least-square fit line occurred for moisture and dry material at a fish weight of 15 g and for total nitrogen at 7.5 g. These shifts in composition were related to fish size rather than to age, environment, or physiological state. During exponential growth, moisture, dry material, and nitrogen increased with increasing weight but whereas moisture increased at a slower rate than weight, solids and nitrogen increased more rapidly.