Effects of castration on the smoltification of precocious male masu salmon Oncorhynchus masou.

Abstract

Underyearlings of precocious male masu salmon were castrated in August and then were reared until next April under natural conditions to study the effect of castration on the induction of smoltification. Forty seven out of 196 castrated fish which survived until April were completely castrated. Of these fish, larger ones became smolt, whereas smaller ones did not become smolt. Other 149 castrated fish had a small piece of remaining testis and smoltification did not occurr. All the sham operated fish had a small testis and were parr. Plasma androgen levels were low (about 1ng/ml) in both partially castrated and sham operated fish in April. These results indicate that precocious maturation inhibits the parr to smolt transformation and plasma androgen concentration of about 1 ng/ml is probably high enough to prevent the transformation. Growth rate is also an important factor in inducing smoltification as well as gonadal immaturity.