Early morphological predictors of maturity in one-sea-winter Atlantic salmon

Early separation of maturing from immature fish would be very valuable in Atlantic salmon (Salmo salar) culture. Possible morphometric predictors of maturation in one sea-winter Atlantic salmon were investigated from measurements of body weight, length and depth and adipose fin length undertaken at 14 day intervals from May until August in a population of 80 individually tagged fish, 61 (male and female) of which were subsequently found to be maturing (i.e. grilse). Stepwise discriminant analysis revealed that a combination of forklength and condition factor proved to be the strongest and most consistent predictor of maturation (over 88% accurate) until August. In August when maturing fish could be easily distinguished by shape and colour, forklength and condition factor no longer had any discriminatory power, but body depth at the anal and dorsal fins were good predictors. A second trial was performed on a separate stock of fish for which the same predictors proved robust. The discriminant functions derived from one stock of fish did not, however, produce the same discriminatory power when applied to the other stock. Hence there does not appear to be a simple mathematical means for the general discrimination of maturing and immature salmon. However the morphological predictors described above could be used for visual discrimination when ‘grilse grading’.