PRECOCIOUS MALES OF CULTURED ATLANTIC SALMON, SALMO SALAR L. IN THE SECOND SPAWNING SEASON

Background. Early maturation of salmon males (Salmo salar L.) affects the reduction of fish physical condition and culture results. On the other hand, precocious male can fertilize mature eggs. The aim of this study was to estimate the magnitude of precocious maturation and to evaluate semen characteristics in a group of cultured 1-year-old salmon. Material and methods. A total of 145 salmon males belonging to a group of low growth rate specimens that had not smoltified during the first spawning season were sampled from the “Aquamar” Fish Farm (Miastko, Poland). The study was based on light microscopy examination of histological sections and a standard procedure of milt quality evaluation. The gonadal development stage was determined with Billard and Escaffre’s 9-grade scale modified by Dziewulska. Results. The mean fork length of males was 10.45 cm. Three groups of males were distinguished: non-maturing (stage I); beginning spermatogenesis (inactive substage II); and precocious (stages VI to IX plus maturing males classified as undergoing “attempted spermatogenesis”). The groups contained 72.4, 4.8, and 22.8% of the males examined, respectively. The gonadosomatic index recorded in the three respective groups ranged from 0.010 to 0.164 (mean 0.040); 0.050–0.155 (0.089); and 0.058–6.219 (1.358). The gonadosomatic index is not an accurate indicator of gonadal activity. The precocious males semen contained from 6.1 to 23.0 million spermatozoa per mm3 (13.41 million on the average). Spermatozoa performing progressive movements constituted 80–90%. Conclusion. Among non-maturing males and males beginning spermatogenesis, precocious individuals were detected, the latter produced semen of good quality.