Studies on precocious maturity in artificially reared 1+ Atlantic salmon parr Salmo salar L.

Abstract

ABSTRACT Studies on the testicular cycle of commercially reared precocious male Atlantic salmon have shown that spermatogenesis began in late spring/early summer and was completed by November/December. The spermatogenic cycle could be divided into three phases: (i) Formation of spermatogonia which takes place during spring and summer. (ii) Development of spermatids which occurs from August to December. (iil) Spermiation which takes place from December until the next cycle begins. Androgen secretion increased during the sexual cycle and was at a peak in November. Two types of basophils were identified in the meso-adenohypophysis and these resembled the gonadotrophic cells that have previously been described in salmonida. These precocious males had a greater interrenal activity than immature parr. Both immature and mature 1+ parr in this study exhibited epidermal hyperplasia during the winter and the epidermis was considerably thicker amongst the precocious males. The skin thickness varied from area to area being thickest in the caudal peduncle and thinnest in the pectoral area. There was a positive correlation between it and the body weight. There was a seasonal cycle in the superficial mucous cell concentration and the numbers of goblet cells increased in both sexes during autumn and winter. From October onwards, precocious males had significantly more mucous cells than immature females. The secretions of these cells were acid mucopolysaccharide with a large sialic acid component. The pectoral fin and tail were the regions most frequently infected with Saprolegnia and these had the greatest concentration of mucous cells. When 0+ parr were treated with exogenous steroids, it was found that androgens caused epidermal hyperplasia and that cortisol increased the mucous cell concentration in the fins and tail. Cortisol and oestrogen experimental groups had the greatest prevalence of Saprolegnia and the areas most frequently parasitised were the pectoral fin and tail. Comparison of mucohistochemical results from untreated 1+ parr and 0+ parr fed steroids suggests that the endocrine status of the animal may influence the chemical composition of the mucus secretion. Methallibure and cyproterone acetate failed to prevent gonad maturation when they were administered during the later stages of spermatogenesis. They did,' however, lower plasma 11-ketotestosterone levels and caused a decrease in the gonadotrophic cell size. When these drugs were fed at the beginning of the sexual cycle from early spring to August all the male parr in the methallibure-treated group were immature but 40% of the males in the cyproterone acetate group had developing gonads. None of the males in the methallibure experimental group showed any sign of gonad development 20 weeks after cessation of the treatment. The results of these experiments show that it is possible to prevent precocious sexual maturity in intensive aquaculture units using chemotherapeautic agents and that central inhibitors of gonadotrophic secretion are the drugs of choice.