Growth and gonadal development in triploid sea bass ( Dicentrarchus labrax L.) during the first two years of age

Abstract

The aquaculture of the sea bass is now well established in Mediterranean countries. However, under culture conditions the sexual maturation is an undesirable process that reduces growth. In addition, spermiation around the time of marketing of fish is another inconvenience. In an attempt to solve these problems, triploidy was induced in the sea bass according to a previously published protocol. A cold shock applied starting 5 min after fertilization for 10 min at 0°C was used for the triploidisation of a large volume of sea bass eggs. Control and shocked fish were reared in duplicate groups under natural conditions of temperature and photoperiod using standard procedures for sea bass aquaculture. Triploidy was determined in a sample of fish at 3 months of age by karyotyping (2 n=48; 3 n=72), and individually checked at 15 months of age by measurement of the major axis of erythrocytes (1.2–1.3 times larger in shocked fish than in controls; P<0.001). These latter analyses verified that the shocked groups were 95–97% triploid. No statistically significant differences in mortality and growth performance were found between ploidies from 5 to 23 months of age (2 n=172.02±2.73 g vs. 3 n=158.94±2.68 g). Macroscopic observation of the gonads showed marked effects in triploid females, which had filiform ovaries. Histological analysis showed that diploid females had ovaries with developing oocytes, while triploid females had rudimentary gonads. In contrast, diploid and triploid males showed similar gonadal development. However, at 1 and 2 years of age testes with spermatozoa were observed in diploid males, while in triploid males this condition was never achieved. At 2 years of age, gonadosomatic index (GSI) in triploid females was equivalent to only 16% that of diploids while that in triploid testes GSI was reduced to 40% that of diploids. The sex ratio in triploids was not different from that usually found in cultured diploids of this species (i.e., 3 males:1 female). Thus, the results obtained show that until 2 years of age triploids present a growth performance similar to that of diploids and that the triploidisation markedly affects the gonadal development of females. Nevertheless, it is required to determine the comparative growth between diploids and triploids in older and larger fish, as well as to analyze the gonadal development of larger triploids in this species.