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

The aquaculture of the sea bass is now well established in Mediterranean co untries. However, under culture conditions the sexual maturation is an unde sirable process that reduces growth. In addition, spermiation around the ti me of marketing of fish is another inconvenience. In an attempt to solve th ese problems, triploidy was induced in the sea bass according to a previous ly published protocol. A cold shock applied starting 5 min after fertilizat ion for 10 min at 0 degrees C was used for the triploidisation of a large v olume of sea bass eggs. Control and shocked fish were reared in duplicate g roups under natural conditions of temperature and photoperiod using standar d procedures for sea bass aquaculture. Triploidy was determined in a sample of fish at 3 months of age by karyotyping (2n = 48; 3n = 72), and individu ally checked at 15 months of age by measurement of the major axis of erythr ocytes (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 performa nce were found between ploidies from 5 to 23 months of age (2n = 172.02 +/- 2.73 g vs. 3n = 158.94 +/- 2.68 g). Macroscopic observation of the gonads showed marked effects in triploid females, which had filiform ovaries. Hist ological analysis showed that diploid females had ovaries with developing o ocytes, 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, go nadosomatic 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 d iploids. The sex ratio in triploids was not different from that usually fou nd 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 marke dly affects the gonadal development of females. Nevertheless, it is require d to determine the comparative growth between diploids and triploids in old er and larger fish, as well as to analyze the gonadal development of larger triploids in this species. (C) 1999 Elsevier Science B.V. All rights reser ved.