The management of broodstock Atlantic halibut (Hippoglossus hippoglossus) and the influence of nutrition, holding conditions and hormonal manipulation of spawning on gamete quality

Abstract

The Atlantic halibut {Hippoglossus hippoglossus) is an important marine species for aquaculture in higher latitudes. Its excellent growth rate in relatively cold waters, high market price and the decreases in wild captures, have provided the incentive for an expansion in interest for the farming of halibut. However, much work remains to be carried out to identify the nutritional, husbandry and environmental requirements of broodstock in order to establish a reliable supply of good quality gametes and larvae. This thesis describes the dynamics of oocyte growth and ovarian development of Atlantic halibut; the development of pelleted broodstock diets and the influence of levels of essential fatty acids in these diets on egg and larval quality; the effects of stocking density and recirculation technology on egg quality; and the uses of gonadotropin-releasing hormone analogue (GnRHa) implantation for the manipulation of gamete production. Histology, stereology, electron microscopy and ultrasound scanning were used to study ovarian and oocyte development. Oocyte growth followed the same general processes described for many teleosts. including primary growth, folliculogenesis, cortical alveoli formation, vitellogenesis, and final maturation. Vitellogenesis started from late September, continued during the spawning season, and accounted for some 13% of the final egg size. Final oocyte maturation included yolk coalescence and hydration; this was responsible for some 80% of the final egg size and produced a transparent, buoyant egg. GSl began to increase from late September, from a minimum value of 2.1 in pre- vitellogenic fish (September), to average values of 15.0 in pre-spawning fish (March). The maximum GSI value recorded for spawning fish (March) was 21.6. Ultrasound was used extensively for sexing mature and immature fish, studying ovarian growth throughout the year and monitoring final maturation of discrete batches of eggs. It provided a discriminating non-intrusive tool for the study of ovarian development. Pelleted broodstock feeds provided a similar performance, as far as egg and larval quality were concerned, as the wet-feed widely used by commercial farms. Atlantic halibut eggs selectively accumulated docosahexaenoic acid (DHA) and arachidonic acid (AA). Improved fertilisation rates, egg quality scores and hatching rates were achieved at concentrations of 16% DHA and 1.8% AA in the lipid fraction of the diets. In addition, relative concentrations of DHA and AA with respect to eicosapentaenoic acid (EPA) were important. DHA:EPA and EPA;AA ratios of 2 and 4 respectively appeared to provide improved egg and larval quality. No differences were found in the spawning performance and egg quality of broodstock Atlantic halibut maintained at stocking densities of 2.5 kg/m' and 5.21 kg/m^ respectively. Recirculation of chilled water in Atlantic halibut broodstock tanks had no deleterious effect on spawning performance or egg quality. Implants of 10 pg GnRHa/kg of body weight were effective in improving the spawning performance of previously unsuccessful female Atlantic halibut. Free oestradiol, sulphated 11-deoxycortisol, free androstenedione and sulphated 3p, 17, 20p-P-5p were the main reproductive steroids present in plasma of pre-spawning and spawning females. GnRHa implants were successful in reducing the spermatocrit (viscosity) of milt from male Atlantic halibut. This improved milt motility and fertilisation success. 17, 20p-P is suspected to play a major role in spermatogenesis of male Atlantic halibut. The role of 17-20-a-P is unclear and further research is required on its possible participation in spermiation and sperm motility acquisition. Implants of 25 pg GnRHa/kg body weight towards the end of the spawning season of male Atlantic halibut, once spermatocrit values are avove 90%, are recommended to enhance milt availability and quality. Key words: Atlantic haliby, oocyte growth, ovarian, ovarian development, ultrasound; broodstock diets; DHA; AA; EPA; stocking density; recirculation; GnRHa implantation; sex steroids