Factors affecting the early life stages of hatchery-reared greenback flounder (Rhombosolea tapirina, G‚àö¬¿nther, 1862)
Creators: name: Hart, PR
Publisher: name: University Of Tasmania
Year: 2023

description: The major parameters affecting the viability of hatchery-produced eggs, larvae and newly metamorphosed juveniles of the greenback flounder (Rhombosolea tapirina, G‚àö¬¿nther, 1862), were investigated in order to design a suitable method of production. The production of good quality eggs was considered to be a major problem requiring further investigation. A fertilisation rate of approximately 50% was obtained with most batches of eggs. However, it was found that the broodstock matured normally, even in a completely artificial environment with temperature and photoperiod control. Consistent ovulation of mature eggs was induced artificially, using 'Ovaprim'. Fertilisation of eggs was carried out successfully both dry and in seawater. A salinity of 35-45%0 was found to be optimal for initial fertilisation with incubation able to be carried out in salinities ranging from 15-45%0 with no effect on subsequent hatch rates. The optimal temperature for incubation of eggs was 9-12¬¨‚àûC and incubation at 6 or 21¬¨‚àûC resulted in complete mortality. The time to 50% hatch ranged from 1,089 to 1,212 degree hours, depending on the incubation temperature. Fertilised eggs were buoyant in salinities above 27-28700. At the tail-bud stage eggs could be handled without affecting hatch rates. The optimal temperature for incubation of larvae at the yolk-sac stage was found to be 15¬¨‚àûC, as this temperature resulted in the fastest growth rates and the maximum length at complete yolk absorption. First-feeding occurred at approximately 96 hours post hatch (day 4) at 15¬¨‚àûC, after complete yolk absorption, but before the absorption of the oil droplet. The 'point of no return' occurred at day 6 post-hatch, and mortality of unfed larvae occurred at day 8 post-hatch. Rearing of the larvae to metamorphosis, was carried out successfully in black fibreglass tanks of 3, 25 and 1601 capacity, contained in recirculating systems and provided with light intensities ranging from 300-1,699 lux. The optimal photoperiod for maximum growth was found to be 18-24 hours light and the optimal temperature was 18-20¬¨‚àûC. Culture in salinities of 15%0 resulted in a slight increase in mortality, compared to higher salinities. Rotifers were found to be a suitable first-feed, with enriched instar II Artemia suitable from day 9 post-hatch onwards, when the gape height reached 0.69 mm and the larvae were 4.7 mm in length. The larvae commenced feeding at the water surface but moved to the tank base at approximately day 15 post-hatch at 15¬¨‚àûC. At this time they developed a dark colouration. They became lighter at approximately day 20 post-hatch and developed the juvenile colouration at approximately day 25 post-hatch. Metamorphosis was complete by day 30 post-hatch at a length of 12.4 mm and weight of 25 mg, under optimal conditions. The stomach was fully formed by day 20 posthatch. Enrichment of live feeds with artificial diets, rather than microalgae, resulted in considerable improvements in the growth rate but a higher incidence of malpigmentation in some trials. Weaning was carried out successfully from day 23 post-hatch using a 10 day changeover period from live food to artificial food. The weaning diets tested, and the stocking density (between 5-20 individuals/I), had no effect on survival or growth rates. Changeover periods of 10 or 20 days had no significant effect on growth or survival, but a 5 day changeover period resulted in poor growth. Weaning before day 50 post-hatch resulted in the highest survival rates. Larvae showing the highest growth rates prior to weaning were easier to wean, and therefore, larvae fed with live feeds enriched with commercial diets (containing high levels of highly unsaturated fatty acids), showed the best weaning response. Enrichment of live feeds with algae, for the first 15 days of larval rearing, resulted in the maximum post-weaning survival and growth rates. Growth rates and food conversion rates after weaning were good, and it appears that the most valuable market size (500 g), may be attainable in a 2 year growout period. However, there is a problem with dark pigmentation on the blind sides of most fish and a high incidence of skeletal deformity, possibly caused by a vitamin C deficiency at some stage in the life cycle. Early maturation of 1 year old fish may also be a problem if it results in reduced growth rates.