Improvement of intensive larval rearing and evaluation of inland saline groundwater for aquaculture of snapper, Pagrus auratus

This thesis identified optimal physical rearing regimes for Australian snapper, Pagrus auratus larvae, the suitability of saline groundwater for snapper culture, and the effects of salinity and potassium-deficient saline groundwater on osmoregulation of snapper. The effects of photoperiod, salinity and temperature on growth, survival, onset of feeding, swimbladder inflation, presence of urinary calculi and tail flexion of first-feeding (3 days after hatching; dah) to pre-metamorphosis (21-32 dah) snapper larvae were determined in a series of factorial experiments conducted in specially designed, replicated, 100-1, cylindroconical tanks. The optimal photoperiod changed during larval ontogeny based on success of initial swimbladder inflation and subsequent growth. Snapper larvae tolerated a wide range of salinities from near-isoosmotic to hypersaline environments but optimal salinity for growth and development was from 20 to 35%o. Snapper larvae tolerated a relatively narrow range of water temperature from 15 to 24°C and larval growth increased as temperature was increased. The performance of snapper larvae from 4-33 dah under a "new" regime that combined optimal salinity (20-35%0), temperature (24°C) and photoperiod (12L:12D to swimbladder inflation, then 18L:06D), determined systematically in a series of experiments, was compared with a previous "best-practice" regime of salinity (35%0), temperature (21 °C) and photoperiod (14L:10D) in 2000-1 commercial-scale larval rearing tanks. Larvae reared in the "new" regime grew and developed more quickly than larvae in the previous "best-practice" and by 33 dah were fully weaned from live feeds to a pellet diet. Approximately eleven hatchery cycles per year are possible when larvae are reared under the "new" regime compared with seven hatchery cycles per year for the previous "best-practice" regime. The suitability of saline groundwater (SG; —20%0) from inland New South Wales for growth and survival of juvenile snapper was determined in a series of replicated 7-8 d survival bioassays and 42 d growth studies in tanks of 2-1 or 100-1, respectively. Raw SG was very deficient in potassium compared with similar salinity coastal seawater (CS) and snapper died within 2-3 d after transfer from CS to rawSG. However, growth and survival of snapper was the same in SG and CS, provided the [K+] in SG was increased to 60-100% of the [K+] in CS by adding KC1. It was not possible to acclimate snapper to rawSG by dilution of [K+] over time. The effects of rapid transfer of juvenile snapper from seawater (30%0) to near-isoosmotic (15%0) and hypersaline (45%0) seawater, and from seawater (30%0) to SG (30%0) fortified with 0 (rawSG), 40% (SG40) and 100% (SG 100) of [K] in seawater on serum osmolality, [Na+], [K+], [Cl-], blood haematocrit and branchial chloride cell morphology were assessed during 168 h after transfer. Changes in serum chemistry occurred rapidly in fish transferred to 15%0 and 45%0 but had returned to near initial levels after 168 h. Restoration of homeostasis was concomitant with changes in the number and size of immunoreactive chloride cells. Serum chemistry of fish transferred to SG40 and SG100 was similar, in general, to the initial levels. However, serum osmolality, [Cl-], [Na+] increased and serum [K+] decreased rapidly after transfer to rawSG. The morphology of chloride cells was unaffected by SG treatments. The research described in this thesis has provided an experimental base for new culture conditions to greatly improve survival and growth of snapper larvae through to fingerlings and survival and growth of snapper juveniles in inland saline groundwater. These new culture conditions represent major cost savings to hatchery operations and improve the aquaculture potential of snapper in Australia.