Improvements in yellowtail kingfish larval survival and juvenile quality

Jaw malformation assessment was completed by IMAS (formerly TAFI) for all Yellowtail Kingfish (YTK) larval tanks for five Clean Seas Tuna (CST) hatchery runs (from the Arno Bay and Port Augusta hatcheries) in 2008, and for most larval rearing tanks in five hatchery runs at CST in 2009. The overall proportions of fish with commercially significant jaw malformations at the end of the larval stage in each of the hatchery runs were 27 to 29% in 2008, and 32 to 35% in 2009. These values correlated with IMAS assessments using the same jaw malformation classification system at the end of the nursery phase of 17 to 35% and 25 to 38% in 2008 and 2009, respectively. As in 2007, there was considerable variation in malformation rates between larval tanks (7 to 64%), even when they were stocked with larvae from the same batch of eggs. In 2008 and 2009, fertilised YTK eggs were transported from CST to the Darwin Aquaculture Centre (DAC) and South Australian Research and Development Institute (SARDI) for rearing trials. Samples of larvae (from 1, 5, 10, 15, 20 DPH, and at transfer to the nursery and at handsorting) were sent to IMAS for assessment of jaw and other malformations. There was a high incidence of jaw (44%) and spinal (>30%) malformations in the YTK cultured at DAC in Jan-Feb 2009. Significant notochord (spinal) malformations were observed from Day 1, supporting other evidence such as low hatching rate, that the batch was compromised before larval rearing commenced. There was no effect of the Artemia feeding regimes tested at SARDI in 2008 on jaw malformation (25-41%) which was similar to the incidence in commercial hatcheries. Bacteriology methods were demonstrated to CST staff for diluted plate counts of water and homogenised rotifers, and serial dilution was recommended for ongoing testing. Biochemical analyses were completed for 20 fertilised egg samples and 36 samples of live feeds, with lipid composition, lipid class and vitamin content analysed. There were indications of variability between different batches of fertilized eggs and enriched live feed profiles, although it is not known how much these may have been due to differences in the way and duration for which the samples were stored prior to analysis. The implications (if any) of differences in these parameters to YTK culture are unknown without further experimentation. Histopathology of over 2,000 larvae and juveniles from commercial production did not indicate any significant pathologies, with the most notable finding being non-inflated swimbladders.