Addressing causes of early mortality in hatchery produced southern bluefin tuna larvae. Chapter 6: development of vision and feeding behavior of yellowtail kingfish larvae

High levels of variability in the survival, growth and malformation rates of YTK larvae (produced by different hatcheries, or even between different hatchery runs at the same hatchery), have been problematic in trying to better understand the causes of the bottleneck issues in YTK larval rearing. One of the key parameters that has differed between hatchery sites or runs with variable rearing outcomes has been the lighting regime used during early larval rearing. To address this, a series of experiments focussed on the effect of light and lighting on the feeding response of YTK larvae from 3 dph to 9 dph. Feeding response was measured as both the proportion of larvae eating the enriched rotifer diet, and the feeding intensity (mean number of rotifers consumed per larva). No 3 dph larvae fed in the dark, but at 6 and 9 dph some larvae did feed in darkness. An increasing light intensity (up to 101 μmol s^-1 m^-2) had a significant beneficial effect on the proportion of larvae feeding and the amount of food eaten, and this effect got more pronounced as the larvae aged suggesting that the larvae quickly became more effective feeders at lower light intensities. Another experiment compared the ability of 3 dph and 6 dph YTK larvae to feed in light that differed in spectral qualities. The result in 3 dph larvae was striking, with light spectrum having a major effect on feeding response with blue light inhibiting and red light stimulating feeding proportion and intensity. The effect was still evident, but less obvious, in 6 dph larvae. Prey density (ranging from 0.5 to 25 rotifers mL^-1) had no effect on feeding response in 3 dph larvae, but at 6 and 9 dph the higher feed densities increased the proportion of larvae feeding. Finally, an experiment testing the effect of a range of water turbidity values (0 to 60 NTU) showed some effects on feeding response with the 3 dph larvae not eating in clear water, and the highest level of algal turbidity (60 NTU) slightly reducing the proportion of 6 and 9 dph larvae feeding.