Is sinking mortality in southern bluefin tuna larvae caused by high light intensity?

The southern bluefin tuna (Thunnus maccoyii) aquaculture industry in Australia is based on fattening wild-caught juveniles and industry development is constrained by a quota system. Land-based aquaculture (from egg) is now being investigated to increase production and the scope of the industry. T. maccoyii experience major mortality during the first two weeks of culture - up to >95% of the cohort - and high larval mortality is not uncommon among cultured tuna species (Margulies, 1997). In general, marine fish larvae are visual feeders and, therefore, require light to feed and avoid predators; consequently, light has been identified as an important factor affecting survival. Larvae generally possess a pure cone retina (simplex retina), which presumably limits visually guided feeding to conditions of relatively high light intensity. However, unlike the majority of marine fish larvae, the culture of T. maccoyii under high ambient light intensities may not be conducive to early survival Cultured tuna larvae are often documented to "sink" to the tank base where the subsequent exposure to high detritus and bacterial loads and hard surfaces are thought to result in major mortality (Tanaka et al., 2009). We hypothesise that sinking mortality in T. maccoyii is due to the larvae actively migrating away from areas of high light intensity.