The effect of dietary DHA and taurine on rotifer capture success, growth, survival and vision in the larvae of Atlantic bluefin tuna (Thunnus thynnus)

Abstract

The severe to complete mortality that occurs during the larviculture of Atlantic bluefin tuna (ABFT; Thunnus thynnus) may be due, in part, to sub-optimal neural and eye development. The adult and larval ABFT eyes are rich in docosahexaenoic acid (DHA; 22:6n-3), which facilitates key intra-membrane reactions in the photoreceptors of the retina. Another critical nutrient is taurine, which plays vital roles that include bile salt conjugation as well as development and function of visual, neural and muscular systems. The objectives of the present study were to (1) determine the pattern of conservation and loss of fatty acid groups and their constituent fatty acids during egg and pre-larval development as well as in food deprivation. (2) Determine the effect of rotifer (Brachionus rotundiformis) DHA on hunting success, growth, and retinal opsin abundance in 2–14dph ABFT larvae. (3) Evaluate the effect of supplemented taurine in rotifers enriched on the most effective DHA level from objective (2) on larval survival and growth.During the egg and yolk sac larval stages, there was a significant decrease (P<0.05) in SAT, MONO and PUFA that can be expressed as 46.7%, 50.3% and 57.1%, respectively. Similarly, the levels of DHA, EPA, and ArA were markedly (P<0.05) reduced that can be expressed as 59.8%, 52.5% and 59.5%, respectively. In the DHA study, there was a rotifer DHA dose dependent (P<0.05) effect on prey consumption by 3–7dph ABFT larvae where the highest DHA level (11mgg−1 DW rotifer) elicited significantly (P<0.05) higher rotifer consumption compared to the control and moderate DHA diets (2mgg−1 DW and 5mgg−1 DW rotifer, respectively). Moreover, larvae with the highest DHA level (7.01mgg−1 DW) exhibited a significantly (P<0.05) higher opsin protein concentration (25.27 unit area−1) compared to the 2.83mgDHAg−1 DW and 1.26mgDHAg−1 DW fish (20.32 and 16.33 opsin protein unit area−1, respectively). Although there was a significant (P<0.05) taurine modulated increase in larval length in 10dph fish, there was a non-significant (P>0.05) growth advantage, in terms of dry weight, as a result of moderate dietary taurine supplementation at the end of the study. Nevertheless, the moderate 6.44mgtaurineg−1 DW larvae exhibited markedly (P=0.024) better survival and >4 times higher (P=0.0018) average tank biomass (273.6mg) than the low (1.97mgg−1 DW) and high (12.62mgg−1 DW) taurine fish (62.14 and 56.90mg, respectively). Overall, the data suggests that supplementing effective levels of DHA and taurine contributes to an array of physiological processes resulting in enhanced vision and prey acquisition to markedly improve ABFT larval performance during early development.