Pacific bluefin tuna, Thunnus orientalis, larvae utilize energy and nutrients of microbial loop

Abstract

Prey–predator interactions among planktonic organisms in larval tanks of bluefin tuna and red sea bream were investigated quantitatively, and carbon flow models were constructed from the results of feeding experiments. Two different carbon pathways were uncovered in the rearing water of both fish species. One was from autotrophic nano-plankton (ANP) to rotifers to fish larvae (artificial food chain) and the other was from bacteria to heterotrophic nano-plankton (HNP) to heterotrophic micro-plankton (HMP) (a microbial loop). Surprisingly, bluefin tuna selectively consumed HMP, especially dinoflagellates, throughout the experiment. In the bluefin tuna tank, the microbial loop was linked to the artificial food chain. On the other hand, red sea bream larvae did not consume HMP and the pathway in the red sea bream tank was more straightforward than in the bluefin tuna tank. Energy and nutrients transferred to red sea bream larvae via the artificial food chain only, except for 3 days after hatching. Biomass of micro-protozoa were high (3.2–158.4 ng C ml − 1) in the bluefin tuna tank with higher water temperature (26.0–27.6 °C) and low (3.0–4.9 ng C ml − 1 ) in the red sea bream tank with lower temperature (19.8–20.1 °C). The study suggests that difference in water temperature is a significant factor for the microbial loop in rearing tanks of fish larvae. This is the first report suggesting that a microbial loop, established naturally, contributes energy and nutrient gain to bluefin tuna larvae reared in an artificially controlled environment.