抄録
The Japanese anchovy Engraulis japonicus is used as live bait in skipjack pole-and-line fisheries. Japanese anchovies are transported to fishing grounds by fishing boats on voyages that can take from 4 to 50 days. Because ammonia excreted by Japanese anchovies might affect their survival during transportation, fishers empirically attempt to maintain them in healthy condition by exchanging water aboard ship holding tanks at a rate of 160–500 % volume h−1. To develop protocols for more efficient and cost-effective transportation of Japanese anchovies, we evaluated their ammonia tolerance and effects of stocking density and variable water exchange on their survival. We estimated median lethal concentrations (with 95 % confidence intervals) of un-ionized ammonia nitrogen (UIAN), at which 50 % of Japanese anchovies had died within 24 h and 48 h, as 0.770 (0.751–0.790) mg l−1 and 0.706 (0.661–0.750) mg l−1 at 15 °C and 0.634 (0.466–0.802) mg l−1 and 0.450 (0.379–0.521) mg l−1 at 25 °C, respectively. While the UIAN concentration increased to lethal levels and severe fish mortality (dependent on stocking density) occurred at 25 °C with no water exchange, the UIAN concentration could be maintained within non-lethal levels at 25 °C and with a one-third water exchange every 12 h. Our results suggest that present-day empirically deduced water exchange rates applied aboard commercial vessels in holding tanks of Japanese anchovies could be reduced, implying a possible cost reduction for the skipjack pole-and-line fisheries.
