VULNERABILITY OF PACIFIC THREADFIN JUVENILES TO PREDATION BY BLUEFIN TREVALLY AND HAMMERHEAD SHARK: SIZE DEPENDENT MORTALITY AND HANDLING STRESS

Reiji Masuda and David A. Ziemann

The Oceanic Institute, 41-202 Kalanianaole Hwy., 
Waimanalo, HI 96795 USA
ziemannda@oceanicinstitute.org

Aquaculture , in press

Predation is hypothesized to be a major cause of post-release mortality in stock enhancement projects. We evaluate critical size and release condition of Pacific threadfin Polydactylus sexfilis juveniles in regard to their ability to avoid potential predators such as bluefin trevallies Caranx melampygus and hammerhead sharks Sphyrna lewini. Four different sizes (70, 100, 140 and 190 mm in FL) of threadfin juveniles were released into experimental tanks with predators (5-6 individuals of one of above species) after 24 hrs of acclimatization. Fish were released either gently (control group) or after the stress of 1-min air exposure (stressed group). When they encountered trevallies, fish released at 70 mm or 100 mm were eaten within 7 min 10 sec in maximum, whereas fish released at 140 mm or 190 mm survived for a minimum of 1 hr. There was no difference between stressed and control group. In the case of encountering sharks, there was no size dependent mortality, although individuals in the stressed group tended to be eaten more easily than individuals in the control group. For both predators heavy predation occurred only in the first hour after the release, suggesting that hatchery-reared fish can learn how to avoid predators in a relatively short period. In the practice of stock enhancement we suggest the importance of assessing critical size and species of potential predators before release.