Vulnerability of Pacific threadfin juveniles to predation by bluefin trevally and hammerhead shark: size dependent mortality and handling stress
Abstract
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 trevally Caranx melampygus and hammerhead shark 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 h of acclimatization. Fish were released either gently (control group) or after the stress of 1-min air exposure (stressed group). When they encountered trevallies, all 70 mm or 100 mm threadfin were eaten in less than 7 min 10 s, whereas, 140 mm or 190 mm fish survived for at least 1 h. There was no difference between stressed and control group survival. In the case of shark encounters, there was no size dependent mortality, although individuals in the stressed group tended to be eaten more rapidly 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 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.
- Publication:
-
Aquaculture
- Pub Date:
- 2003
- DOI:
- 10.1016/S0044-8486(02)00254-5
- Bibcode:
- 2003Aquac.217..249M
- Keywords:
-
- Stock enhancement;
- Predation;
- Size-dependent mortality;
- Polydactylus sexfilis;
- Handling stress;
- Learning