Theme D: Developing optimal release strategies
44. SHELTER ACCLIMATION DECREASES THE POST-RELEASE PREDATION MORTALITY OF HATCHERY-REARED BLACK-SPOT TUSKFISH CHOERODON SCHOENLEINII
Yuuki Kawabata1, Kimio Asami2, Masato Kobayashi2, Taku Sato2, Koichi Okuzawa2, Hideaki Yamada2, Kenzo Yoseda2, Nobuaki Arai1
1 Institute For East China Sea Research, Nagasaki University, Taira-machi 1551-7, 851-2213 Nagasaki, Japan
2 Ishigaki Tropical Station, Seikai National Fisheries Research Institute, Nagasaki University
Black-spot tuskfish Choerodon schoenleinii is a highly prized commercial fish in many areas of Asia, including Japan, Hong Kong and Malaysia. In Okinawa Prefecture, Japan, the black-spot tuskfish has been targeted for stock enhancement since 2000, due to its low total catch in the last decade. Around the coasts of Ishigaki Island in Okinawa Prefecture, hatchery-reared fish of 50–100 mm total length (TL) were experimentally released onto dead coral patches because the fish at least over 90 mm TL utilizes a burrow-like shelter which it excavates at the base of hard substrates, such as dead corals, for predator avoidance. However, some released fish were found in the stomachs of large piscivores and all released fish disappeared from the release site within 2 weeks (Okuzawa et al., unpubl. data, 2009). Post-release predation mortality is a plausible cause for this rapid disappearance; therefore, developing release strategies that reduce post-release predation mortality is a priority for ensuring the success of the stock enhancement of this species.
In this study, we investigated the effect of pre-release shelter acclimation whether it enhances the shelter utilization by tuskfish and consequently decreases the post-release predation mortality. We first performed laboratory experiments to investigate whether acclimation to shelters affects the post-release survival of hatchery-reared black-spot tuskfish in the presence of a reef resident predator, the white-streaked grouper Epinephelus ongus. Tuskfish juveniles were exposed to groupers under three different experimental conditions/treatments: (1) acclimation of fish to shelters prior to their exposure to groupers; (2) no acclimation of fish to shelters, but with shelters available during their exposure to groupers; (3) fish not acclimated to shelters and no shelters available during their exposure to groupers. Tuskfish that were acclimated to shelters utilized shelters more frequently than did non-acclimated fish, and the survival rate of acclimated fish was higher than those of fish in the other treatments.
We then conducted field experiments using acoustic telemetry. We acclimated four tuskfish juveniles to shelters in cages before release, and monitored their movements with six non-acclimated fish. While 67% of the non-acclimated fish showed untypical movements before cease of the detections that suggest the predation event occurred, none of the acclimated fish showed the untypical movements. Based on the detection pattern, survival rate one month after release was estimated. None (0 %), two (50 %) and two (50 %) of the acclimated fish were estimated as preyed, survived and un-known, respectively, while, four (67 %), none (0 %) and two (23 %) of the non-acclimated fish were estimated as preyed, survived and unknown, respectively.
These results suggest that the shelter acclimation decreases the post-release predation mortality of hatchery-reared black-spot tuskfish. Since many sedentary juveniles utilize shelters for predator avoidance, this acclimation approach might be useful for mitigating the post-release predation mortality of the other shelter-dwelling species for stock enhancement.
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