Experimental restocking and seasonal visibility of a coral reef gastropod assessed by temporal modelling

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Purcell, SW & Cheng, YW 2010, 'Experimental restocking and seasonal visibility of a coral reef gastropod assessed by temporal modelling', Aquatic Biology, vol. 9, pp. 227-238.

Article available on Open Access

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Assessing the survival of restocked marine invertebrates is complicated by crypticity and seasonal visibility of animals at release sites. We investigated local enhancement of trochus Trochus niloticus (Gastropoda) on 3 coral reefs in Western Australia. 12 000 tiny hatchery-produced juveniles, 1 to 4 mm basal shell width (BSW), were released onto each of 4 intertidal sites on each reef. Visual surveys were conducted at stocked sites and control sites on each reef at 3-monthly intevals. Juvenile trochus < 25 mm BSW hide within the reef matrix and are found rarely on the upper reef surfaces, so detectable enhancement was expected from 12 mo after release, when they would be at sub-adult size (25 to 50 mm BSW). Abundance estimates of wild sub-adults fluctuated seasonally, and appear related to heat stress; i.e. they tended to be more exposed on the reef surface in periods of higher cloud cover and/or lower temperature. A cyclical trend in the abundance of sub-adult trochus is attributed to seasonal variation in their visibility in censuses. Temporal changes in adult abundance showed no consistent pattern. Due to the lag in detecting stocking of sub-adults and the cyclical nature of population estimates, we tested stocking success using a nonlinear mixed-effects model. From 12 to 18 mo post-release, sub-adults at stocked sites were more abundant (54% increase) compared to control sites. Although statistically significant, the absolute enhancement was small. Variation in the stocking effect among sites underscores the need for multiple sites in restocking. Releases of tiny, cultured trochus juveniles could be used to rebuild nucleus breeding populations for fishery restocking, but improved release methods leading to higher survival of juveniles are needed to make this approach cost-effective.