Castell Perez, Laura, Naviti, William, and Nguyen, Felix (1996) Detectability of cryptic juvenile Trochus niloticus Linnaeus in stock enhancement experiments. Aquaculture, 144 (1-3). pp. 91-101.

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Abstract

Juvenile Trochus niloticus are very cryptic because of their small size and the complexity of the reef surface where they live. In this study we addressed the question of how the cryptic nature of juvenile T. niloticus can affect survival estimates in experiments to investigate seeding as a tool to enhance overfished stocks. We compared recapture rates of cultured trochus with only a numbered tag with that of flagged trochus, which had both a tag and an additional bright coloured piece of tape glued to the shell. One experiment was done at Orpheus Island in Australia, and another at Moso Island in Vanuatu. For each experiment the areas were censused 2 or 3 days after release. At Orpheus Island, 76% of the flagged trochus were recaptured compared with 52% of the unflagged trochus. Corresponding values for Moso Island were 68% for flagged trochus and 56% for unflagged trochus. This pattern of higher recapture of flagged trochus was highly consistent among experimental plots, supporting the hypothesis that a significant proportion of the trochus are overlooked by the observer.

Mean sighting probability of trochus after 2–3 days was 0.69 at Orpheus Island for an average size of 23 mm shell width (SW), and 0.81 at Moso Island for an average juvenile size of 30 mm SW. Without appropriate correction, survival rates could be underestimated by 30% at Orpheus Island and 19% at Moso Island. As indicated by our results, sighting probability will vary with locality and size. Thus, we recommend the estimation of sighting probability of juvenile T. niloticus as an essential component of seeding experiments.

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