DETECTABILITY OF CRYPTIC JUVENILE TROCHUS-NILOTICUS LINNAEUS IN STOCKENHANCEMENT EXPERIMENTS

Juvenile Trochus niloticus are very cryptic because of their small siz e and the complexity of the reef surface where they live. In this stud y we addressed the question of how the cryptic nature of juvenile T. n iloticus can affect survival estimates in experiments to investigate s eeding as a tool to enhance overfished stocks. We compared recapture r ates 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 shelf. One experiment was done at Orpheus Island in Australia, and another at Moso Island in Vanuatu. For each experim ent the areas were censused 2 or 3 days after release. At Orpheus Isla nd, 76% of the flagged trochus were recaptured compared with 52% of th e 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 juve nile size of 30 mm SW. Without appropriate correction, survival rates could be underestimated by 30% at Orpheus Island and 19% at Moso Islan d. As indicated by our results, sighting probability will vary with lo cality and size. Thus, we recommend the estimation of sighting probabi lity of juvenile T. niloticus as an essential component of seeding exp eriments.