METHOD OF ESTIMATING THE STANDING STOCK OF TROCHUS-NILOTICUS INCORPORATING LANDSAT SATELLITE DATA, WITH APPLICATION TO THE TROCHUS RESOURCES OF THE BOURKE ISLES, TORRES STRAIT, AUSTRALIA

The habitat of Trochus niloticus in the Bourke Isles, Torres Strait, w as classified into areas of a Landsat image, using high-ratio values o f green (Band 2) to red (Band 3) light, along the windward reef margin s. These shallow-water (< 15 m) areas have a coral and rubble/algal pa vement cover, which constitutes the optimal habitat for this gastropod . The habitat was sampled to estimate the abundance of T niloticus, Th e proportion of commercial-sized individuals was estimated by measurin g the basal width of all individuals in a sample. A multistage sample design incorporating three spatial scales - 100 m2 (transect), 1500 m2 (site) and 1 km2 (reef) - was used to provide variance estimates for sample-design optimisation and to provide data on the spatial variatio n of abundance. Most variation (68%) in abundance was within reefs and was attributable to differences in reef cover. Variations in abundanc e and time costs for sampling 2 and 4 m transects were compared; the 2 m transect was more efficient than the 4 m transect. The abundance es timates were combined with habitat-area estimates and the proportion o f commercial-sized individuals was estimated at a standing stock of 18 6000 (24% precision), or 14 t of commercial-sized T. niloticus.