STOCK ASSESSMENT OF TROCHUS (TROCHUS-NILOTICUS) (GASTROPODA, TROCHIDAE) FISHERIES AT WEST NGGELA, SOLOMON-ISLANDS

Stock abundance estimates of trochus at four sites at West Nggela were obtained using the Peterson mark-recapture technique. Densities were calculated using reef areas obtained from high resolution aerial photo graphs. Densities on most reefs were apparently low compared to well-m anaged trochus fisheries elsewhere in the Pacific. A further six reefs did not have sufficient numbers of trochus for an estimate to be made using mark-recapture. Egg-per-recruit (EPR) and yield-per-recruit (YP R) analyses were performed using total mortality (Z) estimates derived from size-frequency data from harvests at a number of sites, and two growth rates drawn from previous studies on the Great Barrier Reef. Na tural mortality (M) was varied from 0.2 to 0.6, which covers most of t he range of M seen in previous studies. EPR and YPR were modeled for a range of fishing mortalities (0-2.0) using the same growth rates and range of M. At West Nggela, trochus are typically fished down to a bas al shell diameter of 5.5-6 cm, despite a legal minimum size limit of 8 cm. EPR models showed that egg-production increased, though not drama tically, using the higher (i.e., legal) minimum size limit. YPR showed increases in yield when using the higher size Limit, except at the sl ower growth rate and M=0.6. Both models were sensitive to small change s in growth rate and M. At M=0.2, egg production at all sites was at o r below 20% of virgin stock levels for both minimum size limits and bo th growth rates tested, at the current levels of fishing. Possible man agement measures are discussed, including reduction of fishing effort, reseeding of reefs with cultured juveniles, and establishment of smal l reserves within the boundaries of each ownership group. Enforcement of a higher minimum size limit at the market end is recommended as the most practical and expedient of the available options. (C) 1997 Elsev ier Science B.V.