Research Article
George A Darpaah
Abstract
A new approach to post larval shrimp (PLS) collection using a locally designed PLS trawler has been successfully tried in the Volta Estuary of Ghana for 3 years (2000, 2002 and 2005). The PLS trawler had 4 major components: the pulley system, the post larvae (PL) collector, the control room and a work space. The PL collector had an aperture area of 0.433 m2 and a volume of 0.389 m3. Trawling for PLS was made at designated water depths at trawl speed of approximately 0.25 knots (0.128 m/s). Each trawl session lasted for 10 min and was replicated. The distance traversed by the PL collector in a trawl session was estimated at 77.07 m and covered a swept area of 33.40 m2. The associated volume of water filtered through the PL collector was 33.38 m³. The average number of PLS collected per trawl session ranged from 682 6 604 to 758 6 761 at high tide and from 324 6 273 to 414 6 248 at low tide. The ‘other larvae’ (OL) (by-catch discards) collected in the PL collector comprised of the larvacean, Oikopleura spp.; the chaetognatha, Parasagitta (Sagittidae) spp.; crab larvae and fish larvae. Mean OL numbers collected for each trawl session ranged from 295 6 188 to 349 6 292 at high tide and from 257 6 164 to 396 6 261 at low tide. Total PLS collected over 36 trawl months spread over a period of 3 years was 39,549. The corresponding OL numbers was estimated at 21,543. The OL (by-catch discards) to PLS ratio per trawl session was thus estimated at 54.47%. The comparatively low catches of the OL was attributed to the combined effect of the filtering efficiency of the 500 μ mesh PL collector net, the ability of the PL trawler to sample at variable water depths and the relatively shorter time of 10 min used in a trawl session. This method of PLS collection would provide a viable and reliable alternative to the marine resources (by-catch) destruction which accompanies wild PLS harvest and also ensure biodiversity preservation in the Ghanaian coastal region. The mode of operation of the PL trawler and its operational efficiency is presented and discussed.