Experimental Beam Trawls for Sampling Juvenile Prawns

C. T. Turnbull, R. A. Watson, (1990). “Experimental Beam Trawls for Sampling Juvenile Prawns.,” Torres Strait prawn project: A review of research 1986-88 (Queensland Fisheries, Brisbane, Australia) Chapter 11.

Small fine-meshed beam trawls have been used for conventional sampling of juvenile penaeids in seagrass areas (Coles and Lee Long 1985). As juvenile penaeids are nocturnally active, beam trawl sampling for them occurs at night for a set time or on a trawl track generally marked by lighted buoys. Many factors can affect the efficiency of beam trawls such as water depth, lunar intensity and the type of substrate or sediment being trawled over. Sediments associated with seagrass nursery habitat sampled by Coles and Lee Long (1985) in the Gulf of Carpentaria were usually fine and silty. Sediments of Torres Strait nursery areas (Section 4) are coralline and punctuated by pieces of dead corals weighing several kilograms. This rough substrate interferes with the efficiency of a conventional beam trawl. Low tides at night on the reef-platform nursery areas of Torres Strait made daytime time sampling of juvenile penaeids in this region necessary (Section 4). Alternative sampling gear was designed for daylight sampling, which incorporated a higher clearance from the bottom to minimise the effect df rough terrain on the trawl gear. A variety of gear has been used to sample prawns. Allen and Hudson (1970) described a sledmounted suction device which they employed to quantitatively sample young pink shrimp, Penaeus duorarum duorarum. They found that samples from their suction device compared favourably with those from a more conventional, hand-pulled frame trawl. Penn and Stalker (1975) described and tested an “active” beam trawl which operated by pumping jets of water into the substrate. This action washed inactive buried prawns into the path of the net. Their design allowed quantitative daylight samples of nocturnally active juvenile prawns. Their beam trawl used a large collecting bottle on the cod end of their net which proved ineffective in substrates with a high volume of organic material such as dead seagrass. Electricity has been used to improve catches of fish (McRae and French, 1965), prawns (Pease and Seidel, 1967), and lobsters (Saila and Williams, 1972). It is used routinely in the mariculture harvest of prawns such as Penaeus japonicus in Japan (Lewis and Carrick, 1987). It also can be used for daytime harvest of nocturnally active animals. Electric sampling gear has an added advantage. It can be designed to limit the retention of vegetation and sediment in the collecting bag by relying on the involuntary movement of prawns influenced by pulsed current rather than through the mechanical disturbance of the bottom. In the present study three alterative beam trawls were tested to compare their efficiency. A conventional beam trawl designed for nighttime use was compared with a water jet beam trawl, and an electric beam trawl both designed for daytime use.