Flow-related effects in prawn-trawl codends: potential for increasing the escape of unwanted fish through square-mesh panels

Abstract

Two experiments were done in a flume tank to quantify the effects of codend mesh circumference and weight of catch on water flow. The mesh circumference of the posterior section of prawn-trawl codends (with and without bycatch reducing square-mesh panels) were assessed with three weights of catch (30 kg, 50 kg, and 70 kg). Compared with a codend with a circumference of 100 meshes throughout its entire length, a codend with an anterior section of 100 meshes and a posterior section of 200 meshes in circumference significantly increased the displacement of water forwards (up to 1120 mm from the end of the codend). This result varied with the weight of catch in the codend. The second experiment involved placing composite panels of square-shaped mesh (bycatch reducing devices) into the tops of the anterior sections of two codends with the same configurations as those above (termed the '100 panel codend' and '200 panel codend,' respectively). There was a displacement of water forwards immediately under the square-mesh panel in the 200 panel codend (by up to 2200 mm from the end of the codend). The results are discussed in terms of 1) the probable effects that codend mesh circumference and water displacement in codends have on fish behavior, and 2) implications for the future development of bycatch reducing devices like square-mesh panels in prawn-trawls.