Square-mesh codend circumference and selectivity

Broadhurst, M. K., and Millar, R. B. 2009. Square-mesh codend circumference and selectivity. – ICES Journal of Marine Science, 66: 566–572. Despite the wide-scale assessment and the use of square-mesh codends in demersal trawls, relatively few studies have tested the effects of configurations other than mesh size on their selectivity. We investigated the consequences of increasing the circumference of square-mesh codends used in an Australian penaeid fishery from the expected optimal configuration of ∼33% of maximum diamond-mesh extension to ∼56 and 75%. Three square-mesh designs comprised 27-mm polyamide mesh throughout and had the same length (100 bars, B), but different circumferences (90, 150, and 200 B, respectively). Paired simultaneous comparisons (using twin trawls) of each treatment codend against a small-meshed control revealed significant effects of circumference on the efficiency of the trawl for a small teleost (pink-breasted siphonfish, Siphamia roseigaster) and commercial size classes of school prawns (Metapenaeus macleayi). Compared with the 90 codend, pink-breasted siphonfish catches and prawn count (numbers per 500 g) were both significantly greater in the codends of larger circumference, and these effects are attributed to concomitant (i) convoluted or reduced lateral openings of meshes and (ii) lesser probabilities of organisms encountering meshes in the posterior section. These differences would not preclude the use of codends of larger circumference in the fishery, but they do highlight the need to select appropriate configurations in future studies to reduce the potential for including the confounding effects of different geometries.