Effects of Nordmøre-grid angles, profiles and other industry-developed modifications on catches in an Australian penaeid-trawl fishery

Abstract

In response to claims that conventional Nordmøre-grids used in the Clarence River penaeid-trawl fishery were occasionally being fouled or blocked during fishing, two experiments were done to examine the utility of modified grids with different installation angles, sizes and profiles, and alternative industry-developed, behavioral-type bycatch reduction devices (BRDs). In experiment 1, a conventional Nordmøre-grid (600 mm in length, installed at 45º) was compared against a longer grid (1200 mm in length, installed at 21º) and two locally-developed, behavioral-type BRDs. While slightly more difficult to operate than the conventional design, the longer Nordmøre-grid retained similar quantities of penaeids and bycatch; in contrast, both behavioraltype BRDs retained significantly (up to five times) more bycatch and were considered ineffective for use in this fishery. In experiment 2, the conventional Nordmøre-grid was tested against three modified Nordmøre-grids, including a 600-mm-long grid with a curved profile and installed at 45º, and two grids, both 900 mm in length and installed at 28º, but with flat and curved profiles. Only the curved 900-mm Nordmøre-grid caught significantly fewer penaeids and bycatch, although both of the curvedprofile grids appeared to facilitate the release of debris from the trawl. The results are discussed in terms of fishery-specific factors that probably contributed towards the performances of the various BRDs.