Modifying penaeid-trawl spreading mechanisms to improve environmental efficiency

Abstract

The environmental efficiency of demersal trawls can be defined within measures of three response variables: (i) species selectivity; (ii) extent and intensity of habitat impacts; and (iii) energy efficiency. All three variables affect all fisheries, but especially those targeting penaeid prawns (or shrimp). Penaeid-trawl fisheries are globally distributed in tropical and temperate latitudes and involve vessels (~8−25 m) towing small-meshed, single- or multi- funnel-shaped nets on the seabed, horizontally spread by hydrovanes (called ‘otter boards’), and less commonly, rigid beams. To date, most of the efforts towards improving the environmental efficiency of penaeid trawls have concentrated on gear selectivity specifically, via bycatch reduction devices (BRDs) in the codend (to address i above). Although somewhat effective (30–70% reductions), BRDs offer few perceived benefits to fishers and, in many fisheries, there is resistance to their adoption. It is known that the anterior trawl section affects all three response variables above, which might concomitantly be mitigated through modifications that could potentially provide fishers with a realised benefit—increased energy efficiency (and therefore greater adoption). A primary objective of this thesis was to investigate the potential for holistically improving penaeid-trawl environmental efficiency through research focussed on the anterior section of the gear; specifically the spreading mechanisms (otter boards, sleds, beams and sweeps). The treatments tested comprised three broad categories: (i) revising otter boards and their rigging; (ii) substituting otter boards with a beam; and then (iii) amalgamating key concepts across both methods. Modifications within the first category included removing sweeps and testing a novel otter board (i.e. the batwing). Within the second category, beams were modified in attempts to maintain target catches while minimising bycatch, habitat contact and energy intensity. The third group of modifications involved exploiting the concepts underlying an identified inherently superior species selection of beam trawls to otter trawls via counter-herding devices, termed ‘simple anterior fish excluders’—SAFEs. Where appropriate, the treatments were compared against controls for their utility in mitigating the three response variables. The overall results demonstrated that selectivity can be improved (i.e. bycatch reduction) by using a beam trawl, SAFE or by removing sweeps; habitat impacts will be reduced when using batwings; and, while overall energy intensity was not improved, an important component—drag—can be alleviated using a beam trawl or batwing otter boards. For targeted catches, the conventional otter trawls caught more total penaeids, but the beam trawls had comparable penaeid catches when corrected for fuel-to-catch ratios. A key conclusion from the thesis is that no single treatment will be effective for significantly improving all three response variables; however, anterior modifications lowering drag (the beam trawl and batwing otter boards) provide fishers with the greatest potential benefit—lower fuel usage. Ultimately, because otter trawls are the most ubiquitous demersal trawls, the batwing otter boards were perceived to have the greatest potential with their greater fuel efficiency and lower habitat impacts. Additionally, applying counter-herding devices (i.e. a SAFE) to otter trawls could provide a cost-effective modification that will reduce bycatch without affecting targeted penaeids, when trawl spread ratios (defined as the wing-end spread ÷ headline length) are maintained. It is likely that one of the greatest benefits from spreading-mechanism modifications will be lower unaccounted mortality to bycatch species, by limiting the potential for injury sustained from escape attempts (i.e. through the mesh and/or BRDs), which may occur when individuals enter the trawl. The overall results on improving spreading-mechanism environmental efficiency will complement future work with other anterior-trawl sections—frame lines, ground gear and the body—for a more holistic approach to improving overall efficiency Overall, this thesis provides a comprehensive assessment of penaeid-trawl spreading-mechanism modifications and gear alternatives for improving penaeid-trawl environmental efficiency. The findings are not only applicable to Australian penaeid (and non-penaeid) trawl fisheries, but also similar fisheries overseas.