Abstract
Incremental refinements were made to a generic Nordmøre-grid to minimise bycatches of blue swimmer crabs Portunus armatus and giant cuttlefish Sepia apama, while maintaining catches of western king prawns Melicertus latisulcatus in the Spencer Gulf penaeid-trawl fishery. These refinements involved varying bar spaces, escape-exit areas and guiding-panel lengths, and were compared against a conventional trawl. Catches of teleosts and M. latisulcatus largely remained unaffected by the changes. Maximum reductions in P. armatus and S. apama bycatches (both ~90%) were achieved with a Nordmøre-grid comprising 38-mm bar spaces, 0.81- or 1.05-m2 escape exits and a 2.7-m guiding panel. Catching fewer P. armatus should reduce abrasion and crushing of M. latisulcatus in the codend and so increase the value of this targeted species. While noting some unresolved operational concerns, these results demonstrate the potential improvements in selectivity in this fishery using a Nordmøre-grid, primarily by mechanical separation.
Highlights
Relative to other fishing methods, penaeid trawling is generally considered to be poorly selective and can result in the discarding of large quantities of bycatch, which sometimes includes threatened or endangered species [1,2,3]
While P. armatus and S. novaehollandiae maintained consistent trends between experiments, S. apama were more abundant during experiments 1 and 3
The mean individual weight of P. armatus was significantly less in experiment 1 (100 ± 12 g) than experiment 2 (147 ± 12 g) (LMM and false-discovery rate (FDR), p < 0.05)
Summary
Relative to other fishing methods, penaeid trawling is generally considered to be poorly selective and can result in the discarding of large quantities of bycatch, which sometimes includes threatened or endangered species [1,2,3]. The most common technique for mitigating penaeidtrawl bycatch is to install physical modifications in the posterior trawl, termed bycatch-reduction devices (BRDs), which are designed to exclude organisms mainly based on differences in behaviour (‘behavioural separators’) or size (‘mechanical separators’) [4]. The latter category has been successful in several fisheries, with designs like the ‘Nordmøre-grid’ consistently excluding up to 90% of total bycatch while maintaining catches of penaeids [5,6,7,8]. Many Australian penaeidtrawl fisheries use mechanical separators and virtually all require behavioural separators [10–
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