Disparity among recommended and adopted escape-gap designs and their utility for improving selection in an Australian portunid trap fishery

Abstract

Over the past decade, most efforts at improving the selectivity of commercial, baited traps targeting the giant mud crab, Scylla serrata in south-eastern Australia have focused on testing rectangular escape gaps (46 mm high × 90–120 mm wide) in spatially and temporally restricted manipulative experiments, followed by some promotion among local fishers. Anecdotal data imply wider adoption of escape gaps throughout unassessed regions of the fishery, but there are no data on escape-gap designs, or if performances correspond to predictions from isolated experiments. This deficit was addressed here via a fishery-wide, observer-based assessment. During two fishing seasons, six observers sampled catches from seven estuaries responsible for 80% of the total S. serrata harvest. Most fishers used collapsible netted cylindrical traps (83% of traps and made from 50–55 mm stretched mesh opening); among which ~10% were fitted with one rectangular escape gap, while 37% had one or two circular escape gaps (85–90 mm diameter). Compared to traps with no escape gaps, those with rectangular escape gaps (46 × 120 mm) reduced the numbers of undersized S. serrata by 92% (similar to observations during previous manipulative experiments); but those with circular escape gaps did not reduce any unwanted catches. Rather, traps with circular escape gaps caught more legal-sized S. serrata, which was attributed to possible species-specific behaviours in the traps and less egress through funnel entrances. This result potentially contributed towards fisher opinions of circular escape-gap success (given they want to maximize desirable catches) and facilitated their wider adoption. The data reiterate ongoing fishery assessments are essential to understand the performances of evolving/devolving gear modifications and ensure intended stakeholder and resource management expectations are realized.