Relative ghost fishing of portunid traps with and without escape gaps

Owing to concerns about derelict portunid traps perpetually fishing ('ghost fishing') in southeastern Australia, the utility of escape gaps for minimising temporal catching efficiencies, and damage and mortality, among key species was investigated. Replicate baited traps with (i) no escape gaps ('conventional'), (ii) three escape gaps ('escape-gap' traps), or (iii) three holes where escape gaps would have been if they were biodegradable ('open-hole' traps) were deployed and retrieved after short (one and two weeks), medium (four and six weeks) and long soaks (eight and 10 weeks). No trap had bait remaining after any soak. Total catches comprised 17 species, but 70% was blue swimmer crabs, Portunus armatus-most (78%) of which were smaller than a proposed new legal size (≥65 mm carapace length). Many (62%) P. armatus were injured (typically missing one or two appendages), but had few mortalities (total of 5%). Compared to conventional traps, the escape-gap and open-hole traps retained significantly fewer undersized P. armatus across all soaks (by 93 and 96%) and there were less total injuries (by 86 and 97%) and mortalities trap-1 (by 80 and 85%). Irrespective of the trap, catches of total and undersized P. armatus and injuries trap-1 were greater after short (all by up to 3×) than medium or long soaks. Several traps had broken mesh bars, which was negatively associated with P. armatus catches and injuries. Only three other species had mortalities (n = 7). The data support using escape gaps for minimising ghost fishing among derelict portunid traps.