Performance of industry-developed escape gaps in Australian Portunus pelagicus traps

Abstract

In response to concerns over the discarding and subsequent unaccounted fishing mortality of undersized Portunus pelagicus (<60-mm carapace length) caused by recent changes to baited traps in south-eastern Australia, commercial fishers voluntarily developed and tested various escape gaps. To facilitate prioritising designs for adoption, data were collected on the relative performances of single and multiple round (64-mm diameter), rectangular (33×120mm) and square (50×50mm) escape gaps across three fishing operations. The immediate mortality and exoskeleton damage among trapped P. pelagicus and key contributing factors were also assessed with a view towards predicting impacts among discards. Compared to control traps, those with escape gaps maintained catches of legal-sized P. pelagicus, but caught 51–100% fewer undersized individuals. Generally, rectangular escape gaps and especially multiple configurations were the most effective. Irrespective of the escape gap, there was a negative relationship between the proportion of undersized P. pelagicus and the total number trapped, which was attributed to intra-specific antagonistic interactions promoting escape (or possibly limiting ingress). Minimal observed damage (mostly appendage loss) among all trapped individuals was positively associated with total catch and less frequent among late inter-moults. While there was no damage bias towards undersized P. pelagicus and they only had a 0.2% immediate mortality, escape gaps represent a low-cost option for minimising interactions with unwanted catches throughout the fishery.