Evaluation of a novel research trap for surveys of blue swimmer crab populations

Abstract

Surveying free-ranging crab populations is important for monitoring the health of exploited stocks and predicting future productivity. Here, we present a novel research trap design for use in fisheries-independent surveys of blue swimmer crab (Portunus armatus) populations, and evaluate the trap against some existing approaches within an independent survey framework. Specifically, the trap design aimed to improve efficiency for capturing smaller crabs, without adversely affecting catches of larger crabs. We quantify and report the abundance and selectivity of these traps, relative to co-located samples obtained using beam trawls and standard commercial round traps, to establish whether these small-mesh traps may offer any improvements over existing survey methods. Comparison and evaluation of these small-mesh traps against other existing survey gear, in different places and at different times, showed that the traps are more effective at catching smaller crabs when they are present, and equally or more effective at catching larger size classes of crabs. The beam trawl appeared to be effective at capturing a reasonable size range of crabs; however, the number of crabs caught (using a similar investment of time) was substantially lower than that caught in traps. This novel small-mesh research trap appears suitable for fisheries-independent surveys of portunid crab species.

Patterns in abundance, size and space‐use of Giant Mud Crab (Scylla serrata) in a temperate marine protected area

“…This includes individuals smaller than the minimum legal length (85 mm CL) but excludes recently settled crablets and juveniles (<60–65 mm CL). Future studies could attempt to capture smaller crabs using specialized gear (e.g., Hanamseth et al, 2022); however, few studies have successfully sampled juvenile Giant Mud Crab (e.g., Hill et al, 1982; Hyland et al, 1984). In general, smaller crabs typically avoid areas where larger crabs reside (Alberts‐Hubatsch et al, 2014), and the patterns presented here may not reflect those of smaller individuals.…”

Fleet‐wide acceptance of escape gaps and their utility for reducing bycatch in south‐eastern AustralianPortunus armatustraps

“…One easily accessed decapod family, Portunidae, includes some 500 species, many of which are target or byproduct species in artisanal, recreational, and commercial fisheries throughout tropic and temperate zones (Boutson et al, 2009; Hosseini et al, 2014; Jirapunpipat et al, 2008; Leland et al, 2013; Prince et al, 2020; Vazquez Archdale et al, 2006). Various active and passive gears are used to harvest portunids although, owing to their sporadic distributions in shallow and confined areas among coastal estuarine and river systems, much of the global harvest comes from baited, small‐mesh traps (Broadhurst et al, 2014; Hanamseth et al, 2022; Jirapunpipat et al, 2008; Vazquez Archdale et al, 2006). Concerns over deploying small‐meshed traps in areas associated with large abundances of small fauna, including juveniles of the target species, have led to many efforts to reduce portunid‐trap bycatch, especially in Australia (Barnes et al, 2022; Broadhurst et al, 2014, 2017, 2019, 2020; Broadhurst & Millar, 2018a, 2018b; Butcher et al, 2012; Rotherham et al, 2013).…”