Round scallops and square meshes: a comparison of four codend types on the catch rates of target species and by-catch in the Queensland (Australia) saucer scallop (Amusium balloti) trawl fishery

Abstract

Concern over the amount of by-catch from benthic trawl fisheries and research into the problem have increased in recent years. The present paper demonstrated that by-catch rates in the Queensland (Australia) saucer scallop (Amusium balloti) trawl fishery can be reduced by 77% (by weight) using nets fitted with a turtle excluder device (TED) and a square-mesh codend, compared with a standard diamond-mesh codend with no TED. This large reduction was achieved with no significant effect on the legal size scallop catch rate and 39% fewer undersize scallops were caught. In total, 382 taxa were recorded in the by-catch, which was dominated by sponges, portunid crabs, small demersal and benthic fish (e.g. leatherjackets, stingerfish, bearded ghouls, nemipterids, longspine emperors, lizard fish, triggerfish, flounders and rabbitfish), elasmobranchs (e.g. mainly rays) and invertebrates (e.g. sea stars, sea urchins, sea cucumbers and bivalve molluscs). Extremely high reductions in catch rate (i.e. ≥85%) were demonstrated for several by-catch species owing to the square-mesh codend. Square-mesh codends show potential as a means of greatly reducing by-catch and lowering the incidental capture and mortality of undersize scallops and Moreton Bay bugs (Thenus australiensis) in this fishery.

Performance of square-mesh codends in reducing discards and by-catch in the Shark Bay scallop fishery

“…Further trials would be required to ascertain whether this loss would increase or decrease with larger-sized square-mesh codends. In a similar study in the Queensland scallop fishery (based on the same species), mesh trials using a 50-mm square-mesh codend resulted in a significant reduction in by-catch (77%) and undersized (,95 mm in the Queensland fishery) scallops (39%), particularly in the 60-80-mm size range, compared with their standard 88.9-mm diamond-mesh codend (Courtney et al 2008). Given that the focus of the Queensland mesh trials was to reduce by-catch by using square mesh, alternative square-mesh sizes were not tested; however, our study showed that largersized square-mesh codends than those tested in Queensland have the potential to concurrently reduce by-catch and smallsize scallops, with a risk of a small loss of commercial-sized scallops.…”

“…In Australia, nearly all prawn and scallop trawl fisheries are required to use one or more BRDs such as inclined grids, composite squaremesh windows, fisheyes and other excluder devices (Eayrs 2007). Most recently, the Queensland east coast otter trawl fishery found significant reductions in by-catch and undersized saucer scallops (Amusium balloti) with square-mesh codends and TEDs compared with their standard diamond-mesh codend (Courtney et al 2008). Several studies have shown square-mesh instead of the conventional diamond-shaped-mesh codend to be more selective for some species (Suuronen and Millar 1992;Broadhurst et al 2004;Sun et al 2006), given that twines in square-mesh netting are always at right angles to one another and remain open as the codend fills up and the tension on the mesh increases, whereas the diamond meshes tend to close (MacLennan 1992).…”

Quantifying long-term discards from Queensland’s (Australia) east Coast otter trawl fishery

“…where a e is the intercept, x e and z e are the column vectors of covariates for fixed and random effects, respectively, and b e and b e are their associated coefficients, respectively. A gamma distribution was used because it often best describes the attributes of trawl discard data, including a lack of zero observations (Courtney et al, 2008(Courtney et al, , 2014. For all sectors, x e includes sector, BRD, the interaction between sector and BRD, lunar phase, and lunar phase advanced 7 days, and z e the interaction between sector and sampling trip.…”

“…BRDs generally resulted in significant reductions in discard rates, however, their effects varied across sectors and methods (Table 2). Significant reductions in discard rates have been demonstrated in QECOTF by undertaking research charters to evaluate TEDs and other BRDs (Courtney et al, 2006(Courtney et al, , 2008(Courtney et al, , 2014. It is more difficult to quantify BRD effects while sampling on-board commercial vessels during their normal fishing activities, due to a lack of experimental control needed to obtain repeated measures from nets with and without BRDs.…”

“…Discards from the QECOTF are contentious, not only because of the relatively high annual fishing effort and discard rate characteristic of prawn trawl fisheries, but because most of the fishery occurs in the Great Barrier Reef Marine Park, which is a World Heritage Area (Figure 1). Previous research on the fishery's discards has focused on protected species of sea turtles (Robins, 1995) and sea snakes (Courtney et al, 2010;Elfes et al, 2013), discards composition (Watson and Goeden, 1989;Watson et al, 1990), benthic impacts (Pitcher et al, 2016), and BRD testing (Robins-Troeger, 1994;Robins-Troeger et al, 1995;Courtney et al, 2008Courtney et al, , 2014. Despite this and the implementation of discard management measures, the amount of discards generated by the QECOTF remains unknown (Robins and Courtney, 1998).…”