Benefits of square mesh codends promoted in Queensland scallop and prawn trawl fisheries

A fisheries extension project aimed at assisting Queensland’s trawl fishers to adopt square mesh codends is approaching completion. The project, a collaborative effort supported by the Fisheries Research and Development Corporation (FRDC), the Queensland Department of Primary Industries and Fisheries (QDPI&F), Seanet Extension Services and Ecofish, was initiated after research showed that square mesh codends can be highly effective at reducing bycatch in Queensland’s saucer scallop and prawn fisheries.

Figure 1. The square mesh codend used during an at-sea research charter trial in the north Queensland tiger/endeavour fishery. The 12mm polyethylene rope was used to take the weight of the catch, reducing knot slippage and distortion. The squares were 23mm x 23mm. Note how the squares remain open allowing small fish and other bycatch to escape. Standard diamond mesh codends tend to close up, thus retaining large amounts of bycatch.

Research has shown that square mesh codends in commercial trawl fishing nets can be used to significantly lower bycatch and improve the selectivity of target species. For example, by using a combination of square mesh codend and turtle excluder devices (TED), bycatch rates in Queensland scallop trawls were reduced by 77% compared to a standard diamond-mesh net codends. The square mesh codends were also shown to be very effective at reducing the catch rates of undersize scallops. Similar at-sea research trials have also demonstrated that bycatch rates in the deepwater eastern king prawn fishery off southern Queensland were reduced by 29% by using a combination of square mesh codend and TED. Importantly, these reductions in bycatch were achieved with no loss of marketable sizes of scallops and prawns.

Another recent research charter undertaken further north in the state’s tiger/endeavour prawn fishery showed that bycatch can be reduced by 34% as a result of square mesh codends (see Figure 1) compared to standard diamond-mesh codends, with no significant loss of marketable size prawns (all species pooled). Detailed analyses of the prawn catch revealed that the square mesh codend actually caught significantly more banana prawns than the standard codends (Figure 2).

These reductions in bycatch are significant when one considers that Queensland has the largest prawn trawl fleet in Australia and that the total amount of bycatch produced annually is in the order of several thousand tonnes.

Two factors contribute to the effectiveness of the square mesh codends. Firstly, because the catch accumulates in the codend, the square meshes allow small fish to escape easily from the net without having to expend large amounts of energy trying to find an escape hole or route. This is important when trying to exclude small fish as they generally lack swimming speed and stamina. Further, small prawns, undersize scallops and small crabs, sea urchins, shellfish, etc are excluded from the square mesh codends as they simply fall through the large square meshes.

Secondly, large mesh sizes appear particularly suited to these fisheries as the targeted animals are relatively large compared to the bycatch species. This, combined with the fact that the meshes remain open throughout the trawl, allows most of the small bycatch species to escape, while retaining the marketable scallops and prawns.
Other preliminary research on square mesh codends indicates that they may also reduce drag while being towed through the water, compared to standard diamond mesh codends, thus reducing vessel fuel costs. Reasons for this are unknown at this stage, but results suggest that drag may be reduced as a result of the reduced weight and volume of bycatch that is towed around by the vessel. Drag may also be reduced because the flow of water through the net increases due to the square meshes remaining “open”. As a result, the pressure wave that is being “pushed” in front of the net is reduced compared to nets with standard diamond mesh. At this time of record fuel prices, any reduction in the costs of fishing operations can only be seen as a benefit to commercial fishers.

The main objective of the extension project was to construct square mesh codends and lend them to trawler operators free-of-charge to trial. The project also offered assistance and on-going support to these fishers, as well as providing advice on the construction of square mesh codends to net makers.

Overall, 36 square mesh codends were trialled by fishers during commercial trawl fishing operations in Queensland’s eastern king prawn, leader prawn and saucer scallop fisheries. 28% of fishers reported bycatch reductions greater than 20%, while 46% reported reductions of 10-20%. Importantly, 90% of fishers reported that there was no effect on the catch rates of the target species. The most promising result from these trials was that 82% of the fishers said that they will continue to use the square mesh codends in their chosen fishery.

The extension project is approaching completion. Project staff also intend to produce a DVD on the construction and installation of square mesh codends that fishers and net makers can use. The DVD will also present some research results that show the reductions in bycatch that can be achieved by using these devices.

Figure 2. The number of banana prawns caught per 2 nautical mile trawl during the charter located in the north Queensland tiger/endeavour prawn fishery. The net containing the square mesh codend (SMC) caught significantly more banana prawns than the net containing a standard diamond-mesh codend. The commercial grades refer to prawns of increasing size and are usually measured as the number of prawns per 500 grams e,g., U3 refers to large prawn size classes of fewer than 3 prawns per 500 g.

Further information on the extension project can be obtained from:
Denis Ballam - Queensland SeaNet Officer, Cairns. Phone: (07) 4032 2234
Matthew Campbell or Tony Courtney, Queensland Department of Primary Industries and Fisheries. Phone: (07) 3817 9591 or (07) 3817 9582
Email: Matthew.Campbell@dpi.qld.gov.au or Tony.Courtney@dpi.qld.gov.au