T90 mesh for whitefish discard reduction

T90 mesh for whitefish discard reduction Testing 90mm T90 codends on board Greencastle trawler Foyle Fisher. Photo: BIM

Trials with 90mm T90 codends showed major reductions in catches of undersize roundfish, increased monkfish and no loss in flatfish species, reports BIM Fisheries Conservation Manager Dr Ronán Cosgrove.

As outlined in BIM’s Business of Seafood, nephrops continue to dominate as the highest value species in the Irish demersal fishing sector with landings of 7200 tonnes worth €56m at first point of sale in 2018. Whitefish species also remain economically important with monkfish, megrim, hake, whiting and haddock, predominantly caught in the Celtic Sea, providing a return of €46m in 2018.

These species are primarily caught in mixed demersal trawl fisheries that are impacted by the landing obligation which is largely implemented through the discard plan for North-Western Waters. This plan contains survivability and de minimis exemptions which permit some continued discarding, and prescribed selective gear options that aim to avoid unwanted catches during fishing. The new gear measures came into force in the Celtic Sea on the 1st July 2019.

Vessels targeting nephrops are relatively well prepared to address the discard plan. Gear options such as the 300mm square mesh panel (SMP) and SELTRA panel that greatly reduce unwanted fish catches have been successfully tested, developed and implemented by BIM and the Irish fishing industry. Emanating from a BIM study, the nephrops survivability exemption in ICES subarea 7 also assists – rather than landing and having them deducted from quotas, large numbers of living juvenile nephrops can be put back to sea, with major biological and economic benefits for the fishery. Nephrops and whitefish vessels also benefit from de minimis exemptions of 6 to 7% for cod, haddock and whiting, and survivability exemptions for plaice and skates and rays.

Exemptions aside, a raft of new gear requirements pose challenges for vessels targeting whitefish species in the Celtic Sea. Although yet to be tested, one of the options identified as having major potential to reduce unwanted catches comprises 90mm T90 mesh in the codend and extension piece.

Led by Daragh Browne with assistance from Matthew McHugh, the gear was assessed on board Foyle Fisher from Greencastle. Catches from the 90mm T90 codend were compared against a traditional 80mm diamond codend using a twin-rig trawl configuration deployed in ICES 7j and g in early May.

Results revealed a tale of two stories in relation to effects on different species and associated directed fisheries. Major differences occurred in the size of retained roundfish species. Undersize catches were reduced by around 90% in the case of whiting and haddock, and 60% for cod in the T90 gear.
Due to the increased size and orientation of the T90 mesh, market sized whiting (>31 cm) were reduced by 60%. Other larger roundfish fared better. Likely due to greater water flow and trawl performance, market sized haddock and hake catches increased by 41% and 29% respectively. Results indicate that the gear generally works well for roundfish species, but due to substantial losses of market sized fish, the traditional directed fishery for whiting would likely be better served by one of the other gear options in the discard plan. Previous BIM research suggests that the 80mm T90 codend with a 160mm SMP might be a useful option and warrants further investigation in this regard.

Little difference occurred in catches of flatfish such as megrim, plaice and lemon sole, but monkfish catches increased by 56% in the T90 gear. The latter result is difficult to explain but may be linked to increased water flow through T90 meshes and resulting effects on trawl geometry and spread. Whatever the reason, results are extremely positive for vessels targeting monk and megrim given such increases in a key target species, and major reductions in unwanted catches of undersize roundfish species.

BIM would like to thank Mick Gillen and the crew of Foyle Fisher for an excellent collaboration, and John George Harrington or assistance with on board sampling.

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