Catch and bycatch in the tangle net fishery for crayfish (Palinurus elephas) off the south west coast of Ireland

Abstract

Crayfish or spiny lobster is fished off the south west coast of Ireland. There are also smaller fisheries on the west and north west coasts. Although historically, prior to the 1970s, the main fishing gear used in the fishery was top entrance traps crayfish are now targeted with large mesh tangle nets. The selectivity of these nets is poor and there is known to be a by-catch of finfish, skates and rays and protected species such as grey seal. The fishery usually occurs from mid-March to December. The species composition and size distribution of the catch and by-catch was surveyed over a 4 year period from 2017-2020 from Dingle Bay north to the Shannon Estuary. Additional data south of this area was obtained in 2020. A crayfish tagging programme was undertaken in 2017 and 2018. Between 2 and 6 vessels participated in the programme depending on year Most of the data was reported directly by Skippers and crew who were contracted to supply data. Scientific observers covered a proportion of trips in 2017 and 2019. A total of 1500 nmiles of tangle net hauls were surveyed across 6 vessels. A total of 11792 crayfish and 899 lobsters were measured, 1234 crayfish were tagged and released and 45 recaptures were reported. Spider crab, brown crab and crayfish were the numerically dominant species in the catch. Lobster, pollack, thornback ray, spurdog, monkfish and turbot were caught regularly in low numbers. There was no cetacean by-catch. A total of 200 grey seals were caught over the 4 year period by the vessels participating in the survey. Endangered and critically endangered species, including flapper skate, common skate, angel shark and white skate, were caught in very low numbers. The finfish catch is mainly lost to scavengers or seal depredation before nets are retrieved and because of long soak times which averaged 8 days. This reduces the value of the catch. The average value of the live crustacean (crayfish, lobster, brown crab) catch was approximately €300 per mile of net hauled. Crayfish were tagged and released in 2017 and 2018 and recaptured during the period 2017-2020. Three crayfish tagged by IFREMER off Brittany in 2015 and 2016 were recaptured off the south west and west coast of Ireland in 2019. The northern most recapture was off Erris Head Co. Mayo. The tagging data shows that most crayfish were recaptured locally close to release points even in the years following tagging. It is still unclear if crayfish are resident or if they migrate in and out of the tagging area as the reporting rate of recaptures outside the tagging area is unknown. The by-catch of critically endangered species poses a high risk to the continued presence of these species in Irish waters and indeed in European waters given that the area is known to hold the last European remnant populations of species such as angel shark and white skate. Grey seal by-catch was related to the distance between the fishing event and the nearest seal colony haul out at the Blasket Islands. Sixty % of seals were caught within 10km of the haul out. The probability of capture in a single net haul declined from 30-35% within 10km to 9-14% at distances of 10-30km to 4% at distances of 40-60km and zero at distances greater than 60km (data for 39 hauls >60km). The high by-catch of grey seal is a significant risk to the Blasket Island seal colony. It is unlikelythat the Blasket population can sustain this level of by-catch mortality without inward migration fromother colonies.