The spider crab is the largest crab found in Irish waters, with a carapace width of up to 20cm and a leg span of 50cm or more. Spider crabs inhabit coarse sand mixed grounds and open bedrock from the shallow sublittoral zone to a depth of 120m, although highest densities occur between 0 and 70m. Large migrations of spider crabs occur during the early spring when they move into shallower water to spawn. Female crabs become berried (egg-bearing) from April onwards, and by June all mature females are berried. Hatching occurs from July until November, following which the crabs migrate back to deeper water. Juveniles remain in shallow water close to the coast until they mature in their second year. Carapace widths for mature adults are from 8.5cm-20cm for males and 7cm-17.5cm for females. Spider crabs are known to congregate in large numbers and form ‘mounds’.
Stocks are thought to be abundant as they are not heavily fished, although reports are few and far between. A study by Cornwall Sea Fisheries Committee (CSFC) has shown that there is a high percentage of female crabs above the MLS of 12cm carapace width. There is a lower percentage of males above the MLS of 13cm CW, but there is a large proportion in the region of 10cm-12cm, which should enter the fishery in the next 5 years or so. This suggests that there is a sustainable future for the stock if appropriate management is in place.
Pot caught is a more sustainable fishing method than tangle netting, as with potting there is much less bycatch of non-target species and any bycatch or small crabs may be returned to the sea alive, with a high rate of survival. The minimum landing size for spider crab in EU waters is 120 mm (carapace width), with UK legislation imposing an additional MLS of 130 mm carapace length for males. Within the Cornwall Inshore Fisheres and Conservation Authority district, in both net and pot fisheries, fishing is by permit only. Under new regulations, introduced in 2011, all spider crabs landed within the district must measure at least 130mm.