Biology
With their characteristic Cornish pasty-shaped shell, brown crabs are easily recognisable. They can live for up to 100 years and grow to a maximum size of 27.5 cm shell width and weigh up to 4 kilos. Although the average size adult crab is between 10 and 15 years old. An adult female brown crab lays between 250,000 and 3 million eggs during the winter months. In Cornwall it is illegal to land crabs carrying eggs (known as berried crabs) so they are returned to the sea where they will be allowed to spawn. Rocky shores around Cornwall are teeming with juvenile edible crabs which can be found beneath rocks and seaweeds on low tides. As they get older, they move to deeper water and the largest edible crabs are caught in deep water off the Lizard in Southern Cornwall. Like all crustaceans Brown crabs must moult, shedding their shells as they grow. Juveniles moult several times per year but as they get larger the rate of moulting slows to once or less per year. After moulting the crab inflates every section of the shell with seawater and a chemical reaction takes place, hardening the shell up. The new shell takes a few weeks to harden fully, and it takes longer for the crabs’ muscles to grow to fill the shell. During this time the shell is pale, and the crabs are not landed by fishermen as they are full of water. In the winter months they are less likely to moult so the quality of brown crabs is best at this time of year although crabs are landed all through the year. Brown crabs use their massive powerful claws to crack the shells of mussels and other shellfish and even other crabs. They also scavenge for food and are attracted to fishermen’s pots by the scent of bait. Crabs are caught in largest numbers on and near hard, rocky seabeds.
Stock Info
Potting (baited trap fishing) is the main method used to catch brown crabs. For many years this low impact and selective fishing method, combined with appropriate minimum sizes has consistently yielded good returns of crabs proving itself to be a steady and sustainable fishery. In the past 4 years however catches have been less reliable and landings per unit effort for pot caught brown crab has significantly decreased each year between 2017 and 2016 across the whole district (CIFCA 2021). Fishing effort has been very high since 2016 at approx. 500 pots per km squared. CIFCA data from 2018 showed that potting effort in the Newquay area increased by 84% between 2016 and 2017, and landings per unit effort during this time decreased for both male (69.5%) and female crabs (32.7%). (this may be partly due to EU funding allowing replacement of pots on a large scale).
Effort has not increased in the same way on the south coast but landings per unit effort has still decreased.
According to government scientists (CEFAS) the status of the stock of brown crab in Cornish waters is moderate. In the Western English Channel (off Cornwall's south coast) stocks are around the level required to produce maximum sustainable yield (CEFAS 2019). In the Celtic sea (off Cornwall's north coast) the stocks of Edible crab are lower, but are approaching the level associated with Maximum Sustainable Yield (CEFAS (2019). In both Celtic sea and Western channel the fishing levels are moderate;- fishing effort is slightly above maximum sustainable yield in Celtic sea and around maximum sustainable yield in the Western Channel. The stock assessments are all based on female crabs as there is insufficient data on male crabs.
Management
Commercial crab fishing can only be carried out by boats with a shellfish fishing licence, CIFCA issues potting permits but number of these issued is not restricted. Actual numbers of pots being used are also not restricted.
Cornwall Inshore Fisheries and Conservation Authority has a permit system for crab potting and a larger minimum size than elsewhere in EU waters. Only a maximum of 75kg of brown crab can be caught as by-catch by netters per day.
There is no restriction on the quantity of brown crab being caught as they are not a quota species. There is an ICES working group on Crab and CEFAS carry out stock assessment work on this species, but management could be improved according to the Shellfish Association of Great Britain who strongly argue that pot numbers should be capped. (Bannister 2009) (Nautilus consultants 2009).
CEFAS stock assessment and ICES working group on Crab show that this stock is being monitored for future management.
The CIFCA shellfish licence requires all fishermen to report their catches in detail each month so that the fishery can be monitored.
No more than 30kg of detached brown crab or spider crab claws can be retained on board any vessel or landed at any one time. This limits the practice of removal of claws when unpicking crabs from nets and then throwing back the crabs with no claws.
Minimum landing sizes within Cornwall IFCA district (within the 6 mile limit) are larger than outside, females 150mm and males 160mm.outside the 6nm limit there is a smaller minimum landing size of 130mm, which complicates enforcement. It is also illegal to land berried female brown crabs within the CIFCA district.
Boats operating outside Cornwall's 6 mile limit that are over 15m in length are restricted by DEFRA effort regulations (KW/days). Also known as days at sea regulations. In 2021 boats are limited to 160 days at sea fishing for crabs per year. This allocation can change and in 2021 it increased to 280 days by the end of the year.
Cornwall IFCA are now working on a management plan for shellfish fisheries which we hope will result in better management of this stock. It is also encouraging that nationally Fisheries Managment Plans (FMPS) have been established to improve management in Brown crab fisheries in the wider SW area including Cornish waters outside the CIFCA district.
Project UK inshore crab management workshops have also been held and a detailed report shows that there is support for improved management of this fishery.
Capture Info
The majority of crabs landed to Cornwall are caught in baited crab or lobster pots. This is a low impact, selective fishing method. Any undersized or recently moulted crabs can be returned, unharmed, to the sea to be caught another day.
The gear was, for a long time thought to have very little physical impact on the seabed however a study Rees, Sheehan and Atrill 2021, carried out in Lyme bay showed that high densities of potting does have significant impact on sensitive species such as ross bryozoans (Pentapora) and seasquirts growing on rocky reefs, and that optimal, lower pot densities result in better catches of crab and lobster as well as reducing impacts.
Brown crab are also caught in significant numbers as by-catch in net and trawl fisheries.
Crabs tangled up in gill nets are difficult to un-pick and sometimes to save time the claws are removed to disentangle them. If offered brown crab claws only, as apposed to whole crab this is how they are likely to have been caught. This is poor practice as it is wasteful and there are ethical concerns with this practice as the live bodies of the crabs are often returned over the side and the crab has a long period of up to two years where survival is extremely difficult as they can't feed or defend themselves and have to slowly regrow their claws. Netters have a limit of 30kg of unattached crab claws per day in Cornish waters.
Nets have issues with accidental by catch of other species such as skates, sharks, seabirds and cetaceans. Trawls have a greater impact on the seabed than pots and undersized crabs that are caught in nets or trawls are often much more damaged than those returned from crab pots and less likely to survive. Boats operating outside Cornwalls 6 mile limit that are over 15m in length are restricted by effort restrictions (KW/hours) which restrict fishing effort. Outside the CIFCA district (6 miles offshore) EU minimum landing sizes currently apply which are smaller than those within Cornwalls 6 mile limit.
References
CIFCA Shellfish Summary Statistics 2021 - available on request from Cornwall IFCA
Nautilus consultants 2009 Brown crab management report
Eno N.C. et al, 2001, Effects of Crustacean traps on Benthic fauna, ICES journal of Marine Science
Bannister. R. C. A 2009 On the Management of Brown Crab Fisheries, Shellfish Association of Great Britain
Project inshore pre assessment database http://msc.solidproject.co.uk/msc-project-inshore.aspx?a=CW&s=
MMO landings data to Cornish ports.
