Cefas edible (brown) crab tagging programme in the English Channel, 2007 – 2010

Results from major tagging studies carried out in the 1970s indicated that mature female crabs generally moved from east to west through the English Channel and in a more southerly direction further west (see illustration). Male crabs tended to move less far and with a less discernable pattern. Return movements of adult females were not observed. Studies of planktonic crab larvae distributions have indicated that there are major offshore spawning grounds to the south of Devon and Cornwall from which larvae drift in a north-easterly direction back into the Channel. However, the rate of larval drift is thought to be insufficient to return larvae to fishing grounds in the eastern Channel, which suggests the possibility of separate stocks in the eastern and western Channel.

In 40 years since the last tagging programme much has changed including warming seas, increased fishing effort and the expansion of the fishery to the south of Devon and Cornwall.  To further our knowledge of the structure of crab stocks in the area and to investigate changes, if any, in the distribution and behaviour of the edible crab, Cefas will carry out a new crab tagging programme between 2007 and 2010 using double T-bar tags (which are retained through moulting and cause less injury than the suture tags used in previous studies) and electronic data storage tags (DSTs) which provide continuous records of depth and temperature for up to 2 years. This can be used to determine the location of the crabs during the period between release and capture. The pictures below show the two types of tags when attached to crabs.

Since autumn 2007, 15,522 crabs have been tagged aboard commercial potters at locations throughout the English Channel from east of Beachy Head to Lands End, as well as on the Trevose grounds off north Cornwall. The T-bar tags vary in colour but all are the same design and bear a four digit tag number, prefaced by E04 or E05, followed by "WWW.CEFAS.CO. UK PLEASE RECORD TAG NO. DATE. LAT & LONG, WIDTH, SEX". 128 crabs have also been tagged using DST's. Recoveries of these will provide detailed information on crab movements and behaviour which may help us to identify migration routes and spawning grounds and will complement the double T-bar tagging study.

So far 2,185 T-bar recaptures and 29 DST recoveries have been reported to Cefas. The figure shows the releases and recorded movements up to the end of September 2009. To maximise the information obtained from the programme, rewards are payable (£6 for T-bars, £50 for DST's) for the return of the full recapture details: tag colour, tag number, date, latitude, longitude, sex, carapace width in millimetres. Don't forget your own contact details. There is the opportunity to win up to £1000 in tag number raffles. On each anniversary of the project in 2008, 2009 and 2010 a lottery draw for a £500 prize will take place using those tags returned during the previous year. The winner of the 2008 draw was a skipper working in the eastern Channel and in 2009 he was from Salcombe. An additional final grand draw with a prize of £500 will also be made in 2011 in which all T-bar tags returned during the project will be entered. Returned DSTs will be entered into a separate draw with a single prize of £1000.

For information on returning tags see: What to do if you find a Cefas tagged fish