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 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.
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. Cefas is currently carrying out a 3 year crab tagging programme, using double T-bar tags and electronic data storage tags (DSTs) to improve knowledge of the structure of crab stocks in the area and to investigate the distribution and behaviour of the edible crab.
This project aims to release up to 15,000 double T-bar tagged crabs between 2007 and 2009. These tags are retained through moulting and cause less injury than suture tags used in previous studies. Tagging is being carried out from commercial vessels in various locations throughout the English Channel and off the north Cornwall coast between 2007 and 2009. Tags will vary in colour, but will all be the same design and bear the tag number followed by "WWW.CEFAS.CO.UK PLEASE RECORD TAG NO. DATE. LAT & LONG, WIDTH, SEX" (upper inset).
In 2008 and 2009 around 100 crabs will also be tagged using electronic data storage tags (DSTs). These provide high frequency records of depth and temperature for up to 2 years and the data can be used to determine the location of the crabs during the period between release and capture. DST recoveries 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.
To date 7,130 crabs have been tagged with T-bar tags and released at locations throughout the Channel from east of Beachy Head to Lands End. Around 15% of these haved been recaptured and reported to Cefas by the end of August, 2008. The figure below shows the release sites and the recorded movements up to that date. In addition 32 crabs were fitted with DST's and released in the eastern Channel in August (see picture).
To maximise the information obtained from the programme, rewards (£6 for T-bars, £50 for DST's) are payable for the return of the full recapture details: tag colour, tag number, date, latitude, longitude, sex, carapace width in millimetres (lower inset). 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. 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 grand prize of £1000.
For information on returning tags see: What to do if you find a Cefas tagged fish
