Edible (brown) crab
English Channel tagging programme, October 2007 - 2011
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 right). 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. This suggests the possibility of separate
stocks in the eastern and western Channel.
In the 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 edible crabs, we have been tagging crabs using double
T-bar tags and electronic data storage tags (DSTs). T-bar tags are
retained through moulting and cause less injury than the
suture tags used in previous studies.
T-bar tags vary in colour, but all 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".
DSTs provide continuous records
of depth and temperature for up to two years. This can be used
to predict the location of the crabs during the period between
release and capture.
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.
T-bar tag
fixed on a crab (left) and DSTs attached to the
carapace of crabs (right).
Since October 2007, 15,548 crabs have been tagged from
commercial catches throughout the English Channel, from east of
Beachy Head to west of Lands End, and on the Trevose grounds off
north Cornwall. In addition 144 crabs were released with Cefas G5
data storage tags (DST's) fitted. In addition 144 crabs were
released with Cefas G5 data storage tags (DST's) fitted. So far
2,260 T-bar recaptures and 49 DST recoveries have been reported to
Cefas. The figure shows the releases and recorded movements up to
the end of September 2009.
Releases and
recorded crab movements, autumn 2007 to August 2010
To ensure we obtain as much information as possible
from our tagging programmes, rewards are payable for the
return of the tags: we pay £6 for T-bar tags, and £50 for DSTs. For
each T-bar tag returned we need the full recapture details
- tag colour, tag number, date, latitude, longitude, sex, and
the crab's carapace width in millimetres - and the returnee's
contact details. For DSTs we would also like the crab back for
detailed examination. The tags have a finite battery life, so
please do not return them to the sea and risk losing all the data
collected.
Instructions about returning
tags and
an electronic form make it
easy to supply information online. The instructions also indicate
other ways of supplying tag information.