Crabs

The Brown Crab Cancer pagurus is a large long lived marine crab which can be found from the coast off Morocco (34˚N) to the coastal waters around the Northern Norway (70˚N)(fig1). The Brown Crab is an important species for the inshore fisheries around the UK with landings of 29,793 Tonnes with a value of £39 million in 2014 (1). In Orkney the Brown Crab is the most important species in terms of landings and value, in 2014 there was 2,810 Tonnes landed with a value of £3.7 Million (1).

Crab Distribution
Figure 2 Distribution of the Brown Crab

 

Brown crab can be found over a wide range of substrates such as, gravel, rocky or sand. They also cover a wide range of depth from the inshore littoral zones to the deep offshore areas, usually found between 6- 80m, with some larger crab in deeper water (2). They are nocturnal animals and are both a scavenger and a predator their diet is mainly composed of smaller crustaceans and molluscs.

The life cycle of the brown crab is complex and is divided into a free swimming pelagic phase and a bottom dwelling phase. Reproduction occurs after moulting when the crab shells are ‘soft’. Female crabs undergo a migration to offshore areas where the sediment is soft and they can burrow into it and brood their eggs. Moulting and reproduction occur in the summer months between June and September. The female carries a clutch of eggs in her abdomen and hatch between August and October and the eggs are carried for about 6-8 weeks (3).

Crab carrying eggs
Crab carrying eggs

The larvae are free swimming and are moved inshore by the sea current feeding on zooplankton and phytoplankton in the top layer of the water column, they undergo a moult which transforms them into bottom dwelling juveniles, where they settle living in crevices in rocks and in seaweed. Juvenile crabs moult frequently and reach sexual maturity at about 5 years old. As crab get larger they moult less frequently as they have the trade-off between growing and mating (3).

For more information on crab migration and the work the Orkney Sustainable Fisheries is doing to track the movements of Brown crab see the page on Crab Tagging.

Brown Crabs are not regulated by Total Allowable Catch (TAC) like most pelagic or demersal fish. They are subject to a Minimum Landing Size (MLS). The minimum landing size for Scotland is 140mm Carapace Width or 5.5 inches (CW) (fig 2). Carapace width is defined as the measurement of the width of the crab as its widest point.

Figure 2. The Minimum Landing Size of Brown Crab in Scotland (1).
Figure 2. The Minimum Landing Size of Brown Crab in Scotland (1).

 

(1) Scottish Sea Fisheries Statistics (2014) http://www.gov.scot/Resource/0048/00484499.pdf

(2) http://www.bim.ie/media/bim/content/publications/BIM%20Brown%20Crab%20Handling%20and%20Quality%20Guide.pdf

(3) www.moreforsk.no/download.aspx?object_id…pdf