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Brown crab

Latin name: Cancer pagurus
Common names: Edible crab

Introduction

The brown crab fishery is economically a very important fishery for Scottish vessels with total landings in 2009 of around 7400 tonnes and a value of £9 million. The fishery is long established and landings, although variable, have increased significantly over the last thirty years. Previously, most brown crab was caught inshore in mixed species creel fisheries. From the mid 1980s technological advances allowed the fishery to expand to offshore areas to the west and north of Scotland, which nowadays account for almost a third of total landings. Landings from the four principal brown crab assessment units: the Hebrides, Sule, South Minch and Orkney, made up 69 % of the total in 2009. The majority of crabs are landed in the third and fourth quarters of the year and a large proportion are exported live to markets in southern Europe.

Biology and life cycle

The brown crab is found all around the Scottish coast from the shallow sub-littoral into offshore waters to depths exceeding 100 m. It inhabits rocky reefs, mixed coarse grounds and soft sediments (muddy sand) particularly on the offshore grounds. Brown crabs eat mainly benthic invertebrates, particularly bivalves, small decapods and barnacles; although their capture in baited traps indicates that they will also scavenge for food. In common with other crustaceans, brown crab grow by casting (moulting) their shell and then hardening a new larger shell. Growth rate varies between areas, and animals will typically reach minimum landing size, (140 mm carapace width CW) at four to six years old. Small animals may moult several times in a year, but at larger sizes growth slows down and the inter-moult period can be as long as four years. Female brown crabs in Scottish waters typically mature between 130-150 mm CW. During courtship the mature female is guarded by a male. Mating takes place post moult while the shell is still soft. Females produce up to three million eggs. Fertilised eggs are carried under the female's "tail" for up to nine months, over the winter, until they hatch. The larvae are pelagic and drift with water movements until they settle to the seabed as miniature adults (about 2.5 mm in size) in summer or autumn depending on latitude and water temperature. Juvenile crabs are more commonly found in shallow inshore waters.

Adult crabs, especially females, can undertake extensive seasonal migrations (100s km), behaviour thought to be associated with reproduction. Migration of adults and the dispersal of larvae will both influence how connected stocks are. The structure of brown crab populations around Scotland is poorly understood.