Velvet crab

Latin name: Necora puber
Common names: Velvet swimming crab, devil crab

Introduction

The velvet crab fishery in Scottish waters is a relatively recent development; velvet crabs were once considered a 'pest' species, and only taken in a small scale fishery for a few months in the winter. With the collapse of the Spanish fishery in the early 1980s the Scottish fishery expanded rapidly to supply southern European markets and became the largest velvet crab fishery in Europe. In 2009, the fishery landed 2,300 tonnes of velvet crab into Scotland with a value of £6.2 million. Velvet crabs are caught in the inshore creel fishery along with lobster and brown crab. Very few fishermen fish solely for 'velvets'. Previously the fishery was associated mainly with the west coast (Hebrides and South Minch assessment units) and Orkney, but since 2002 landings on the east coast of Scotland have increased substantially. It is not clear whether this is due to an increase in the population in this area or to more specific targeting of the species. In 2009, the majority of landings came from Orkney, South Minch, Shetland, East Coast, Hebrides and South East. Most velvet crabs are landed between July and November.

Biology and life cycle

The velvet crab is a member of the family Portunidae (Swimming crabs) and is found in waters all around the British Isles. It is a fast moving and aggressive species, most commonly found on rocky substrates down to depths of about 25 m. Velvet crabs feed on both animal and algal material, with brown algae being the dominant item found in gut content analysis.

Females grow slower and to a smaller maximum size than males, differences which are likely to be due to reduced growth during the females egg bearing phase. Growth is highly seasonal and males and females moult at different times of the year; the main moult for males is between April and July whereas females moult between May and August. Velvet crabs typically live for four to six years and recruit to the fishery at around age 3 (65 mm carapace width (CW)). They reach maturity at a carapace width of approximately 40 mm (approximately 1.5 years), although size at maturity varies according to location. This variation may be due to differences in water temperature or other factors such as population density, genetic makeup and fishing pressure. Mating occurs after females have moulted, when the shell is still soft. Studies carried out in Orkney and Shetland provide fecundity estimates of between 5,000 and 278,000 eggs per female. In contrast to brown crabs, there is no evidence that velvet crabs undertake extensive migrations. Their movements are thought to be restricted to a few hundred metres.