INTERTIDAL MIGRATION BY THE SHORE CRAB CARCINUS-MAENAS

Intertidal migration by the shore crab Carcinus maenas (L.) was studied on a seasonal and tidal basis from May to December 1992 on Ynys Faelog, Menai Strait, North Wales, UK. Work was carried out using a specially designed, remotely operated, unbaited directional trap, in conjunction with other conventional methods. Highly oriented migration was detected, with significantly more crabs taken in traps facing up- and downshore tidal flow than when facing against or 90-degrees to flow. Facing-flow crab catches from falling tides were greater than those from rising tides. No significant differences were found in the numbers of daytime and nighttime migrants. Green (early intermoult) crabs were more attracted to bait than were red (late intermoult) crabs, food searching activity being extensive during times of flood tide, and rare during ebb tides. Most crabs caught in the unbaited trap were green male early intermoults, and they were retrieved more abundantly from the top half of the shore; female crabs were virtually absent from the lower shore, Upper-shore migrants were smaller than lower-shore migrants, which contained a higher proportion of late intermoult red crabs. Crabs remaining on the shore at low tide were smaller than high-tide migrants; nearly all were green and they were located most abundantly around the midshore. Numbers of both resident and migrant crabs declined between summer and winter.