Spatial and temporal distribution of shore crabs Carcinus maenas in a small tidal estuary (Looe Estuary, Cornwall, England)

Tidal, diel, and spatial variations in numbers, sex, size, and colour morphology of shore crabs Carcinus maenas caught in baited drop nets during tidal periods, at neap tide and spring tide, were studied in the strongly tidal Looe Estuary, Cornwall, Southwest England. Depth, salinity, temperature, pH, and oxygen tensions were measured simultaneously. High numbers of both genders were caught in the estuary. In total, 61% of adult crabs caught were females. However, the sex ratio (males over females) in the catches significantly increased (P < 0.05) from approximately 0.2 at the station nearest to the mouth of the estuary to approximately 4 at the innermost station. Due to the well-established relationship between carapace colouration and inten-noult duration, catches were analysed with regard to red and green colour forms, besides for sex and size. Green crabs were caught throughout the estuary and constituted 79% of total catches. Green males dominated the shallow stations, whereas green females dominated the deep stations. The catch per unit effort (CPUE) was related to tidal and diel phases, with most adult crabs being caught during high tide and only few during low tide. Also, more adult crabs were caught during night time than during day time. The CPUE increased with increasing depth, and crabs were never caught at salinities below 15parts per thousand and rarely at salinities below 20parts per thousand. Oxygen tension, temperature, and pH exerted no effect on the distribution of shore crabs. Even though conclusions based on these data depend on whether catch-data analysis reflects true population abundances, sex ratios, and colour morphology compositions, the data suggest that the small size, strong current, and high salinity characteristics of the Looe Estuary allow both genders and colour forms to migrate into the estuary during high tide and to return to the shore before low tide, thereby exploiting a marginal feeding habitat.