Feeding ecology of the common shrimp Crangon crangon in Port Erin Bay, Isle of Man, Irish Sea

The diet of the common shrimp Crangon crangon (L.) was studied in Port Erin Bay (Isle of Man, Irish Sea) by analysis of stomach contents, with comparison by season and size class of diet composition and prey diversity. Monthly samples were taken from April 1995 to March 1998. Mysids and amphipods together constituted the dominant prey, accounting for >60% of the diet in both percent occurrence and percent abundance. Mysids were most important irrespective of season or size class. The small size group (< 10 mm CL) tended to be more dependent on epifaunal and infaunal organisms, reflecting ontogenic changes in diet. Trophic diversity and equality of diet varied with season and size class, with highest values in spring. Diet composition differed among seasons and size classes. Niche overlap index was higher between size classes (Schoener index: maximum = 0.83 in summer, minimum = 0.67 in autumn) than between seasons (Schoener index: maximum = 0.70 between summer and autumn, minimum = 0.46 between spring and winter). Shrimp size was significantly correlated with size of certain prey (e.g. Schistomysis spiritus and Gammarus sp.) though not with size of infaunal prey (e.g. Iphinoe trispinosa and Corbula sp.). This is discussed in relation to predator visibility, food availability and energy investment in handling prey. Feeding behaviour was linked to moult stage, ovarian condition and season. During premoult and postmoult there was low foregut fullness. Females carrying eggs and with advanced ovaries also displayed low fullness, suggesting that feeding activity is affected by the reproductive cycle. Fish otoliths in the stomachs showed that larger females (mainly > 10 mm CL) prey on 0-group fish co-occurring in the study area - plaice (Pleuronectes platessa), dab (Limanda Limanda) and sandeel (Ammodytes tobiannus). This suggests that predation by adult C, crangon can affect mortality of young fish in Port Erin Bay.