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 compari son by season and size class of diet composition and prey diversity. Monthl y samples were taken from April 1995 to March 1998. Mysids and amphipods to gether constituted the dominant prey, accounting for >60% of the diet in bo th percent occurrence and percent abundance. Mysids were most important irr espective of season or size class. The small size group (< 10 mm CL) tended to be more dependent on epifaunal and infaunal organisms, reflecting ontog enic changes in diet. Trophic diversity and equality of diet varied with se ason and size class, with highest values in spring. Diet composition differ ed among seasons and size classes. Niche overlap index was higher between s ize classes (Schoener index: maximum = 0.83 in summer, minimum = 0.67 in au tumn) than between seasons (Schoener index: maximum = 0.70 between summer a nd autumn, minimum = 0.46 between spring and winter). Shrimp size was signi ficantly correlated with size of certain prey (e.g. Schistomysis spiritus a nd Gammarus sp.) though not with size of infaunal prey (e.g. Iphinoe trispi nosa and Corbula sp.). This is discussed in relation to predator visibility , food availability and energy investment in handling prey. Feeding behavio ur was linked to moult stage, ovarian condition and season. During premoult and postmoult there was low foregut fullness. Females carrying eggs and wi th advanced ovaries also displayed low fullness, suggesting that feeding ac tivity 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-occ urring in the study area - plaice (Pleuronectes platessa), dab (Limanda Lim anda) and sandeel (Ammodytes tobiannus). This suggests that predation by ad ult C, crangon can affect mortality of young fish in Port Erin Bay.