Temporal and ontogenetic variation in the diet of squid (Loligo forbesii Streenstrup) in Scottish waters

The squid Loligo forbesii is the only cephalopod species currently targeted by fisheries in the northern NE Atlantic. An active predator, it feeds primarily on fish, crustaceans and cephalopods. During 15 years since the only previous large-scale study of the diet of this species in Scottish waters, there have been substantial changes in marine fish abundances. The present study evaluates sources of variation (temporal, ontogenetic) in diet composition and prey size preferences of L. forbesii, including a comparison of contemporary (July 2006-June 2007) and historical (1990-1992) dietary datasets. Results revealed significant size-related and seasonal variation in diet composition and prey size. Teleost fish of the families Ammodytidae and Gobiidae were eaten by squid of all sampled sizes, although occurrence of gobies was generally more frequent in smaller squids, while Gadidae were eaten more frequently by larger squids. Cannibalism was also more frequent in larger squids. Compared to the 1990-1992 dataset, clupeid fish were less important in the diet of squid in 2006-2007, while the importance of gobies increased, and the size of gobies eaten also increased. The trend in gadoids differed according to the index used: their frequency of occurrence was considerably higher in 2006-2007 than in 1990-1992, but their numerical importance was slightly lower. In general, results provided little evidence that changes in the diet of L. forbesii correspond with changes in fish abundance, at least at the scales at which these are measured.