Ontogenetic changes in the diet of L. forbesi: insights from fatty acid and stable isotope analysis

The diet of L. forbesi in Scottish waters was subject to ontogenetic changes as shown by fatty acid and stable isotope analysis. Crustaceans were more frequently found in stomachs of immature squid smaller than 150 mm mantle length. With increasing size ( > 150 mm) and maturity fish becamemore prominent in the diet. Seasonal differences in the diet were also found but seemed to be linked to seasonal changes in squid size. Prey type and species composition also varied slightly between different regions. Differences between size and region were reflected in fatty acid profiles of mainly the digestive gland tissue. High levels of the saturated fatty acid 16:0 and the polyunsaturated fatty acids 20:5n-3 and 22:6n-3, which are typical for fatty acid signatures of many crustacean species, were found in tissues of smaller immature squid. Higher levels of the monounsaturated fatty acids 16:1n-7, 20:1* and polyunsaturated fatty acids of the linoleic family (C18), which are typical forfish prey species, were found in squid of larger sizes and higher maturity stages. Regional differences found for fatty acid profiles of the digestive gland suggested a significant difference in origin of fatty acids thus indicating different diets. In comparing the fatty acid profiles of squid to those of putative prey species of L. forbesi, it was apparent that gadid species Trisopterus minutus, Micromesistius poutassou and Gadus morhua, were an important component of the diet ofL. forbesi. With increasing size of the predator however the composition of fish species in the diet shifted more towardsGadiculus argenteus, Trachurus trachurus and Sebastes marinus. Results of quantitative fatty acid analysis on the estimate of the contribution of each prey species to the diet also reflected this shift in the importance of different prey species with increasing predator size. Due to slower turnover rates in muscle, changes in carbon and nitrogen stable isotope ratios with diet were more pronounced in this tissue. Smaller squid showed the lowest δ15N ratios thus feeding on the lowest trophic level of all squid examined. δ 13C ratios were the most depleted for small squid indicating that small squid feed on prey closer to the carbon source in the food chain. Stable isotope analysis also showed that squid with fish remains in their stomachs showed higher ratios of nitrogen and less depleted carbon ratios than squid feeding on crustaceans. Comparisons of isotope levels of squid and putative prey species identified blue whiting and silvery pout as putative prey species of bigger sized squid. Small squid seemed to feed on the same trophic level and all other prey species examined were always similar or higher in isotope ratios than squid of any size.