Investigations of trophic interactions between marine mammals and marine resources typically use trophic models that are based on food composition, energy content of the prey and energy requirements of the predators. Although the diets are known to vary intraspecifically according to year, season, sex and age, this variation is rarely taken into account in published models. The aim of the present study was to assess the diet of the short-beaked common dolphin Delphinus delphis in the Bay of Biscay (France), examine intraspecific dietary variation and explore its effects on calculated daily food intake. Stomach contents were analysed from 71 common dolphins stranded along the French coast between 1999 and 2002. The composition of the fresh fraction of the diet was quantified by occurrence, relative abundance, reconstructed mass and prey size distributions for all prey taxa. Overall, the most important prey species were sardine, anchovy, sprat and horse mackerel, which represented 44.9, 22.6, 8.0 and 5.0% by mass of the fresh diet, respectively. The diet composition displayed significant variation in relation to season and sex/maturity status, in terms of both prey species composition and prey size distributions. Temporal variation in diet composition was interpreted in relation to prey availability in the Bay of Biscay. Because different prey types have different energetic values, changes in diet composition could lead to changes in the estimated total biomass needed to fulfil energetic requirements. In spite of the main prey species varying extensively, estimated daily food intakes changed relatively little because all diets included a high proportion of fat fish (73 to 93% by mass). KEY WORDS: Diet · Delphinus delphis · Common dolphin · Energy requirements · Prey availability · Stomach content analysisResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 354: 277-287, 2008 However, variation in dietary compositions of cetaceans exist and may have different causes, including differential foraging tactics among different segments of the population or spatiotemporal heterogeneity in prey availability (e.g. Young & Cockcroft 1994, Santos et al. 2004aBrown et al. 2001, Page et al. 2005 for pinnipeds). One may expect some degree of ontogeny in foraging abilities as a consequence of the development of sensorial, swimming and diving performances. For similar reasons, highly dimorphic species could also display sex-related differences in diet composition, and females associated with nursed calves could modify their diet either to face the increased energetic demand or as a response to the limited physical performance of their calf. Seasonal patterns in prey availability are mostly related to the recruitment of juveniles in prey populations. However, the energetic costs and benefits of foraging could be more dependent on factors such as prey aggregation or changes in energy density. Many potential prey species show seasonal changes in their distribution patterns and...

The short‐beaked common dolphin, Delphinus delphis, is abundant in both neritic and oceanic habitats. These two domains differ largely in terms of the nature of forage organisms and their availability to surface‐bound top predators, which suggests that the common dolphin should show extensive variability in foraging strategies as a response to these different habitats. However, although its diet is well known over continental shelves, so far, mostly because of sampling issues, nothing has been published on its diet in oceanic habitats. In this study, the diet of sixty‐three common dolphins bycaught in the French albacore tuna driftnet fishery in the summers of 1992–1993 in the oceanic Bay of Biscay was determined from their stomach contents and compared to neritic studies. The diet was dominated by fish (90% by number [N] and 53% by mass of total diet [M]). Cephalopods were also important in the total diet (9%N, 46%M) but were a prey of secondary importance in the fresh fraction (3%N, 10%M), presumably due to longer retention of cephalopod remains in the stomach. Crustaceans were of minor importance. At the species level, the myctophid fish (Notoscopelus kroeyeri) largely dominated the diet. Prey size ranged from 1 to 68 cm, but the majority of preys were from 2 to 30 cm. The prey characteristics and their state of digestion suggest that the common dolphin forages preferentially on small schooling, vertically migrating mesopelagic fauna in the surface layer at dusk and early night. The diet is taxonomically distinct from results obtained in neritic studies but is similar in terms of prey type and the corresponding feeding behavior.

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