Knowledge about the diet of killer whales, Orcinus orca (Linnaeus, 1758), in the waters surrounding western Europe is scant (O'Brien et al., Reference O'Brien, Berrow, McGrath and Evans2009) with the exception of Norwegian and Scottish waters where killer whale occurrence is strongly linked with that of Atlantic herring (Clupea harengus) (Similä et al., Reference Similä, Holst and Christensen1996) and harbour seals (Phoca vitulina) (Bolt et al., Reference Bolt, Harvey, Mandleberg and Foote2009; Foote et al., Reference Foote, Similä, Vikingsson and Stevick2010). In Britain and Ireland limited research from observational studies (Wilson & Pitcher, Reference Wilson and Pitcher1979; Ryan & Wilson, Reference Ryan and Wilson2003) and from diet content analysis (Santos et al., Reference Santos, Pierce, Wijnsma, Ross and Reid1995; McHugh et al., Reference McHugh, Law, Allchin, Rogan, Murphy, Foley, Glynn and McGovern2007) indicate that killer whales there feed on Atlantic salmon (Salmo salar) and oceanic cephalopods. In contrast, stable isotope analysis of a Scottish specimen suggested a diet of marine mammals (McHugh et al., Reference McHugh, Law, Allchin, Rogan, Murphy, Foley, Glynn and McGovern2007). In one study (Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009), morphological, genetic and stable isotope evidence indicated that two ecotypes exist sympatrically in the north-east Atlantic: generalist and specialist, where stable isotope ratios indicate ecotype (Bolt et al., Reference Bolt, Harvey, Mandleberg and Foote2009; Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009). Ecotypes were reported as generalist (type 1) and specialist (type 2), the latter exhibiting a narrower niche-width and higher trophic status (Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009). The diet of generalists is believed to include herring, Atlantic mackerel (Scomber scombrus) and harbour seals (Foote et al., Reference Foote, Similä, Vikingsson and Stevick2010) while that of specialists may comprise other cetaceans such as baleen whales (Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009).

A stranded adult female killer whale measuring 5.12 m in length found at Doohoma, County Mayo, Ireland was examined on 4 October 2010. Significant apical tooth-wear was noted, suggesting type 1 ecotype (Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009). Part of the mitochondrial DNA control region was sequenced (see (Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009) for methods), indicating that the specimen was an Atl_1_33 haplotype, the most common haplotype found between Iceland and Norway (Foote et al., Reference Foote, Newton, Piertney, Willerslev and Gilbert2009, Reference Foote, Morin, Durban, Pitman, Wade, Willerslev, Gilbert and Da Fonseca2011). The specimen was found to have been pregnant with a near-term foetus. The stomach contents were removed for examination. Siphonostomatoid copepods were found in the stomachs and identified as Cecrops latreilli (Leach, 1816), an ectoparasite on the gills and skin of sunfish, (Mola mola (Linnaeus, 1758) and M. lanceolata (Grabda, Reference Grabda1973)). Cecrops latreilli has a cosmopolitan distribution, following that of its host and has previously been recorded on the gill filaments of sunfish in Irish waters (Cooper et al., Reference Cooper, McGrath and O'Connor1982). Partially digested bony structures consistent in shape and size with the modified toothless premaxilla and dentary of sunfish were also found in the stomachs. It was not possible to calculate the body size of the prey by allometric regression as the spongiform nature of sunfish bones means they are prone to shrinkage (Gregory & Raven, Reference Gregory and Raven1934). While C. latreillii has been questionably recorded from Atlantic bluefin tuna (Thunnus thynnus) (Brian, Reference Brian1912; Grabda, Reference Grabda1973), the presence of bony structures from sunfish in the whale's stomach confirms that sunfish was the copepod host.

Killer whale predation on sunfish has previously been recorded from observational studies in the South Pacific and Gulf of Guinea (Gladstone, Reference Gladstone1988; Weir et al., Reference Weir, Collins, Carvalho and Rosenbeaum2006) and in captivity (Caldwell & Brown, Reference Caldwell and Brown1964). To our knowledge, this is the first recorded incidence of such an interaction in the North Atlantic. It is possible that erratic deep diving (to almost 500 m) observed in a satellite telemetry study of sunfish was an avoidance response to predators such as killer whales (Sims et al., Reference Sims, Queiroz, Doyle, Houghton and Hays2009). Whether sunfish represent a significant proportion of the diet of killer whales in the North Atlantic is unknown. There are no results from stable isotope ratios of sunfish tissues in the literature (Pope et al., Reference Pope, Hays, Thys, Doyle, Sims, Queiroz, Hobson, Kubicek and Houghton2010); however, future studies on both killer whales and sunfish using this and other analytical chemical techniques may shed light on the importance of sunfish in the diet of killer whales. While this is a single observation, it indicates that killer whales do predate on Mola mola which the authors feel merits further investigation.