The trophic link between squid and the emperor penguin Aptenodytes forsteri at Pointe Géologie, Antarctica

Cephalopod beaks retrieved from stomachs of dead emperor penguin chicks at Pointe Géologie, Terre Adélie, provide information on taxonomic and size composition of the penguin’s squid diet, on the trophic range of the squid species preyed upon and on the fractional trophic impact of the penguin on the whole food web. Emperor penguins prey upon four squid species (Psychroteuthis glacialis, Kondakovia longimana, Gonatus antarcticus, Alluroteuthis antarcticus) and do not take squid larger than 480 mm mantle length. Larger squid live either below the penguin’s diving range or are beyond its handling capacity. Nitrogen stable isotope ratios indicate that squids cover a range of about two trophic levels (2.5–8& dN). The impact of the emperor penguin, however, concentrates on the upper part of this range, about 68% of its squid prey being >6& dN. The principal components of the emperor’s diet, fish, krill and squid, differ distinctly in average trophic level. Consequently the trophic position of the emperor penguin changes accordingly with diet composition and may differ by almost one trophic level between different emperor penguin colonies. Introduction Cephalopods play an important role in the ecology of the Southern Ocean, having been identified as key species in the diets of many higher predators, including penguins, various seabird species, seals, cetaceans and fish (Collins and Rodhouse 2006 and References therein). Cephalopod beak samples from predator stomachs provide information on the trophic position of cephalopods in the Antarctic food web and on their migration patterns and ontogenetic dietary shifts (Cherel and Hobson 2005). Therefore, studies of predator diet and foraging are a major source of information on Southern Ocean cephalopod species composition and distribution (Clarke 1980; Cherel and Duhamel 2004; Cherel et al. 2004). Among the Antarctic penguins the deeper diving king and emperor penguins are thought to consume significant quantities of cephalopods (Williams 1995). Antarctic top predators such as emperor penguins catch larger specimens and a greater diversity of squid than fishing trawls during research cruises (Rodhouse 1990). The share of squid in the emperor penguin diet, however, is subject to substantial geographical variation, ranging from 3% at Pointe Géologie to 69% (wet mass) at Taylor glacier colony (Offredo and Ridoux 1986; Klages 1989; Gales et al. 1990; Robertson et al. 1994; Pütz 1995; Cherel and Kooyman 1998) and 74% in diet samples collected in the Weddell Sea (Ainley et al. 1992). Comprising about 3,000 breeding pairs, the emperor penguin colony of Pointe Géologie is average sized among the 46 documented colonies (Woehler 1993; Mellick and Bremers 1995; Messick et al. 1999; Coria and Montalti 2000; Todd et al. 2004). So far, feeding and alimentation of emperor penguins were studied once in this colony some 20 years ago (Offredo et al. 1985), a good reason to conduct our dietary study at this breeding site. Communicated by M. Wahl. I. Zimmer (&) T. Brey Alfred Wegener Institute for Polar and Marine Research, PO Box 120161, 27515 Bremerhaven, Germany e-mail: [email protected] U. Piatkowski Leibniz-Institut für Meereswissenschaften Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany 123 Mar Biol (2007) 152:1187–1195 DOI 10.1007/s00227-007-0766-1