ORIGINAL PAPER Yoshinari Endo ® Hiroshi Asari ® Yutaka Watanuki Akiko Kato ® Maki Kuroki ® Jun Nishikawa Biological characteristics of euphausiids preyed upon by Ade ´ lie penguins in relation to sea-ice conditions in Lu ¨ tzow-Holm Bay, Antarctica Received: 2 November 2001 / Accepted: 3 June 2002 / Published online: 14 August 2002 Ó Springer-Verlag 2002 Abstract We examined the biological characteristics of euphausiids found in the stomachs of Ade´lie penguins in relation to sea-ice conditions in Lu¨tzow-Holm Bay over three seasons. Euphausiids, especially Euphausia superba, provedtobeastaplefoodforAde´liepenguinsirrespective of the ice condition. Body length and maturity-stage compositions of euphausiids were different among sea- sons,probablyreflectingsea-iceconditioninsummer.The mean body length decreased and maturity regressed dur- ing each season in E. superba, which was partly attribut- able to the selective feeding on large, mature female krill by Ade´lie penguins. The 1995/1996 year class of E. superba,whichwasspawnedwhentheseaicewasmost developed, was strong and conspicuous in the 1996/1997 and 1997/1998 seasons. This vigor indicates that sea ice provided females with good spawning conditions and larvae with good growth and survival rates. Introduction As part of the Sea Ice and Penguin Study (SIPENS) program, we investigated biological characteristics of euphausiids under fast sea ice at Hukuro Cove in Lu¨ tzow-Holm Bay, using Ade´lie penguins as a biological sampler. The bay is usually covered with landfast sea ice, which breaks up every few years (Ohshima et al. 2000). The bay never becomes entirely ice free, but the area of tidal cracks and open water expands in warm years. The sea-iceconditionisreportedtoaffectthebreedingsuccess of Ade´lie penguins through access to their foraging grounds in the Ross Sea (Ainley et al. 1998) and at Be´chervaise Island (Irvine et al. 2000; Clarke et al. 2002) during the chick-provisioning period. The sea-ice condi- tionisalsoknowntoaffectthegrowthofkrill:thegrowth ofjuvenile Euphausiasuperba isbetterinyearsofheavyice cover in the Antarctic Peninsula region (Kawaguchi and Satake 1994; Loeb et al. 1997). The differences in the sur- vivalandgrowthratesofpenguinchicksmayberelatedto the quality and quantity of the diet. For instance, Ainley et al. (1998) examined the diet of Ade´lie penguins at three colonies on Ross Island in the Ross Sea and found that the fish Pleuragramma antarcticum was the most important prey, especially during years or periods within a year when little pack ice was present. E. crystalloro- phias formed the remainder of the diet; more juveniles were consumed in a year of heavy pack-ice cover but more adults were consumed in 2 years of sparse pack ice. We analyzed biological characteristics of euphausiids found in the stomachs of Ade´lie penguins in three con- secutive breeding seasons from 1995/1996 to 1997/1998 with very different ice conditions, and obtained data on species composition, size-frequency distribution, and maturity-stage composition. We examined the relation- ship between the ice condition and the biological char- acteristics of the euphausiids in the diet of Ade´lie penguins. Ade´lie penguins eat euphausiids, amphipods, and fish. Around oceanic islands, E. superba is the main food item (Volkman et al. 1980; Lishman 1985; Trivelpiece et al. 1990), and in shelf areas a mixture of E. superba, E. crystallorophias, and fish is eaten (Emison 1968; Puddicombe and Johnstone 1988; Thomas and Green 1988; Ridoux and Offredo 1989). Lu¨tzow-Holm Bay is unique in that E. superba is the main food item of Ade´lie Polar Biol (2002) 25: 730–738 DOI 10.1007/s00300-002-0410-y Y. Endo (&) ® H. Asari Graduate School of Agricultural Science, Tohoku University, 1-1, Amamiya-machi, Tsutsumi-dori, Aoba-ku, Sendai 981-8555, Japan E-mail: yendo@bios.tohoku.ac.jp Tel.: +81-22-7178734 Fax: +81-22-7178734 Y. Watanuki Faculty of Agriculture, Hokkaido University, Kita-9 Nishi-9, Sapporo 060-8589, Japan A. Kato ® M. Kuroki National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan J. Nishikawa Ocean Research Institute, University of Tokyo, 1-15-1, Minamidai, Nakano-ku, Tokyo, Japan