] Jfar. Bio/ . .-iJS. l".K ;_2002!. 82, 9!3 916 Printed in thc l'nited Kingdom Changes m the cephalopod diet of southern elephant seal females at King George Island, during EI Nifio-La Nifia events Uwe Piatkowski*, Daniel E Vergani 1 and Zulma B. Stanganelli 1 *Institut für Meereskunde, Universitäl Kiel, Düsternhrooker \\'eg 20, D-24105 Kid, Germany. tProyeno Cyber Atlas Centro Nacional Patag6nico, Boule\·ard Alm. Browu 3500, 9120 Pm·rto Madryn, Chubut, Argentina. E-mail: atlas(dcenpat.edu.ar *Corresponding authnr, e-mail: upiatkowski(r.i"ifm.uni-kiel.de Possiblc of "El Nüi.o' Southern Oscillation (ENSO) components 'EI Nillo' and 'La Niiia' on popu- lations of southern elephant seals, A1irounga leoninn, wcrc analysed. Changcs in the ccphalopod dict composition of moulting fCmales at King Gcorge Island, Antarctic Peninsula werc considcred. The dict of f(;male elephant seals sampled in l99J .... l992 and 1992 .... 1993 (EI Niiio ycars) were compared \Vith those taken in 1995 .... 1996 (La Niiia year) at the same site and employing the same methodology. The squid Ps_ychroieuthis glacialis constituted the main cephalopod prey of the seals. A reduction in the 'Index of Biomass Ingcsted' by female elephant seals (IBIF) of this prey species was observed in 'EI Nii\o' years (1992, 1993) compared witb the 'La Niii.a' year (1996). This reduction in biomass applied to all squid species in the seals' prey with the exception of Galiteuthis glacia!is, which occurred in 1ow numben, but was more abundant during EI Niiio years than in the La Niii.a year. INTRODUCTION Thc life cyclc of southern dephant seals comprises two pcriods on land, one for breeding the other for moulting. Both cvents are extremely exigent in terms of energy. Fernales have to store enough encrgy reserves to last the bH·cding period fast du ring which thcy increase the birth mass of thcir pups over an average of23.7 days of!actation (Arnbom et al., 1997). Ahead of the füllowing moulting period thcy have to recowr, at least in part, the biomass lost du ring brccding to store enough reserves to cope with a sccond starvation period. Although in terms of energy storcs required, breeding is approximately t\vice as expen- siw· as the moult (Boyd et al., 1993} the feeding pcriod ! March·-· August) prior to breeding (\ergani, 1985; Vergani & StanganeHi, 1990; Bornemann et al., 2000) Iasts thrce timcs Ionger than the fceding period prior to the moult (::"ovcmber--December). Female elcphant seals brccd and mou!t annually at King George Island, South Shetland Islands. Breeding females arrin· betwcen 8 and 26 September and reach breeding scason pcak between 21 and 31 Octobt:r (\:ergani & Sranganclli. 1990; \'ngani et al., 2001) . .Moulting fema!es that had brcd at King Gcorge Island as \Yell as sorne from the South Gcorgia brccding population start arriving early Deccmber and dcpart from the beadws in early April (\Crgani, 1983; :\lcConndl et al.. !992; & Fedak. 1996:. Thc group offt·males 13 of !2J which bred at South Georgia f{·d to thc south-\vt'St of Adelaide bland :·Amarctic Peninsula). At thc time ofmoulting (Decemher- January) these seals wcre approximately ISOO 3000 km distant from South Gcorgia (Figurc 1\ and must have completed their moult on beaches in the vicinity of the .-\ntarctic Peninsula: tagged individuals in South Georgia werc found moulting at King George Island. Although ]rmmat of thr .\larin.r Biologira! A.rsociatian. qf lhe l'n.ited A"ingdam (2002) there is no infonnation on the feeding t:cology of post- breeding elephant stals at King George Island most of thc post-moulting females vvhich had tran!led south- west along the coast of the Antarctic Peninsula into the Hellingshausen Sca reaching the area of Alcxander Island (Borncmann et aL, 2000). Possible effects of 'El Sout hern Oscillation ( ENSO) on hreeding population trends (Vcrgani & Stanganelli, l990) and pup weaning rnass in fonale dephant seals havc becn cxaminrd, with weaning mass higher during 'La ]\;'iüa'. and lower during 'El Niti.o', the cold and warm pha.scs of .ENSO respectively (Vergani et al., 2001). :Major effects of EI La Niiia anomalics on the Antarctic e-cosystem may bc related to the seasona1 variabilit y of the pack-icc zone. This zonc is thc most productive- in the Antarctic region on an annual basis (Hcmpel, !985) and variation in pack ice cxtenl may cause pronounccd changcs in fOod availability for tCmale elephant seals. Although monthly ENSO fluctua- tion appears to bc linkcd with month1y fluctuations in \Vcstern Antarctic Peninsula (\\'AP) sea ice extent (Smith ct aL, 1996), no direct cvidencc of changes in food aYail- ability to the fema!e elcphant scals has as yet lwen fi)Und in th<.·sc pcriods. :VL\TERIALS A"\D :\ way in which to exp!ore changcs in fOod m·ai\ability is to <:ompare biomass ingcstcd by f(·ma!c c1ephant seals during El ::'\ii'i.o and La NiJi.a ycars. The first prdiminary studi-es on dirt wcrc carried out at King Gcorge Islancl in 1991·· 1992 and 1992· during thc mmdting Sf'ason (Piatkowski & Vergani, 2000). A complementary study was carricd out in 1995·--1996 ( Dancri ct al., 2000). In thi.s study \V<' rcport on thc ccpha!opod diet of moulting