Mass Natural Mortality of Walruses (<i>Odobenus rosmarus</i>) at St. Lawrence Island, Bering Sea, Autumn 1978

In October-November 1978, several thousand living walruses came ashore in at least four localities on St. Lawrence Island where they had not been present before in this century. They hauled out also at two other sites which they have occupied annually but in much smaller numbers. At least 537 animals died on the haulout areas at that time, and approximately 400 other carcasses washed ashore from various sources. This was by far the greatest mortality of walruses ever recorded in an event of this kind. At least 15% of the carcasses on the haulouts were aborted fetuses, 24% were 5-6-month-old calves; the others were older animals ranging in age from 1 to 37 years old. About three-fourths of the latter on the haulouts were females; in the non-haulouts areas, the sex ratio was about 1:l. Forty of the best preserved carcasses were examined by necropsy. The principal cause of death was identified as extreme torsion of the cervical spine, with resultant cerebrospinal hemorrhage, apparently due to traumatization by other walruses. Nearly all of the dead were extremely lean, having less than half as much subcutaneous fat as healthy animals examined in previous years. RfiSUMfi. En Octobre-Novembre 1978, quelques milliers de morses vivants echouaient dans au moins quatre IocaIitCs de I'ile St. Laurent, qu'ils ne frequentaient jamais avant, depuis le debut du sitcle. 11s se trainaient aussi jusqu'B deux autres sites qu'ils frequentaient chaque annte mais en nombre bien moins important. Au moins 537 animaux mourraient sur 1es plages d'echouage, B cette epoque et approximativement 400 autres carcasses etaient rejetees sur le rivage provenant de sources variees. C'ktait de loin la plus grande hkecatombe de morses jamais enregistree dans un dvenement de ce genre. Au moins 15% des carcasses sur ces aires d'echouage etaient des foetus avortes, 24% Ctaient des jeunes de 5 A 6 mois; les autres etaient des animaux plus -6s dont I'age allait de 1 B 37 ans. Environ les V4 de ces derniers ttaient des femelles. En dehors de ces points d'tkhouage, le rapport des sexes etait environ de 1 B 1. Quarante des carcasses les mieux conservees, tstaient examinees avec autopsie. Celle-ci revelait qu'une extreme torsion de I'tpine cervicale etait la principale cause de mortalit& avec comme consequence une hemorragie cerebrospinale, apparamment due B une traumatisation par d'autres morses. Presque tous les morts Ctaient tres penchks car ils avaient moins de la moitie du gras souscutann6 qu'ont les animaux en bonne sant6, examines les annkes prtcedantes. Traduit par Alain de Vendegies, Aquitaine Company of Canada Ltd.