A demographic comparison of two southern elephant seal populations

Abstract

Summary To estimate concurrent age‐specific survival for southern elephant seals at Macquarie and Marion islands, seals were marked from 1993 to 1997 in the first 3 weeks of life and resighted (recaptured) on return to their natal islands (1994–2001). These recaptures formed the basis for the survival analysis in the mark–recapture program mark. Weaning masses were collected at each location. Recapture probabilities were (= 376·480, P < 0·0001) higher at Marion Island than at Macquarie Island. There are two possible reasons: (1) the population at Marion Island is smaller and less dense than at Macquarie Island and (2) seals hauled out along a smaller section of the coast at Marion Island than at Macquarie Island, which: (1) facilitates the detection and individual identification of seals and (2) increases access to hauled out seals. Age‐specific survival estimates (corrected for preweaning mortality and tag loss) differed ( = 22·264, P < 0·05) at the two islands and were consistently higher at Macquarie Island. The survival estimates for male and female seals were different at Macquarie Island (= 34·657, P < 0·0001) and Marion Island (= 20·373, P = 0·002). Female survival estimates were higher than male survival estimates. The combined survival estimates for juvenile seals (1–3 years) differed between islands but survival of older seals (4–6 years) did not. The inclusion of gender in the survival models did not improve model performance and hence male and female estimates were considered jointly. The mean wean masses of male and female seals combined from 1993 to 1998 were not different between islands (T6837 = 1·169, P = 0·242). At Macquarie Island the mean annual wean mass was 118·8 kg (SD = 27·2, n= 6504) while at Marion Island it was 120·6 kg (SD = 24·7, n= 335). The mean age at first breeding was different (P < 0·001) at the two island populations. At Macquarie Island the mean age of first breeding was 4·68 years, and at Marion Island it was 3·95 years. More (= 67·39, P < 0·0001) 3‐year‐old females breed at Marion Island (28·7%) than at Macquarie Island (1·2%) and the proportion of seals that had bred at least once by age 7 was greater at Marion Island than at Macquarie Island. We conclude that the observed decreases in elephant seal numbers between the 1950s and 1990s in the Pacific and Indian Ocean sectors were driven principally by resource limitation in the Southern Ocean. A conglomerate of factors including local predation by killer whales and intraspecific resource competition is postulated as a cause for the inter‐island (regional) differences in population trends. It appears that more resources are available per capita to the Marion Island population than are available to the Macquarie Island population.