Stable Isotope Analysis of New Zealand Fur Seal Vibrissae Shows Deceased Juveniles Were Weaned Early

The New Zealand fur seal/kekeno ( Arctocephalus forsteri , NZFS) is a marine predator native to Aotearoa/New Zealand. Little is known about its fine‐scale dispersal around New Zealand in the nonbreeding season. As an alternative to external marking or electronic tracking, stable isotope analysis was trialed to determine the origin of stranded (deceased) NZFS young‐of‐the‐year (YOY). Vibrissae were collected opportunistically from YOY at nonbreeding sites on the east and west coasts of Northland and Hawkes Bay, New Zealand. Vibrissa growth rates and estimated vibrissa lengths at birth were calculated using samples from pups at established breeding colonies and stranded YOY. The δ 13 C and δ 15 N values at breeding colonies and in YOY were not sufficiently isotopically distinct to determine the colony of origin. Isotopic niche widths were greater for YOY than for pups from the same breeding season. For YOY, isotopic niche widths were greater after weaning than preweaning, implying weaned NZFSs foraged more diversely than their mothers. Based on the assumed median pupping date, the estimated maximum age at weaning for stranded YOY was 223–247 days, which is considerably less than previously published weaning data for NZFSs. Early weaning may contribute to increased juvenile mortality.