Abstract:
The presence of unmarked individuals is common in mark–recapture study populations; however, their
origin and significance in terms of population dynamics remain poorly understood. At Marion Island, southern
Indian Ocean, where virtually all southern elephant seal Mirounga leonina pups born annually (1983–2008) were
marked in a long-term mark–resight study, large numbers of unmarked seals occur. Unmarked seals originate either
from marker (tag) loss or from immigration. We aimed to identify patterns in the occurrence of marked and
unmarked individuals that will allude to the possible origin and significance of the untagged component of the
population, predicting that tag loss will add untagged seals to mainly adult age categories whereas migrating
untagged individuals will be mostly juveniles.We fitted a generalized linear model using the factors month, year and
age-class to explain the relative abundance of untagged seals (tag ratio) from 1997 to 2009. Site usage of untagged
seals relative to tagged seals was assessed using a binomial test. Untagged seals, predominantly juveniles, were
present in the highest proportions relative to tagged seals during the winter haulout (tagged seals/total seals less
than 0.3) and the lowest proportion (approximately 0.5) during the female breeding haulout, increasing in relative
abundance from 1997 to 2009. Untagged seals were distributed evenly across suitable haulout sites while tagged
seals displayed high local site fidelity and occurred in greater numbers at or near large breeding beaches. Untagged
seals are considered to be mostly migrant seals that disperse from other islands within the southern Indian Ocean
and haul out at Marion Island during non-breeding haulouts in particular. Some of these seals immigrate to the
breeding population, which can be a key component of the local population dynamics.We emphasize the need for
mark–recapture studies to evaluate the role of the unmarked component of a population, thereby inducing a more
confident estimation of demographic parameters from the marked sample.
