Galimberti,F. & Sanvito,S. (2001)
Modelling females haul out in southern elephant seals (Mirounga leonina)
Aquatic Mammals 27(2): 92-104.
Abstract
In Pinnipedia species is difficult to achieve good estimates of
population size by direct counts, because a part of the population is at
sea at any time. In southern elephant seals (Mirounga leonina) the
estimation of population size is carried out starting with the number of
females hauled out during the breeding season, by applying a correction
factor calculated from life tables. Different models were proposed in
the literature to estimate the total number of females hauled out. In
this paper, we consider the model proposed by Rothery & McCann (1987)
for the South Georgia population, applying it to a five years data set
for the population of Sea Lion Island (Falkland Islands). We test the
assumptions of the model, finding them reasonable. We fit the model to
our data set, obtaining an excellent fit in all cases, better than or
equal to other models proposed in the literature. The precision of the
estimation depended mostly on the length of presence on land of females,
which is a constant of the model. A 1 day variation in length produced a
4% variation in the estimated total number of females. When a good model
of the haul out process is already available for the population, even a
single count close to the peak of the season is enough to estimate total
females in the ± 2% range. When such a model is to be estimated from
data, at least 8 counts are needed to have a good estimate. The model
was not only a good description of the haul out process at population
level, but its application to a set of daily counts of single harems
demonstrated its usefulness also at sub-population level.