© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
At-sea distribution of female southern elephant seals relative to variation in ocean surface properties
a Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania Private Bag 05, Hobart, Tasmania 7001, Australia
b Antarctic Cooperative Research Centre and Institute of Antarctic and Southern Ocean Studies, University of Tasmania Private Bag 77, Hobart, Tasmania 7001, Australia
c School of Mathematics and Physics, University of Tasmania Private Bag 37, Hobart, Tasmania 7001, Australia
*Correspondence to Corey J. A. Bradshaw: Current address: Key Centre for Tropical Wildlife Management, Charles Darwin University, Darwin, Northern Territory 0909, Australia; tel: +61(0)8 8946 6713; fax: +61(0)8 8946 7088. e-mail: corey.bradshaw{at}cdu.edu.au.
We examined the relationships between physical oceanography (sea surface temperature – SST, sea surface height anomaly – SSH, ocean colour – OC, bathymetry – BA, sea-ice concentration – SI, and their associated gradients) and the foraging distribution (time at sea) of female southern elephant seals using generalized linear and generalized additive models (GLM and GAM). Using data from 28 separate foraging trips (22 unique individuals) over two years, we found that during the post-lactation trips (summer), the best GLM demonstrated a negative relationship between time at sea and SST and BA, but a positive relationship with SST gradient and SSH. During the post-moult (winter) trips, there was a negative relationship with OC gradient, SSH, and BA. The best post-lactation GAM identified a positive relationship with OC gradient, negative relationships with OC and SST gradient, and a non-linear relationship with SST. For the post-moult trip there was a negative relationship with OC, SST, BA and BA gradient, and a positive relationship with SST gradient. The relationship between the predicted time and observed time at sea was significant only for the post-lactation GAM, although predictability was low. That SST and its gradient predicted a small, but significant proportion of the variation in time at sea is indicative of the frontal zones within this area that are generally more biologically productive than surrounding regions. It appears that coarse-scale oceanographic configuration influences foraging behaviour in southern elephant seals only subtly. Nonetheless, some of the mechanisms influencing predator foraging are congruent with expectations of distribution of marine food resources at coarse spatial scales.
Keywords: bathymetry, elephant seals, environmental variation, generalized additive model, generalized linear model, oceanography, sea surface temperature
Received 27 January 2004; accepted 16 June 2004.
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