UC Santa Cruz

The foraging behavior of top predators is linked to the distribution and abundance of prey, which in turn is determined by oceanographic features. Thus, the identification of the specific foraging behaviors associated with different environmental conditions is of primary relevance to understanding the foraging behavior of top predators. The southern elephant seal (Mirounga leonina) and crabeater seal (Lobodon carcinophaga) are important top predators of the Southern Ocean, and as such, they can integrate information about several layers of primary and secondary productivity. Moreover, these two species are part of different trophic pathways of the Southern Ocean. Elephant seals are part of the northern slope and oceanic waters food web, where copepods, mesopelagic fish and squid occupy the mid-trophic levels. Crabeater seals are part of the southern food web, where the Antarctic krill, Euphausia superba, is considered the dominant species in the seasonal pack ice zone.

The main goal of my study was to determine what level of flexibility, if any, is displayed by the southern elephant seals and crabeater seals from the western Antarctic Peninsula (wAP) in their foraging behavior and habitat utilization patterns, using a combination of stable isotope data (δ13C and δ 15N ), satellite telemetry data (tracking and diving) and environmental data (from animal-borne sensors, oceanographic models and satellite oceanography) to address two main aspects of the ecology of the two species: (a) Feeding habits and trophic ecology, and (b) Habitat utilization in relation with the oceanography.

Combining stable isotope analysis and satellite telemetry, I studied the variability in individual foraging strategies of adult female southern elephant seals analyses (Chapter 1). Most individuals were specialists, with half of the individuals utilizing 31% or less of their available niche. I found 8 different foraging strategies for these animals. Hence, female elephant seals from the wAP are a diverse group of predators with individuals utilizing only a small portion of the total available niche, and therefore have the potential to expand their range to exploit new niches that will potentially become available as a consequence of environmental change.

Due to the high specialization of crabeater seal to forage on Antarctic krill Euphausia superba, the species is particularly vulnerable to the environmental changes that are already occurring in the Southern Ocean as a consequence of the climatic global change. Using stable isotopes, I studied the trophic ecology of crabeater seals from the western Antarctica Peninsula (Chapter 2). The median (range) contribution of Antarctic krill to the diet of the crabeater seal from the western Antarctica Peninsula (wAP) is 87.9 (81.2 - 94.8) %, however the results showed that crabeater seals are capable of displaying trophic plasticity, supplementing their diet with other prey that might be available in the environment (i.e. fish).

I also investigated the foraging behavior of adult female elephant seals from the wAP in relation with the in situ oceanographic conditions that they experience during their ca. 8-months post-molt migration at sea (Chapter 3). There was a wide diversity in the areas utilized by elephant seals contradicting the general idea of elephant seals as being part of the northern slope and oceanic waters food web of the Southern Ocean, where copepods, mesopelagic fish and squid occupy the mid-trophic levels, and extends their range into the southern/coastal food web, where the Antarctic krill, Euphausia superba, is considered the dominant mid-trophic species.

Finally I present a suit of habitat models for a conspicuous predator of the wAP: the crabeater seal, likely the largest consumer of krill in the world (Chapter 4). Crabeater seals from the wAP presented a coastal distribution, occupying water of the inner continental shelf preferably, and rarely venturing beyond the break of the continental shelf (1,000 isobath). The best habitat model, included variables associated with bathymetry and water column features. Crabeater seals preferred shelf areas between 100 and 200 km from the shelf break, with bathymetric slope of about 5° (~10%), and shallow depths (<500 m deep). As well I found a negative trend between crabeater seals and zones of intrusions of warm, off-shelf Circumpolar Deep Water (CDW).