Dispersal of female southern elephant seals and their prey consumption during the austral summer: relevance to management and oceanographic zones

Abstract

Summary Numerical models that predict trophic structure require both accurate information on prey consumption rates and estimates of spatial and temporal variation. In the Southern Ocean little information exists on the spatial and temporal patterns of resource use by predators, so we attempted to examine these patterns for an important Antarctic predator, the southern elephant seal. We (i) defined the area of the ocean used by the adult female component of the elephant seal population at Macquarie Island; (ii) quantified the time these seals spent in the different regions of the Southern Ocean; and (iii) estimated the biomass of fish and squid prey consumed per fortnight and per region. We used data from 42 post‐breeding females collected from 1992 to 2001. The data consisted of locations determined by geo‐location (based on light intensity) recorded using dataloggers. A randomized, incremental analysis of at‐sea locations indicated that a sample of 25 individuals was required to provide 95% coverage of the total area of ocean used. The greatest amount of time (44·6%) was spent in the region between the Antarctic Polar Front (APF) and the Subantarctic Front (SAF). Up to 20% of time was spent south of the Antarctic Circle or within Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Statistical Subareas, indicating that seals from Macquarie Island are also important summer‐time predators in high Antarctic waters. The adult female population was estimated to consume 122·73–125·81 × 106 MJ for the post‐lactation foraging trip (31 142–31 925 tonnes of prey). Of this, 47·2–53·4% was consumed within the CCAMLR Statistical Subareas and the Australian and New Zealand exclusive economic zones (EEZ). Synthesis and applications. Our study emphasizes that (i) a large sample of individual seals (25) can estimate spatial trends in prey consumption; (ii) much of the estimated prey consumption occurs within fishery‐managed zones, therefore elephant seals should be included in models predicting trophic structure in the Southern Ocean; and (iii) recent commercial fishery catch within these zones is minimal relative to the prey consumed by elephant seals, but increases in fishing activity in these zones may result in competition for marine resources.