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

Abstract

Summary1. 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. 2. 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. 3. 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. 4. The adult female population was estimated to consume 122·73-125·81 × 10 6 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). 5. 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.

You are what you eat: describing the foraging ecology of southern elephant seals (Mirounga leonina) using blubber fatty acids

“…Although the taxonomic resolution of the diet was coarse (i.e. fishes versus squid), the ability to make this distinction has important implications for the modelling of spatially and temporally explicit prey consumption rates by large marine predators (Thompson et al 2000;Hindell et al 2003). Understanding broad-scale diet structure and the associated variance will permit more precise estimates of prey Proc.…”

“…The exploitation of Antarctic marine resources is increasing because of the growing sophistication in fishing technology and increased market demands. Mirounga leonina is known to be an important consumer of squid Santos et al 2001;Hindell et al 2003), and many of the known squid species have rather small, but developing, fishing industries (Gonzalez et al 1997). Most cephalopods have short life cycles and rapid growth rates, so they may be highly susceptible to overfishing, but also capable of rapid recovery (Pierce & Guerra 1994).…”

“…northern Ross Sea and Antarctic continental shelf) during the winter than during the summer (see also Hindell et al 2003). Following the hypothesis that elephant seals foraging mainly in the vicinity of the Antarctic continental shelf (SW region) were likely to encounter different prey communities from the more pelagic foragers (SE, E-APF and W-APF), we applied a second DFA using this distinction as the classification variable.…”

“…Their capacity to ingest large quantities of prey (fishes and squid) and the considerable size of the world population make them important consumers of Antarctic marine resources Santos et al 2001;Hindell et al 2003). Santos et al (2001) estimated that the total annual consumption by M. leonina is ca.…”

“…Part of this process requires information on both the spatial and the temporal aspects of predator diet (Brown et al 1999;Shurin 2001), and this is particularly important for wide-ranging species that occupy vastly different ecological zones during the course of their regular movements (Hyrenbach 2001). In the case of wideranging top-level marine predators, such as whales, seals and seabirds, there is scant information on diet structure with respect to individual, geographical and temporal variation (Iverson et al 1997b;Hindell et al 2003). This can lead to inaccurate predictions arising from numerical models used to predict consumption rates as part of the management of marine resources such as commercial fisheries (Thompson et al 2000;Bjørge et al 2002).…”