Project 0809/5

Project Title: Quantifying trophic links in several Antarctic marine predators

Chief Investigators: Prof Mark Hindell

To date, this project has provided information on the relative trophic position of two important Antarctic pack-ice predators: the southern elephant seal (SES; Mirounga leonina) and Antarctic fur seal (AFS; Arctocephalus gazella).

Evidence suggests a causal link between prey resource availability and recent declines in some SES populations, whereby the availability or quality of food impacts on the survival and recruitment of juvenile seals into the breeding population and/or adult (female) fecundity (McMahon et al. 2005). Thus, the focus of this component of the project was to increase our understanding of the diet and trophic links of the juvenile SES population of which we know little about. Using stable isotope and satellite telemetry data, we examined the foraging habitat and diet of sub-yearling SES (n = 15) from Macquarie Island (MI; 54˚30’S, 158˚57’E) during their first trip to sea. Growth rates of whiskers, which act as a temporal record of feeding, were estimated using carbon (δ13C) and nitrogen (δ15N) isotopic values along the length of the whisker. Seals were also equipped with satellite-relayed data loggers which measured seal location and foraging behaviour at approximate time of whisker growth (0.22 ± 0.07 mm d-1). Analysis of consecutive sections of whisker allowed us to reconstruct a time series of isotope data for >7 months per seal. Most animals fed in southern waters around the Polar Front (PF) or along the Antarctic Circumpolar Current (ACC; 60–70˚S) as indicated by their lower δ13C (<-20‰) values, and validated by tracking data. However, two pups had higher δ13C (>-19‰) values. Tracking data confirmed that these seals travelled southwest of MI, feeding in waters north of the ACC Front (>60˚S) and close to the PF (~55˚S). Whisker δ15N values ranged from 7.9 to 14.5‰ suggesting seals occupied a range of trophic levels within the two broad-scale foraging habitats identified. The combination of stable isotope analysis and satellite telemetry data revealed considerable spatial and temporal variation in foraging habitat and diet for these pups.

The Antarctic fur seal (AFS: Arctocephalus gazella) is one of the best studied Antarctic marine predators. However, core winter foraging habitats and diet preferences remain largely undocumented for this abundant top marine predator. By using a combination of stable isotopes in whiskers and blood and satellite telemetry data, we were able to document spatial and temporal variation in winter foraging habitat and diet of Antarctic fur seals from Cape Sherriff (CS, Antarctic Peninsula, 62˚28’S, 60˚46’W) and Marion Island (MI, Southern Indian Ocean, 46°54’S, 37°5’E) in 2008. Preliminary analysis indicates that whisker isotopic values reflect the winter migration patterns of AFS seals between sites, as corroborated by tracking data. By comparing the isotope ratios along the length of the whisker with those of suspected prey items, changes in food sources and habitat can be surmised for the temporal span represented by the growth of the whisker. Seals which migrated north from Cape Sherriff (~ 62˚S) to forage in waters off the Chilean and Patagonian coasts, South America (~ 45-50˚S) showed substantial variation in both δ13C and δ15N along the length of the whisker corresponding to the winter foraging period.  δ13C values ranged from -23.4 to -17.3‰ and δ15N values ranged from 7.4 to 11.0‰, reflecting both variation in foraging habitat (from high Antarctic to the subtropics) and prey (from Antarctic krill to mesopelagic fish and squid), respectively. In contrast, isotopic values along the length of the whisker from Marion Island seals reflected migration patterns south of Marion Island (~ 46˚S) to higher latitude waters (>60˚S). δ13C values ranged from -18.2 to -23.7‰ and δ15N values ranged from 11.2 to 7.0‰, reflecting variation in foraging habitat from subantarctic to Antarctic waters and prey from mesopelagic fish and squid to Antarctic krill, respectively