Spatially Explicit Estimates of Prey Consumption Reveal a New Krill Predator in the Southern Ocean

Abstract

Development in foraging behaviour and dietary intake of many vertebrates are age-structured. Differences in feeding ecology may correlate with ontogenetic shifts in dispersal patterns, and therefore affect foraging habitat and resource utilization. Such life-history traits have important implications in interpreting tropho-dynamic linkages. Stable isotope ratios in the whiskers of sub-yearling southern elephant seals (Mirounga leonina; n = 12) were used, in conjunction with satellite telemetry and environmental data, to examine their foraging habitat and diet during their first foraging migration. The trophic position of seals from Macquarie Island (54°30′S, 158°57′E) was estimated using stable carbon (δ1 3C) and nitrogen (δ15N) ratios along the length of the whisker, which provided a temporal record of prey intake. Satellite-relayed data loggers provided details on seal movement patterns, which were related to isotopic concentrations along the whisker. Animals fed in waters south of the Polar Front (>60°S) or within Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Statistical Subareas 88.1 and 88.2, as indicated by both their depleted δ1 3C (<−20‰) values, and tracking data. They predominantly exploited varying proportions of mesopelagic fish and squid, and crustaceans, such as euphausiids, which have not been reported as a prey item for this species. Comparison of isotopic data between sub-yearlings, and 1, 2 and 3 yr olds indicated that sub-yearlings, limited by their size, dive capabilities and prey capture skills to feeding higher in the water column, fed at a lower trophic level than older seals. This is consistent with the consumption of euphausiids and most probably, Antarctic krill (Euphausia superba), which constitute an abundant, easily accessible source of prey in water masses used by this age class of seals. Isotopic assessment and concurrent tracking of seals are successfully used here to identify ontogenetic shifts in broad-scale foraging habitat use and diet preferences in a highly migratory predator.

Vibrissal growth parameters of southern elephant seals Mirounga leonina: obtaining fine-scale, time‑based stable isotope data

“…Vibrissae are often subsampled into 2 mm segments for isotopic analyses (e.g., Newland et al 2011;Walters et al 2014), and we therefore reported the average number of days represented by a 2 mm vibrissal segment to emphasize the temporal resolution of the biogeochemistry data obtainable from sequentially sampled vibrissae.…”

“…However, such a combined approach requires higher resolution SI data (Beltran et al 2015; also see Banks et al 2014). The SI signature captured along the length of keratinous tissue, such as feathers (Grecian et al 2015), nails, hair or vibrissae, provides a time-series of the species' dietary history, if analysed chronologically (Hückstädt et al 2012;Robertson et al 2012;Walters et al 2014;Beltran et al 2015). Numerous studies focussed on ascertaining the exact, species-and tissue-specific growth rates of slow-growing, biologically inert keratinous tissue of various species in different habitats (e.g., Greaves et al 2004;Rohwer et al 2009;Beltran et al 2015).…”

“…In contrast, the half-life of an isotopic tracer injected in captive harbour seals Phoca vitulina, was 47.1 days (Zhao & Schell 2004), although isotopic changes were visible after four days which implies that the marker incorporates rapidly into the protein synthesis of the growing vibrissae (Hirons et al 2001). As a result, vibrissae are increasingly utilised to study the diet of pinnipeds (Greaves et al 2004;Hückstädt et al 2011Hückstädt et al , 2012Newland et al 2011;Hindell et al 2012;Walters et al 2014). Nevertheless, the vibrissal growth rates of several phocid species remain to be determined, despite the recent interest in the utilisation of vibrissae to increase the temporal resolution of the dietary data (e.g., Hirons et al 2001;Greaves et al 2004;Zhao & Schell 2004;Hall-Aspland et al 2005;Beltran et al 2015).…”

“…Southern elephant seals (Mirounga leonina, hereafter SES) are the most abundant phocid in the sub-Antarctic region, and although their vibrissae have been used in dietary studies (Cherel et al 2008;Hückstädt et al 2011;Newland et al 2011;Walters et al 2014), the growth rate and shedding pattern (Ling 1966) of their vibrissae remains poorly understood.…”

“…Also not yet enumerated, although generally accepted, is that individual vibrissae display asynchronous growth and replacement patterns (Ling 1966;Walters et al 2014) (see Supplementary figures, plate 1S). The ability to obtain fine-scale, temporal integrated biogeochemical data was demonstrated in a similar study conducted on a single, captive adult female northern elephant seal, Mirounga angustirostris (Beltran et al 2015).…”