Foraging behaviour of Weddell seals, and its ecological implications

Abstract

Time-depth recorder data of eight adult Weddell seals (Leptonychotes weddellii) provided simultaneous dive records over 8 days in the Drescher Inlet, eastern Weddell Sea coast, in February 1998. The seals primarily foraged within two depth layers, these being from the sea surface to 160 m where temperature and salinity varied considerably, and near the bottom from 340 to 450 m where temperature was lowest and salinity highest. While both pelagic and benthic diving occurred during daylight, the seals foraged almost exclusively in the upper water column at night. Trawling during daytime con®rmed that Pleuragramma antarcticum were by far the most abundant ®sh both in the pelagial and close to the bottom. Pelagic night hauls at 110±170 m depth showed highly variable biomass of P. antarcticum. The temporal changes in the local abundance of P. antarcticum, particularly in the pelagial, may explain the trends in the seals' pelagic and benthic foraging activities. This study describes the jaw movements of a hunting seal, which are presumably indicative of feeding events. Trophic links from the Weddell seal to ®sh, zooplankton and krill, Euphausia superba, are discussed.

Dive depths of Weddell seals in relation to vertical prey distribution as estimated by image data

“…Therefore dives to 50-100 m observed in this study might correspond to foraging dives focused on benthos. This is consistent with the suggestion of Plötz et al (2001) that Weddell seals might primarily forage within 2 depth layers. Plötz et al (2001) suggested that Weddell seals might prefer to forage deeper during daylight and shallower during darkness, and that the diurnal pattern in dive depth might be finely tuned to vertically migrating prey (e.g.…”

Hydrographic influences on the summer dive behaviour of Weddell seals (Leptonychotes weddellii) in Atka Bay, Antarctica

“…2), with high frequencies of shallow dives to depths less than 50 m. Deep dives were concentrated around 100 m for seals AF4, JM5 and JF6. One adult male (AM2) and one adult female (AF3) displayed high numbers of deep dives to depths [200 m, while the remaining seal (AM1) concentrated its dives at depths between 100 and 150 m. The bimodal patterns of dive depths are similar to that reported for seals tagged at the Drescher Inlet (Plötz et al 2001), although seals there targeted deeper water layers close to the seafloor in the region of 400 m, where evidence for foraging (based on jaw movement data) was also reported by Liebsch et al (2007). Seals AM1, AF4, JM5 and JF6 displayed increased bottom times when diving to similar depths, compared to the depths they most often dived to (Fig.…”

“…This can likely be explained by potential prey species occurring in more concentrated numbers within preferred temperature layers when there is more stratification in temperature through the water column, whereas prey species are more likely to be scattered in the water column when the temperature structure is relatively well mixed (Takahashi et al 2008). Indeed, the vertical distribution of Pleuragramma antarcticum, a known prey species of Weddell seals, was associated closely with the vertical structure of the water column when a clear pycnocline was present (Plötz et al 2001). Seals may therefore take advantage of increased concentrations of prey by maximising their forage effort in appropriate depth layers.…”

“…Sato et al 2002Sato et al , 2003, Drescher Inlet (e.g. Bornemann et al 1998;Plötz et al 2001;Watanabe et al 2006) and Dumont D'Urville (e.g. Heerah et al 2013) (Fig.…”

Capital Expenditure and Income (Foraging) during Pinniped Lactation: The Example of the Weddell Seal (Leptonychotes weddellii)

“…Sensors have been glued to the jaw of the seal that detect opening of the mouth (Bornemann et al, 1992;Plötz et al, 2001), but prey capture may be diffi cult to distinguish from other jaw movements during social behavior (e.g., threats and bites). Temperature-transmitting thermistors have been introduced into the stomach to monitor changes in the temperature of stomach contents associated with ingestion of cold items, such as ectothermic fi sh ( Bornemann, 1994;Hedd et al, 1996;Austin et al, 2006aAustin et al, , 2006bKuhn and Costa, 2006).…”