Underwater observations of foraging free-living Atlantic walruses (Odobenus rosmarus rosmarus) and estimates of their food consumption

Abstract

Food consumption of Atlantic walruses (Odobenus rosmarus rosmarus L.) was quantified by combining underwater observations of feeding with satellite-telemetry data on movement and diving activity. The study was conducted between 31 July and 7 August 2001 in Young Sound (74°N–20°W) in Northeast Greenland. On ten occasions, divers were able to accompany foraging walruses to the sea floor and collect the shells of newly predated bivalves (Mya truncata, Hiatella arctica, Serripes groenlandicus) for determination of number of prey and biomass ingested per dive. Simultaneously, the activity of a 1,200-kg adult male walrus was studied by use of satellite-telemetry during an entire foraging cycle that included 74 h at sea followed by a 23-h rest on land. An average of 53.2 bivalves (SE=5.2, range: 34–89, n=10) were consumed per dive, corresponding to 149.0 g shell-free dry matter (SE=18.9, range: 62.4–253.1 g), or 2,576 kJ per dive (SE=325.2, range: 1,072–4,377 kJ). During the foraging trip, the walrus spent 57% of the time diving to depths of between 6 and 32 m, and it made a total of 412 dives that lasted between 5 and 7 min (i.e. typical foraging dives). If the entire feeding cycle is considered (97 h), the estimated daily gross energy intake was 214 kJ per kg body mass (95% CI: 153–275 kJ), corresponding to the ingestion of 57 kg (95% CI: 41–72 kg) wet weight bivalve biomass per day, or 4.7 (95% CI: 3.3–5.9%) of total walrus body mass. Due to ice cover, walrus access to the plentiful inshore bivalve banks in the area is restricted to the short summer period, where walruses rely on them for replenishing energy stores. It is hypothesised that the documented decrease in the extent and duration of Arctic sea ice may increase food availability for walruses in eastern Greenland in the future.