Foraging ecology of southern elephant seals from Heard Island
Creators: name: Slip, DJ
Publisher: name: University Of Tasmania
Year: 2023

description: 1. The foraging ecology of the southern elephant seal population at Heard Island was examined. The study aimed to quantify the current status, diet, movement patterns, and foraging behaviour of the population in order to use this information to make an estimate of the population's energy requirements and food consumption. 2. I compared intravenous and intramuscular administrations of ketamine and diazepam to immobilise juvenile (8 to 24 months old) southern elephant seals (Mirounga leonina), to determine the most appropriate method for immobilising seals to a level required for stomach flushing or attaching electronic activity recorders. With intravenous injections, time to induction was shorter and less variable, the duration of immobilisation was shorter and less variable, and dose of ketamine was lower and less variable. Eight of 32 seals (25%) injected intravenously had apnoeas ranging from 8 to 20 minutes (mean= 16 ± 4.5 min), and 6 of 27 seals (22%) injected intramuscularly were apnoeic for more than 5 minutes. Seals which became apnoeic after intravenous injection began breathing before the theoretical aerobic dive limit was reached. 3. I surveyed the southepi elephant seal population at Heard Island regularly from February 1992 until March 1993, and the haulout patterns of the major components of the population were determined. During the breeding season 14277 adult females were counted. Raw counts were corrected using two models, one purely mathematical and the other based on the haulout behaviour of female seals. Total pup production was estimated at between 17000 and 18000 for 1992. Previous counts of elephant seals from 1949-51, 1985 and 1987 were corrected using the same models. The population declined by about 50% between 1949 and 1985 but there appears to have been little change from 1985 to 1992. 4. Stomach contents were lavaged from 76 southern elephant seals at Heard Island between July 1992 and March 1993. Eighty-six percent of stomachs contained cephalopods from 17 species. Numerically the most important was Psychroteuthis glacialis (21.1%), and from estimated biomass the most important was Kondakovia longimana (40.4%). Three other species were also common prey: Moroteuthis knipovitchi (19.4% by estimated biomass), Moroteuthis ingens (13.0% ), and Alluroteuthis antarcticus (10.2% ). Sixty-six percent of stomachs contained fish remains, and four species, Dissostichus eleginoides, Electrona carlsbergi, E. antarctica, and Gymnoscopelus nicholsi, were identified from otoliths. The diet of adults differed from that of juveniles, particularly pups in their first year. Martialia hyadesi was the most important prey of juveniles and represented 57.1% of estimated biomass consumed. Furthermore, smaller seals ate smaller squid. The species and size of cephalopods eaten by southern elephant seals are similar to those of other Southern Ocean predators, particularly species of beaked whales. 5. I investigated the foraging ranges and diving behaviour of adult southern elephant seals from Heard Island using archival geolocating time depth recorders. Most seals moved south to Antarctic waters, migrating long distances to foraging grounds that are related to oceanographic features, such as the Antarctic continental shelf and the Kerguelen Plateau. During the post moult migration adult females moved an average of 2502 ± 939 km, and adult males moved an average of 1749 ± 929 km away from Heard Island. Post moult adult males moved further south than adult females and foraged mainly over the continental shelf, while females foraged mainly in pelagic waters of the pack ice zone and only occasionally moved over the shelf area. During the post breeding migration adult females moved an average of 1186 ± 673 km away from Heard Island and concentrated their activity at the edge of the Kerguelen Plateau. Seals travelled faster in the first week away from Heard Island than at other times. Diving activity was concentrated between 200 and 800 m depth, and was occasionally related to thermal discontinuities in the water column but at other times was not. 6. Diving behaviour of two juvenile southern elephant seals from Heard Island, 9 to 11 months of age, during their second trip to sea was examined using time depth recorders. Dive behaviour was recorded for 76 days for the juvenile male and 66 days for the juvenile female with 6652 and 4566 dives recorded respectively, which represented about 65% of the trip to sea. Mean dive duration was 15.5 ± 5.1 minutes with a maximum of 39 minutes for the juvenile male (departure mass 168 kg, return mass 192 kg), and mean dive depth was 323 ± 157 m with a maximum of 834 m, while for the juvenile female (departure mass 163 kg, return mass 188 kg), mean dive duration was 21.1 ±6.4 min with a maximum of 58 min, and mean c;live depth was 416 ± 147 m with a maximum of 1270 m. Diving was continuous and over 90% of the time at sea was spent underwater, with only brief periods at the surface. Mean surface intervals were 1.67 ± 2.54 and 1.63 ± 0.94 min, for the male and female respectively, and extended surface intervals greater than three minutes represented less than 2% of all surface intervals for either seal. Both animals remained at sea for the entire period of August to November and showed patterns of continuous, deep, long duration diving with short surface intervals. Dive profiles consistent with pelagic foraging were common. Locations determined from sea temperature profiles suggest that these animals were foraging in mesopelagic waters in mid ocean, both north and south of the Antarctic Polar Front, and at times over the edge of the Kerguelen Plateau. 7. A model of energetic costs of reproduction, foraging, growth, moult and haulout was used to calculate energy. expenditure for different age classes of southern elephant seals that breed at Heard Island. This was combined with data on age structure and population size to estimate the energetic requirements of the population. Estimates of energy consumption were converted to food consumption based on current knowledge of the diet of the Heard Island population. Data on the foraging behaviour and movements of elephant seals from Heard Island were used to estimate the amount of food consumed from the Kerguelen Plateau. Energy expenditure associated with foraging contributed 56.2% and 65.3% of total annual energy costs for males and females respectively. The total gross energy requirements of the population was 1.24 x 10<sup>9</sup>) MJ or 17 .2 x 10<sup>3</sup> MJ per capita. The total biomass of fish and squid prey consumed annually by the population was 311 x 10<sup>3</sup> tonnes. The population consumed about 34% of prey from the Kerguelen Plateau including 23.8 x 10<sup>3</sup> tonnes of the commercially targeted Dissostichus eleginoides. This suggests potential for competition between commercial fisheries and the southern elephant seal population at Heard Island.