(Presented by Jennifer Trofimovics and Rebekah Harris)

If non-gestating, adult female southern elephant seals, Mirounga leonina, from Peninsula Valdes, Argentina, forage on the wide continental shelf off the peninsula during the post-breeding trip to sea, or whether they forage in deep water, where gestating, post-moult animals have been shown to do.

Specifically, it was asked whether non-gestating post-breeding females show the same diving pattern and forage in the same location in deep water as gestating post-moult females, or whether they forage on the continental shelf either exclusively or en route to deep water.

During both trips females showed deep and long-duration diving. PB females crossed the shelf in 3-7 days. Deeper and longer dives were observed during the day, consistent with foraging on daily migrating prey in the water column.

Female elephant seals make two foraging trips per year. One after the breeding season which lasts about 70 days and the other after the moulting period which lasts about 8 months, the entire gestation period. Both northern and southern elephant seals ranging from juvenile to adults have been seen to forage in deep waters off the continental shelf.

The reproductive condition of the female may determine if she would start to forage on the shelf or wait until she reaches deep water. PB females may forage on the shelf before reaching deep water due to the high-energy requirement during lactation. The shelf supports a high biomass of fish and squid on which to prey.

To obtain data from the elephant seals, eight PB females were fixed with GLTDRs (geographic-location depth recorders) and VHF transmitters. Two PM females were fixed with the same equipment. All of this was done in 1993 and 1994. Two PB and PM females had Argo-linked ST6 Platform transmitter terminals placed on them to determine the traveling routes. This was done in 1996 and 1997. Satellite tags were also used to transmit locations of the seal as it surfaced. These instruments measured dive depth, ambient temperature, and ambient light. The instruments were picked up from the PB females when they returned to moult 70 – 78 days later, and the PM seals 7 – 8 months later when the returned to breed.

To estimate migratory paths and location of forging areas sea-surface temperature (STT) data were matched with daily light-level curves. The STT data was obtained from NASA Physical Oceanography Distributed Active Archive Center and the Jet Propulsion Laboratory, California Institute of Technology. For dives being classified as on the shelf, they had to be less than 200 meters.

There was a record of 16,673 dives during 273 days at sea for PB females and 19,453 dives during 336 days for PM females. The data collected do not support the hypothesis that PB females forage over the continental shelf before reaching the deep waters. Crossing the shelf occurred in 7 days or less for all females and 89% of the time was recorded being in deep waters.

There were similar dive patterns for both PB and PM females off the shelf. Females were probably feeding on daily migrating prey based on the number of dives and duration of those dives during the day compared to those at night. This was the most common dive type off the continental shelf.

Once off the shelf, the females spent most of the time in deep waters not returning to the shelf until heading back to the rookery. Most foraging was concentrated in temperate waters of the SW Atlantic within 36° and 46° S and up to 1223 km away from land. PB females were found in the same area as PM females. The only difference was that PM females reached further south.

One PM female, PM-H, was the most pelagic recorded for both females and males. She reached a distance of 2281 km from Punta Delgada traveling an estimated 11,599 km spending 65% of the time deeper than 5000m.

The PB females studied followed the prevailing deep-water foraging patterns of PM, non-gestating individuals. This behavior is consistent with southern and northern seals from other colonies.

There were 3,300 dives recorded as being on the shelf. The behavior here was different in dive rate, depth, and duration but was alike in the patter of continuous diving. The prediction that seals swimming faster utilize oxygen at a higher rate and therefore exhibit an elevated dive rate and relatively shorter dive durations were shown throughout the short duration of dives for PB and PM females.

For PB and PM females, the deep dives were similar to those found from colonies from the Macquarie Island in a previous study. Also found in the deep dives were D-dives (diel dives). These were when the seals migrated in the water column during the day because the prey did that also.

Geolocation tracks and satellite locations support the conclusion that adult Patagonian females feed in temperate waters off the southwestern Atlantic Ocean but do not reach the Antarctic Polar Front (APF) during both pelagic phases of the annual cycle. The Peninsula Valdes is the only colony found that is increasing in pup population that does not feed in the sub Antarctic or Antarctic waters.

Sex differences do make a difference in where the forage. Male seals have been found to stay close to their activity (on the island at the edge of the continental shelf). The males do not travel to the open ocean but forage along the border of the continental shelf. It was also found that males were more pelagic than males.

With all this information it could be said that overlooking the resources available to them on the continental shelf may not be, too costly for these apparently obligated deep diving animals.

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