Finding the fat
Elephant seal research on Macquarie Island
1 March 2002
Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, has been running a research program on female southern elephant seals at Macquarie Island since 1999.
We are investigating the following:
- Southern elephant seals travel thousands of kilometres from their breeding home on Macquarie Island to obtain enough food (fish and squid) to feed themselves, and to provide enough milk for their pups during the breeding season (September – October).Since elephant seal numbers have been declining for some time, we are attempting to determine how changes in food supply affect their ability to provision themselves with enough fat to survive and breed.
- By examining the relationship between foraging behaviour, range and body condition, and the vagaries of the Southern Ocean, we aim to identify those areas of the Southern Ocean important for the seal population on the island.
What we're doing
Our main activity is attaching data loggers known as Timed-Depth Recorders (TDRs) to the elephant seals before they leave the island to forage in the Southern Ocean.
These little devices collect up to 16 MB of information. We program them to collect data such as depth, temperature, light intensity and swim velocity (using a small turbine) every 30 seconds. For the 7.5-month winter foraging trip, this amounts to over six million sampling intervals alone.We must choose very carefully which animals to capture for this project.
We started in 1999 with the females born in 1993 – that made them 6 years old in the 1999 breeding season.
They've just crept up to the 8.5 year-old mark, and we have another two seasons planned.
By doing this, we can control the effects of different ages on foraging ability (i.e. all our seals are the same age). In conjunction with a previous program on elephant seals run by the AAD, we also know the number of times each seal has bred.
Captures happen at maximum of four times per year:
- at the beginning of the breeding season (after giving birth)
- at the end of lactation (approximately 24 days later)
- at the beginning of the annual moult (January)
- at the end of the moult (approximately 30 days later)
Other data collected include samples of blubber to test for traces of rare fatty acids. These fatty acids, the building blocks of all fats, can give an indication of what the seals have been eating. Each species of squid and fish have a certain combination of these fatty acids, and their signatures remain intact within the seal's blubber. This is a novel approach to assessing what they are eating out there!
We also use an ultrasound machine to measure the thickness of the blubber layer – this tells us the body condition of the animal (i.e. the thicker the blubber, the better the animal's condition).
Some salient facts we've discovered since the inception of the program
- The deepest dive ever recorded by a seal happened this past winter by an 8-year old female. She dived to 1631 metres – that's over one mile deep! This particular female was also one of the heaviest weighed thus far – 691 kg when she came back to breed in October 2001.
- As amazing as it is for these animals to travel thousands of kilometres (the farthest trip ever recorded was over 10,000 km round trip) from Macquarie Island, and then to find the island again each year, we have discovered another related phenomenon. Both during the summer and winter trips, individuals return year after year to almost the same areas of the ocean in which to feed. How they do it is still unknown.
- A novel technique developed recently is the ability to determine not only where a seal forages at sea, but how well she forages.
- During a foraging trip, seals turn fish and squid into fat and store this as blubber. As the seal increases its fat content, its sinking (drift) rate decreases (i.e., more fat = more flotation). Using depth-measuring data loggers, we can track changes in the rate of downward drift. When a seal finds a lot of food, we can identify these changes and plot them on a map. For the first time in history we have a spatial summary of foraging success!
- Until quite recently, it was believed that southern elephant seals roamed only the subantarctic regions of the Southern Ocean. With the improved technology of the data loggers we now use, we have shown that female elephant seals spend most of their time during winter near or in the pack-ice zone of the Southern Ocean. Many females feed almost exclusively on the Antarctic continental shelf during winter.
- We have now collected enough summer foraging records to estimate the spatial distribution of prey consumption by this species.
- Our calculations indicate that during the summer foraging trip alone (~70 days at sea), the adult female component of the Macquarie Island elephant seal population (~20,000 individuals) consumes ~17500 tonnes of squid and ~14000 tonnes of fish.
- Approximately 50% of this prey biomass is consumed in the Economic Exclusion Zones (EEZ) of Australia and New Zealand and the CCAMLR management zones south of Australia.
These figures are vastly superior to the commercial fisheries' takes in these zones in recent years. Clearly, southern elephant seals are important predators of the Southern Ocean and Antarctic marine ecosystems.
Corey Bradshaw, Antarctic Wildlife Research Unit, School of Zoology, University of Tasmania, 2002.