Counting Antarctic animals
In order to assess and predict the possible effects of environmental change and human impacts on Antarctic wildlife, it is important to have reliable data on the distribution and abundance of the key species in this ecosystem before changes occur.
Antarctic krill are probably the most abundant species of multi-celled animals on earth. Krill occur in vast swarms, often containing tens of thousands of tonnes of krill. There are estimated to be some between 300 billion and 1 million billion (1,000,000,000,000,000) individual krill in the Southern Ocean at any one time with a total weight, or biomass, of between 150 million and 500 million tonnes. The total weight of humans on the planet at the moment is around 160 million tonnes. These figures can be used to create some interesting problem solving and number work.
To manage the krill fishery, we need to assess krill populations accurately. The abundance of krill is currently estimated using scientific echo-sounders that measure the reflection of sound from objects below a ship. The distribution of krill occurs over an area of 30 million square kilometres (compared with the area of Australia - 8 million square kilometres). In a summer survey a single ship can only survey less than 1 million square kilometre - it's impossible to survey the whole area. And krill swarms are patchy in their distribution.
- To help students understand the principle of surveying, including its 'hit and miss' nature, divide them into pairs and have them play a variant on the game 'Battleships'. Provide them with two sheets of paper each marked with a grid 10 cm by 10 cm, numbered horizontally 1 to 10 and vertically A to J or print a grid [PDF] such as that used for krill. Student A places at random eight swarms of krill of varying sizes as follows: two x four squares, two x three squares, two x two squares and two x one square. Student B then tells Student A the grid route to be followed by the ship, seeking maximum coverage with minimum passes (it might be row B, then back via row D, two rows down, then row F and so on, then vertical passes at similar intervals). Each time there is a 'hit', Student B will have to work out location and size of the swarm by deviating from the track, and then must return to their agreed tracking route. The students then change places and repeat the exercise. The object is to find the krill swarms with the smallest number of passes.
- What effect does it have if the swarms are all heavily biased in one direction?
- What effect does swarm size have?
- What are the possible real-life implications?
- How likely are you to estimate the numbers correctly if you use only one pass of the ship? Two? Ten?
Biologist Graham Robertson describes some early experiments designed to find the best way of counting penguins.
The best time to count emperor penguins is in the middle of winter when the males are enduring their long incubation fast. At this time of year the females are at sea feeding, and since there are only a few non-breeding birds hanging about, each bird represents one breeding pair… Unfortunately for the counters, winter presents its own special problems - the birds cram tightly in hectare-size huddles to keep warm making it difficult to count them, day-length is only three hours of twilight and wind-chill temperatures occasionally reach minus 50°C.
Fifteen people from the stations assisted with the counts, and we tried every method we could think of. We made snap estimates (gut feeling of numbers with no attempt to actually count). We counted them from a three-metre high step ladder, then from 11 metres up on a scaffold tower mounted on a sledge that we towed behind a vehicle along the flanks of the birds. We counted them by eye at the colony and later on from photographs taken from our lofty vantage points. We measured with a rangefinder the area of huddles and multiplied this by the density of bird in huddles (estimated by measuring the circumference of incubating males - we achieved this by passing a cloth tape measure around birds while duck-waddling behind them as they shuffled along, egg-on feet, on the sea-ice) to derive the total number of huddling birds.
We used reference groups of 100 or so birds and estimated the number of these groups in the whole mob. And we gently herded small groups of birds between two people and counted them sheep-like as they passed by (after one brief attempt we declared this method too disruptive). Finally we floated a remotely controlled camera from a helium-filled balloon about 50 metres above the colony to get aerial pictures from which we could count the birds.
- Have the students list the various ways of estimating the numbers of birds.
- Can they think of any methods besides those in the text above?
- Which method would be the most reliable?
Penguin biologist Barbara Wienecke describes how the AAD counts Emperor penguins at one particular colony in Antarctica.
We have been monitoring the emperor penguin colony at Taylor Glacier (~ 90 km west of Mawson station) since 1988. Our aims are to find out how many pairs of emperor penguins decide to breed each year and also how many chicks they manage to raise. That means we have to count the penguins on two occasions each year: in winter when they males are incubating their eggs, and in spring when the chicks are getting ready to fledge (leave the colony).
Winter is the best time of year to find out how many pairs are active in a season because only the males are in the colony taking care of their eggs. The females are in the pack-ice feeding. Each male represents one breeding pair so all we have to do is count them! We do that by taking overlapping photographs of the whole colony. At Taylor Glacier, that is relatively easy because the colony is located on land and next to a rocky ridge from where we take the pictures. One has a great overview from there. The only problem is that one has to sit very still in very low temperatures.
Once the images are taken, we download them to our computer and put them together like a puzzle. Then we can mark each penguin with a coloured dot and count the whole lot. The great thing about photographs is that we can count the penguins with great accuracy and we have a record that can be checked in future. And the best part is that we don’t have to disturb the penguins!
- Give the students a similar situation (such as the numbers of spectators at a football match) and have them come up with various counting methods.
- Have students examine the aerial photograph of the penguin colony above, and ask them to come up with ways of estimating total numbers. (They could estimate, then sub-sample, and then verify their result by actually counting the number of birds).