For the last 25 years we have been using underwater sound (acoustics) as a tool for studying the Southern Ocean ecosystem.
Scientific echosounders are installed in the hull of the Aurora Australis. They transmit pulses of sound (pings) down into the water and then measure the strength of the echoes that are returned. Objects that cause detectable echoes include:
- physical features such as the seafloor
- biological features such as fish and krill.
Acoustics is a non-invasive way of detecting what lies beneath the ship as it travels through the ocean. Because the pings can be sent at rates of around one per second, a very detailed picture is made. This picture is known as an echogram. It is built up from a composite of consecutive pings and can be thought of like an image from a digital camera — each pixel of the echogram image represents a small volume of water beneath the ship.
For krill, the echogram is used to give information on the location, depth, size and density of aggregations.
Given that twice as much krill will produce twice as much echo, it is possible to estimate the amount of krill in the water using acoustics.
The only thing we need to know is how much echo a single krill produces, and this can be done by direct measurement or theoretical modelling. We have applied this principle to estimate the distribution and abundance of krill in the waters off East Antarctica, covering a total area of 2.2 million km².
This information is used to:
- help us understand where and when krill are found
- enable the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) to set precautionary catch limits for the expanding krill fishery.
Read more about echosounding in The Heard Island echosystem: eavesdropping on the food web (Australian Antarctic Magazine 9: 18–19, 2005).