Deep sea lighthouse lures Antarctic krill

Rob King looking at the shutters on the light trap (Photo: Wendy Pyper)
Rob King looking at the shutters on the light trap (Photo: Wendy Pyper)
The reflective prism at the bottom of the light trap (Photo: Wendy Pyper)Rob King and the krill light trap (Photo: Wendy Pyper)

10th September 2012

A deep sea ‘lighthouse’ 2000 to 3000 m below the sea ice will be used to lure Antarctic krill into a research trap during a spring sea ice voyage departing Friday.

The light trap is designed to capture live krill on the sea floor to use in experiments on growth rate and metabolism and to learn more about their diet, population structure and winter survival strategies.

Australian Antarctic Division krill aquarium manager, Mr Rob King, says if the trap works, it will be the first time live krill have been collected from the ocean floor at depths approaching the abyssal region.

‘We want to collect the krill to understand how they survive the winter,’ Rob says.

‘Recent footage of krill on the sea floor, filmed by deep-sea cameras, suggests that they may be feeding on marine snow – the debris raining down from above – and other sea floor fauna. We want to get a sample of these krill so that we can compare them to krill living in the sea ice above. Are they the same population and how do they compare in terms of what they eat, and their growth and metabolism?’

Rob and Antarctic Division instrument and electronics technicians customised the light trap to attach to a deep sea oceanographic instrument – the conductivity, temperature and depth (or CTD) rosette. Cameras will also be mounted on the CTD to film the krill being trapped.

‘This footage will allow us to know exactly where and on what kind of habitat the krill were caught,’ Rob says.

‘This will enable us to relate the condition of the krill to the habitat in which they were caught. For example, in the past, egg-laden krill have been observed at 3500 m depth but it was not possible to sample them.’

The cylindrical light trap works by beaming a light from the top, down onto a prism. The prism reflects the light out of four small, open windows, much like a lighthouse. In theory, the krill will be attracted to the light and will swim through the windows. Once inside the cylinder they will encounter a funnel that directs them into a chamber from which escape is unlikely.

 ‘The trap’s entrance funnel is very similar to eel traps that Indigenous Australian’s have used for centuries,’ Rob says.

‘We don’t know yet whether the krill will be attracted to the light, but there is some evidence from deep sea cameras that they will be. And hopefully they will survive the 2000 or 3000 metre vertical lift back to the surface.’

Tests of the light trap in the krill aquarium were a success. But there’s no guarantee that there will be any krill to sample in the trapping zone.

 ‘We don’t know if our light trap will work and we don’t know what density of krill we’ll be able to sample. But the light trap is just one tool in a quiver of equipment we’ll use and hopefully one of our approaches will work,’ Rob says.

[Video]

Krill light trap video

Video transcript

Rob King – Krill biologist

Recently reports have been turning up where krill have been seen down in the deep ocean. And it’s thought they may be feeding on marine snow, the debris raining from above, and also the fauna down on the bottom of the ocean floor. So what we would really love to have is a sample of krill, living krill from the ocean floor, that we can then pump into that metabolic research that we can do up on the surface, growth rates and physiology.

So we are taking a light trap that mounts directly onto the CTD rosette. So that is the oceanographic device that is usually used for taking water samples from the deep ocean. The krill will hopefully be attracted by that.

Then the light trap works by just capturing krill who stray through a funnel into a cone that they find harder to get out of than it was to get in. Then we shut the door on the light trap remotely from above at the surface by firing a little trigger on the CTD and then we bring the thing back to the surface. Then we find out how krill like travelling 2000 metres vertically in the water and whether they are usable or not because we don’t know the answer to that either.

[end transcript]