New ways to catch krill

Krill researcher Rob King has spent many frustrating hours on the deck of the Aurora Australis observing overturned ice floes peppered with krill, as the ship ploughed relentlessly south.

“I’ve spent a lot of time looking at the ice and wishing I had access to the water column as we were bashing our way to station, knowing there were krill under there,” he said.

“But you can’t stop and dig a hole to get them out. It’s intensely frustrating.”

As one of the collectors for the Australian Antarctic Division’s krill aquarium, Rob needs to catch enough wild krill each year (about 25 000) to sustain important experiments on the animals’ life cycle, behaviour and response to climate change.

To do this he deploys a special ‘rectangular midwater trawl’ net into the open ocean, when a krill swarm is detected with the ship’s acoustic echosounders. His ability to do this, however, often depends on how it interferes with other scientific activities or station resupply timeframes and priorities. And krill are often found close to the ice edge, or within the sea ice zone, where deploying a net may be difficult.

So Rob came up with a solution that he hopes can be incorporated into the design of the new icebreaker (see Future-proofing icebreaker science); to build a watertight room, or “wet well”, deep in the heart of the ship that can process large volumes of seawater.

“The wet well would be located below the water line and connected to the outside by inlets, at two and five metres below the surface and in the keel,” Rob explained.

“Water would gravity feed to large viewing tanks and filter tables and adult and larval krill would end up at the end of the filter tables, where we could collect and transfer them to a temporary aquarium in the wet well space.

“The idea of the viewing tanks is that if we see a very fragile organism, such as a jellyfish, we could shut down the water flow and remove the animal from the viewing tank before it reaches the filter table and disintegrates.”

Locating the water inlets at different depths would allow scientists to see how the krill and plankton community changes over time — as many marine organisms are known to rise to the surface at night to feed, and descend during the day — while the ability to catch fragile organisms would open up new research opportunities.

“There are many fragile species that we know little about because only divers have been able to collect them in perfect condition,” Rob said.

“Some of these species could become more prominent ecosystem players if krill are negatively affected by climate change in the future. So we’ll hopefully be able to collect them live and in perfect condition, and study their growth rates, physiology, reproduction and behaviour.”

While the idea is novel, Rob has a high degree of confidence in its success, based on previous work that catalysed his vision. This included work on Aurora Australis and on the German icebreaker Polarstern in 2013, where Rob set up a filter table in the moon pool area of the ship.

“We were able to pump about 900 litres per minute on to the filter table and we caught krill larvae in really good condition; so we proved the idea works,” Rob said.

“We expect this new design would work well when the ship is stationary or moving slowly through ice, rather than at cruising speed.

While the wet well may not catch krill as quickly as a big trawl, it could provide access to previously inaccessible water (due to sea ice cover or rough conditions), and at times when krill are in very high abundance. Critically, it could enable scientists to capture the animals in perfect condition, without losing ship time to do it.

Wendy Pyper
Australian Antarctic Division