Insightful approach to aging Antarctic krill

Krill biologist Dr So Kawaguchi with a beaker of krill
Krill biologist Dr So Kawaguchi (Photo: Australian Antarctic Division)
Krill Krill eyesA cross section of a krill eye stalk showing the annual bands by which age can be determined

23rd February 2017

For the first time scientists have been able to determine the age of Antarctic krill by counting the growth bands in their eyestalks.

The research, published today in the journal PLOS ONE, found krill grow annual bands in their eyestalks, much like growth rings in trees, and these correlate directly with their age.

Australian Antarctic Division krill biologist, Dr So Kawaguchi, said it’s a remarkable finding.

“Despite more than 50 years of research, until now it’s been impossible to accurately assess the longevity of krill and the age structure of their populations,” Dr Kawaguchi said.

“Krill don’t have any hard parts, such as ear bones, shells or scales, so we can’t determine age using these calcified structures.

“Additionally, there’s almost no size difference in krill beyond two years of age, and their regular moult means they can actually shrink in size, depending on the time of year and food availability.”

The new method, pioneered on larger crustaceans such as lobsters and crabs, involves looking at the eyestalks under a microscope.

“We look at a longitudinal section of the eyestalk to identify the light and dark growth bands and count exactly how many years the specimen has been alive.”

The aging technique has also been successfully used on formalin-preserved samples, which means we are now able to accurately determine the age of preserved krill from the early 1900s.

“The ability to retrospectively age krill allows us to compare length-at-age over time and across environments to examine changes in the Southern Ocean ecosystem,” Dr Kawaguchi said.

The scientists studied both wild and known-age captive krill, bred in the Australian Antarctic Division krill aquarium and the Japanese Port of Nagoya Public Aquarium.

The age-based assessment methods will provide information on stock structure to assist with catch limits and management options for the krill fishery through the Commission for the Conservation of Antarctic Marine Living Resources.

“Krill are a keystone species in the Southern Ocean, predated by penguins, seals, flying seabirds and whales, so any fishery needs to be carefully managed.”

“The Southern Ocean is also undergoing major changes in the sea-ice zone, in primary production and through ocean acidification, so better understanding how long they live will help us more accurately predict the potential impacts of climate change on krill.”

The research is a joint project of the Australian Antarctic Division, the National Ocean and Atmospheric Administration (USA), University of New Brunswick (Canada), Port of Nagoya Public Aquarium and the National Research Institute of Far Seas Fisheries (Japan).

It was partly funded through a grant from the Antarctic Wildlife Research Fund.

[Video]

Insightful approach to aging Antarctic krill

Video transcript

Dr So Kawaguchi: This is very exciting. Knowing the age of krill is a very very long research questions - more than 50 years, because krill doesn't have any hard parts that record their age. To find out their age, we used eye stalks.

What we did was to slice those eye stalks into really thin slices and then polish it, and then count the annual bands that have been created. The concept is the same as the annual tree rings.

Krill is the fundamental food source for most of the higher predators, like whales, penguins, seals, in the Antarctic Ocean.

If there's any change in krill populations, that will certainly have a fundamental impact in the structure of the ecosystem itself. So it's really important to know how old the krill are, because that will be used for the fisheries management. We can actually retrospectively go in to preserve samples, like about a hundred years ago, and then compare with the recent krill - we'll be able to better predict what may happen in this changing environment into the future. 

[end transcript]