Antarctic krill, Euphausia superba. Photo: Tonie Leonora Torgimsby
Antarctic krill, Euphausia superba, has a circumpolar distribution south of about 50°S. Within this large region the abundance of krill differs a lot. It is said that about 70% of the krill biomass occur in about 30% of the distribution area. Its distribution is really patchy and related to the presence of sea ice and major current systems. Krill belongs to the taxon Crustacea. Their lifespan is up to 6 years they grow to a size of about 6 cm. Within their life they do some migration after hatching and for spawning.
The Antarctic ecosystem is comparable simple, because krill has a key role, since it is the main primary consumer and only 1-2 trophic links connected to its major predators such as humpback-whales, mesopelagic fish and seabirds. Humans are also interested in harvesting krill to produce nutritional supplements, beauty products, pharmaceutics, and fish food for aquaculture. Therefore it is important that we get more knowledge about krill, its behaviour and abundance to set reasonable catch limits to prevent overfishing and preserve the stock size to avoid effects on the whole ecosystem.
Tonie and Anna checking the boxes with krill Photo: Tonie Leonora Torgrimsby
Sometimes we got a lot of krill: 200 kg in one trawl is not that bad for scientific purposes. Some of the caught living animals were provided to the different scientists in buckets, filled with seawater: Knut Jørstad wants the genes of krill, Peter Wiebe and Dezhang Chu make experiments to get information about the sound speed and target strength of krill to improve acoustic estimations. For echosounder scrutinizing length and weight distribution of krill are also needed. Finally, we need krill for our growth experiment.
Krill has an exoskeleton, which means that it not has bones or fishbones, but a hard shell. This shell has to grow with the krill to gain body-size. But the exoskeleton does not grow instantaneously, so the krill builds a new slightly bigger exoskeleton every now and then to be able to gain body size. To get rid of the old exoskeleton it moults and then sheds the old one.
Krill feeds on phytoplankton and to a lesser extent on zooplankton. When food availability is low and krill starve, they shrink to prevent death. So the age of krill cannot be estimated just by measuring the body-size of krill. But to estimate the growth within a certain time the difference between moult-size and body-size can be used.
Moulting krill. Photo: Anna Hoelter
We collected at least 104 living krill from the trawl and kept them for 5 days, to get an impression of how much krill grows. Each krill swam in a plastic jar with small perforations together with 7 other animals in a tube, with 12 other tubes put in a big water flow tank. To provide the animals with sufficient oxygen and right water temperature, they were kept at approximately 0°C.
The little guys were checked every single day, for moulting, dead and escaped krill . The moulted exoskeleton is a size record of the krill prior to moulting. Therefore, we look at the size differences between the shed moult and animals with new exoskeletons. But unfortunately we cannot just measure the size of the whole moult because it gets destroyed during the moulting-process. So we have to measure just the uropods, which are the last pair of abdominal appendages and form a part of the tail fan. The uropod growth is linear correlated to the growth of the total body length. Since the size difference of the uropods is so small we cannot measure it on board. We are therefore looking forward to get the results of these measurements when coming back to our laboratory on land.
We collected krill in different areas of the cruise area, so it will hopefully be possible to compare the growth rates from different areas, age classes, size classes, and nutritional conditions.
Tonie Leonora Torgimsby (22)
Bachelor in biology from the University of Oslo, finished spring 2008. Starting her master degree in marine biology at UiO, fall 2008.
Anna Hoelter (25) er master student in marine biology at the University of Bergen, Norway. Her thesis is on acoustic detection of Antarctic krill and its behaviour. She obtained her bachelor in biology from the University in Oldenburg, Germany. Member of the International Biological Association (IBA).