Krill larvae underneath the winter sea-ice
In winter, the larvae of the crustacean Antarctic krill are often found directly underneath the sea-ice. For the first time, researchers have been able to investigate the distribution and abundance of krill larvae underneath the sea-ice of the Southern Ocean on a large scale, using the ice-net SUIT (Surface and Under Ice Trawl). This information can help to establish a policy for the management of Antarctic krill stocks. The research was recently published in the journal Polar Biology.
Photo: Antarctic krill (Euphausia superba). By Jan Andries van Franeker
Worldwide, there are many species of krill. One species, however, is very abundant in the Southern Ocean: Antarctic krill (Euphausia superba). This species is an important food source for many species of whales, birds, seals and penguins, which live in the Antarctic region permanently or parts of the year. In addition, young krill are eaten by fish, jelly fish or other crustaceans. Despite these species also having other food sources, Antarctic krill is crucial for the maintenance of the ecosystem.
Because Antarctic krill contains rare nutritious substances, they are commercially fished. They are used as a food source for the cultivation of fish and shrimp or livestock farming, in the pharmaceutical and cosmetic industries and processed into krill-oil capsules. Using fisheries catch data and results from scientific research, the international organisation CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources) makes sure that krill is being fished in a sustainable way. Not only the effect of fisheries is taken into account, but also the effect of other factors that can have an influence on the krill population. An example is the possible sea-ice decline as a result of climate change, which plays an important role in the life of Antarctic krill.
To make fishery sustainable, it is important that the part of the population that is fished away, is replaced with new young krill. In other words, the krill has to become old enough to have a chance to reproduce (older than two years). That is why research is being done on the survival and life of krill larvae and young krill. The winter months form the first obstacle for krill larvae that were born in the previous summer, because in this period there is not a lot of food in the sea water. One way to deal with this, is by feeding on algae and other small organisms that are being ‘scraped’ from the underside of the sea ice.
Research under sea ice is not easy
It is not easy to do research in the Southern Ocean. The sea ice and ever changing weather conditions make it hard to sample organisms from underneath the ice, and especially on the survival of species over winter there is relatively little information. In general it is known that young krill reside right underneath the sea ice in winter. For scientists, this is a very difficult place to collect samples. On a small scale research on krill larvae has been done before, for instance by using divers, however with ice net SUIT researchers of IMARES, the University of Britisch Columbia and AWI (Alfred Wegener Institute) have been able to investigate a krill larvae population on a large scale for the first time. The population structure was examined, for example by looking at abundance, distribution and age/size structure. The research was performed in the northern Weddell Sea, where a large part of the total krill population resides.
Large differences in populations, body size, and age classes
The encountered krill larvae stock appeared to consist of several small populations. The krill larvae differed in age and size within age. This means that they were not only born at different times within the spawning season, they also grew up under different circumstances. For example, one part of the population may have encountered less food than the other, resulting in less growth within the same timeframe. Apart from that there were indications that younger, smaller krill larvae remain closer to the sea ice than older and larger larvae.
Comparing the population structure between several years can give insights in the reproduction success of the krill, the start and duration of the spawning season and the effect of the state of the environment, for instance of the sea-ice. The study shows that it is important to investigate the sea water directly underneath the ice to get a better knowledge of the processes, as the population structure in deeper water layers is most likely different from the upper water layer.
Fokje Schaafsma, Carmen David, Evgeny Pakhomov, Brian Hunt, Benjamin Lange, Hauke Flores, Jan Andries van Franeker (2016). Size and stage composition of age class 0 Antarctic krill (Euphausia superba) in the ice–water interface layer during winter/early spring. Polar Biology. doi:10.1007/s00300-015-1877-7