Krill – The King of the Southern Ocean? | Polarjournal

No animal has as much influence in Antarctica as Euphausia superba, the Antarctic krill. Almost all the inhabitants of Antarctica, from penguins to blue whales, feed on the crustacean, which is almost 6 cm long. But despite intensive research, much of what makes this crustacean tick, is still unknown. For example, the amount of krill in the Souther Ocean is just a rough estimate. A new study recently concluded that Antarctic krill could be of global importance in terms of the carbon cycle. Therefore, this importance must be taken into consideration when it comes to the control of krill fishing by the CCAMLR (Convention for the Conservation of Antarctic Marine Living Resources). We would like to introduce you to the work and the krill in more detail.

(C) Uwe Kils, Wikipedia

Euphausia superba, the Antarctic krill, is the dominant of the seven krill species of the Southern Ocean in terms of biomass. They have a circumpolar distribution that is largely in line with the extent of sea ice in winter. Typically, E. superba lives in the wild for 5-6 years and is up to 65 mm long. As a result, they are larger than other common krill species, some of which (e.g. Meganyctiphanes norvegica and Euphausia pacifica)play a crucial role in Arctic ecosystems. These northern species are key elements of much more diverse ecosystems and do not dominate pelagic biomass as strongly as Antarctic krill. This important ecological role is reflected in the way E. superba is represented in food web models of the Southern Ocean. Unlike the other krill species, E. superba is depicted alone and is not summarized. Although all krill species are much larger than many other planktonic species, they are usually assigned to plankton. However, the strong swimming ability of adult krill is a characteristic feature of the nekton. Krill form some of the largest monospecific clusters (swarms) in the animal kingdom, making it the main food source for whales, seals and seabirds and the target of the largest fishery in the Southern Ocean. E. superba itself is the main part of the “pasture animals” of the primary producers, the plant plankton in the Southern Ocean.

(C) from Cavan et al (2019) Nature Comm 10 (4742)

The Southern Ocean is divided into different sectors (red here), which in turn are divided into smaller units. For each sector, the CCAMLR draws up specific rules on extraction and protection. These rules also apply to krill fishing.

The enormous spatial distribution of the adult Antarctic krill to about 19 millionkm2 currently makes it very difficult to carry out a more accurate survey of the total quantity, which leads to very uncertain estimates of krill biomass. The preferred method for assessing krill biomass is hydroacoustics, which, however, entails methodological uncertainties and does not cover surface waters < (20 m) or deep waters. The best estimates from the acoustics of the E. superbadensity by larvae in the Southwest Atlantic region of the South Ocean in 2018 were 29 g / m2 (biomass = 60 million tons) and 5.5 g / m2 (1996) or 23 g/m2 ( 2006) at two different locations in the Indian Ocean/South Polar Sea sector.

(C) from Cavan et al (2019) Nature Comm 10 (4742) based on Data by CCAMLR

The main fishing areas of Antarctic krill are primarily in the south-west Atlantic sector of the Souther Ocean (Area 48). To prevent overfishing, the CCAMLR has used two limits: the “Trigger Level” and the “Catch Limit”. The trigger level sets when this certain amount has been reached in the sub-areas and thus the fishing is suspended. For example, the area around the South Shetlands (Area 48.1) has a limit of 155,000 tonnes, after which the area will be closed for the rest of the season.

The Australian Antarctic Division video gives an idea of how krill is caught and how to measure swarms of krill with acoustic methods these days.

Circumpolar estimates of E. superbafrequency are based on catch net data, for which (since the 1920s) there is a much longer historical data record than for acoustics. Networks also have their own limitations in terms of animal bypass, low sampling frequency and limited sampling from water depths up to 200 m. Data from several scientific surveys with networks over several decades estimate the circumpolar postlarvale biomass at 379 million tonnes, with the highest biomass being in the Southwest Atlantic. However, the combination of grid and acoustic data gives a slightly lower estimate of the circumpolar biomass of 215 million tonnes. The high biomass of E. superba in the Southern Ocean is further confirmed by the large number of E. superba-dependentpredators. In combination with body size, the high biomass of E. superbameans that they are probably very important for the recycling of nutrients and the transport of carbon on a large scale.

Source: Cavan et al (2019) Nature Comm 10 (4742):

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