Kils, U., & Klages , N. 1979: Der Krill, Naturwissenschaftliche Rundschau 32 Heft 10 Seite 397-402 


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The Krill

Uwe Kils & Norbert Klages, Kiel

Naturwissenschaftliche Rundschau

1979

An animal, whose existence was familiar to only whalers and zoologists a few years ago, recently became known to many: the krill. The worldwide endeavors about Antarctica, the last unclaimed continent, but also the quest for new food resources, made krill popular. Information about this small crustacean can not be found in general textbooks yet but only in a few scientific publications (for example 1 - 5). Most is still in the stadium of exploration, and also some of the following knowledge about krill derived from the 1976 and 1979 "German Antarktis Expeditionen".

Habitat

Krill is thronging the surface waters of the Southern Ocean in gigantic quantities, has a circumpolar distribution with highest concentrations in the Atlantic sector (5 page 244). The Antarctic convergence defines more or less the northern boundary. That is the circumpolar front where the cold Antarctic surface water submerges below the warmer subantarctic waters.

The Southern Ocean with its Atlantic, Pacific and Indian sectors stretches from the polarfront at ca. 55 degree South to the edge of the continent, covering 32 million square kilometers. That is 65 times the size of the North Sea. Whereas during winter more than three fourth are covered by ice, vast areas (24 million square kilometers) become icefree in South summer. The water temperatures are between - 1.3 and 3 degree Centigrade.

The waters of the Southern Ocean form a system of currents. In the West Wind Drift the surface strata is traveling round Antarctica eastwards. Close to the continent the East Wind Drift runs counterclockwise. At the front between both large eddies develop, for example in the Weddell Sea. The surface water become enriched by nutrient-carrying depthwater, originating from lower latitudes, while a the iceedge water from the surface descends down to the bottom and spreads worldwide into all oceans. Plant plankton need nutrients in order to grow, which generally are in short supply in most part of the oceans and consumed quite fast. Close to Antarctica, however, nutrients are available in high quantities, so that they become not a limiting factor for phytoplankton blooms. Together with the long daylight during summer this is the basis for strong plant growth. However strong winds, causing deep reaching turbulences, often hamper development when plankton gets transported below the euphotic zone. The production is very variable in time and space. In spite of high values close to the iceedge the Southern Ocean is ranking more towards the low productive oceans.

Systematic

The krill, Euphausia superba, belongs to the class of crustacea (euphausia, euphausiidae, euphausiacea, eucarida, malacostraca, crustacea). The order euphausiacea, also called lightshrimps, are shrimplike eucarida, whose carapax is joined with all thoracomers and so short on the sides that the gills are visible. None of the thoracopods is formed into a gnathopod, differentiating this order against the decapoda.

Bodystructure

The name "lightshrimp" points to the fact that these animals are outfitted with light organs (one pair at the eyestalk and the hips of the 2nd and 7th thoracopods and single ones at the four pleonsternites). These lightorgans transmit from time to time a yellowgreen light for 2 to 3 seconds and are so highly developed that they can be compared with a torchlight: A concave reflector in the back and a lens in the front guide the produced light, and the whole organ can be rotated by muscles. The origin of the light and the biological function are not known yet.

Abb. 1 und 2 Antarktic Krill, Euphausia superba (male)  lenght 58 mm, weight 1.5 g

Habitus and size of the krill is comparable with the North Sea shrimp (Crangon crangon), both are also close related (Eucarida). The krill (fig. 1 and 2) reaches a size of about 6 cm, from eyes to tail, and a weight of 1.7 gram. Notable are the different development of the 11 pairs of legs. The caudal 5 (pleopods) are constructed very sturdy and carry at their ends large paddles. They ought to beat twice per second continuously in order to carry the relatively heavy animal in the water and prevent sinking. They are driven by five large muscle complexes, so the caudal body part represents a large meat packet. The forward six pairs of legs (thoracopods) are long and delicate; they carry thousands of bristles, comparable to a comb, and are arranged in such clever way, that they form a finemeshed net (fig.3.). The meshsize of this feeding net is below 0.01 mm, powerful enough to utilize smallest plant plankton. The plankton is then "combed" towards the mouth and reaches the "gastric mill". In this muscular organ, outfitted with ridges, the solid shells of the plankton are broken up. The useful components are guided for processing into the many branches of the hepatopankreas, which is colored intensely green in the living animal. The solid silica shells become ejected through the strait gut very fast.

These biological peculiarities render the krill so interesting for human utilization: The frontal half of the animal collects the -for us- not accessible primary production and converts it into valuable substances, for example protein; in the caudal half these substances are compacted in a form interesting for humans - meat.

Abb. 3. Filtersetae at the head of krill.

Development

Main spawning time of krill is from January to March, over the shelf, but also in oceanic areas over deep waters. As typical for euphausiaceans the male attaches a sperm package to the genital opening of the female. For this purpose the first pleopods of the male are constructed as tools. The fertilized eggs, 2000 to 7000, are spent close to the surface into the open waters. According to the classical hypothesis of MARR 1962 (1), which he derived from the results of the great Discovery-Expedition, the development is this: Gastrulation sets in during the descent of the 0.6 mm eggs, on the shelf at the bottom, in oceanic areas in depths around 2000 m. From the egg hatches the 1st nauplius and starts the migration towards the surface with the aid of its three pairs of legs ("developmental ascent"). The next two larval stages, 2nd nauplius and metanauplius, do not eat but are nourished by the yolk. After three weeks the little krill has finished as 1st calyptopis the ascent (fig. 4). Growing larger additional larval stages are following (2nd and 3rd calyptopis, 1st to 6th furcilia). They are characterized by increasing development of the additional legs, the compound eyes and the setae. At 15 mm the juvenile krill resembles the habitus of the adults. After two, maybe three years krill reaches maturity. As characteristic for all crustaceans krill must molt in order to grow. Approximately every 13 to 20 days krill ejects from its chitin skin and leaves it as exuvia behind.

Abb.4. Vertical distribution of the early developmental stages of krill. After [1].

Lifestyle

The lifestyle too shows some peculiarities, which make a utilization by humans possible: The animals form huge and dense schools, with up to 30 000 animals per cubic meter. Because fisheries nets with small meshes have a high drag only schools are worthwhile objects, otherwise the costs (time, gasoline) outrun the value of the caught krill. When and why krill form dense schools are objects of the current research, because also random distribution has been observed. Also the schools swim close to the surface. Even though the water of its habitat is generally more than 3000 meter deep nearly no krill has been found below 300 meter. The reason for that is the relatively high demand for oxygen, which can only be satisfied in the upper oxygenated strata of water. This circumstance makes krill easy accessible for fisheries.

Krill is a pelagic animal, a free-swimming animal. Krill never rests on substrates or holds on to substrate, but is an extraordinary swimmer, capable of cruising day after day at velocities of 9 meter per minute, during escapes reaching 60 cm per second over short times. Krill is therefore not plankton but nekton.

The capability of hovering in free water allows krill to utilize the vast planktonrich areas of Antarctica, which spread predominantly over depths of 3000 meter. The effort for this however is high: Because krill did not develop a buoyancy aid like a swim bladder or oil drops it must protect itself from sinking by intensive swimming activity and consumes constantly energy, like a flying bird. In case krill finds not enough food or any weakening it will sink into the abyss where it cannot survive.

Food

During summer krill feeds on plant plankton. Preferred food is small diatoms and dinoflagellates. Also other food is taken, even cannibalism has been observed frequently in aquaria observations. Possibly E. superba utilizes even detritus in the dark winter months under the ice.

Position of krill in the Antarctic ecosystem

In the Antarctic ecosystem krill is a key species. Euphausia superba is the main consumer of the phytoplankton production in the sea and on the other side main food for the large predators. Its huge schools convert the microscopic algae into proteinrich meat packages. When in dense schools krill is a lucrative goal for birds, fish and marine mammals. A remarkable characteristic of the Antarctic ecosystem is the faunistic unity. Many species are endemic. In the food chain only few species are dominant. Crabeater seals (Lobodon carcinophagus) comprise 80 % of all Antarctic seals, Adelie penguins (Pygoscelis adeli) 90 % of all penguins. Both species are krilleater. For the fish fauna nototheniiformes comprise nearly three fourth of all neritic fish species.

Characteristic is also the shortness of the food chains. One important track covers only three steps, when diatomeating krill is consumed by marine mammals, like baleen whales. However there are also other pathways in the food chain, including salps, cephalopods, fish and benthos organisms. In comparison with Antarctica the food chain relationships of the other oceans are far more interactive and run over more steps. This is indicated by the comparison between the Southern Ocean and the North Sea.

Remarkable in this comparison are the huge weight differences of representatives of similar trophic strata. These straight relationships in Antarctica render this ecosystem very sensitive for uncontrolled anthropogenic actions. Too much krill fisheries could easily lead to serious changes. Because of the poorly regulated whale hunt over the past decades not only the whale stocks have been depleted to a level that some species are endangered. We have indications that crab eaters seals and birds have increased in numbers, even though we are in lack of precise data, because the food-competitors of the whales have not been investigated before the start of the whale hunt. Fig. 5 shows a simplified food chain of the Southern Ocean.

Fig. 5. Position of krill in the pelagic food chain in Antarctica. In 1 000 000 t wet weight. Theoretical sustainable yield for synchronous optimal fisheries for krill and whales: krill 80 million tonnes, baleen whales 2 million tonnes. The values especially for krill are very unreliable. After HEMPEL 1877 (4)

Catches and processing

Krill fisheries are difficult in two aspects: Because a krill net needs to have very fine meshes it has a very high drag, producing a bow wave, deflecting the krill to the sides. Also fine meshes clog very fast. A fine net is also a very delicate net, and the first krill nets exploded while fishing through the schools. Another problem is how to bring the catch on board.

During hauling of the full net out of the water the organisms, especially the lower ones, compress each other, and much juice is lost. During the last expedition experiments were carried out to pump krill, still in water, through a large tube on board. A special krill net is under development too.

The processing of the krill has to be very quick because it deteriorates within a few hours. One goal is to split the muscular hind part from the front part and to separate the chitin armor, in order to produce frosted products and concentrate powders. Its high protein and vitamin content makes krill interesting for direct human consumption and for the animal-feed industry.

How high can the fisheries yield of krill be?

The previous chapters indicate that Euphausia superba is a key species in the Southern Ocean. In spite of its extreme abundance it is necessary to gain as much exact data about its biomass and anual production as possible. Such figures are prerequisite for predictions of how much krill can be taken from the seas for human consumption without persisting damage. A direct estimation however is virtually impossible. For such an approach it would be necessary to combine echo-surveys with test-fisheries over representative areas, an extreme task considering the size and unaccessibility of its habitat. Approaches to gain an estimation via indirect methods face difficulties too.

Fig. 6. Members of the Antarctic Treaty. Voting members: Argentina, Australia, Belgium, Chile, France, Great Britain, Japan, New Zealand, Norway, South Africa, UdSSR, USA, Poland. Non-voting members: CSSR, Denmark, Netherland, Rumania, DDR, Germany. Recently the Bundestag had agreed to join the "Antarktis-Club". The Antarctic Treaty was establised at December 1st 1959 by 12 states, and rules that the 6th continent can only be used for peacefull goals and that freedom of science is granted. The simple membership without voting power a state achieves by depositing a membership application at one of the founding members and by construction of a research station. For full membership with voting rights it is prerequisite that the research station has produced essential scientific results. Dots: research stations.

Such an estimation is deducted from figures of primary production or of krill consumption by higher levels (for example whales, seals) extrapolating towards annual krill production. The knowledge about the transfer coefficients from one trophic level to the next is very limited though. Not much is known about the percentage of krill consumed by predators. Especially data about predation by fish and cephalopods are missing.

If we consider a total biomass of about 350 millionen tonnes wet weight we should keep in mind the very uncertainty of this figure. This is even more true for the annual krill production, which has been estimated to be more than a billion tonnes (Everson 1977 [2]).

How much of this can be used? A first idea would be to utilize the percentage that baleen whales consumed before their breakdown. Laws 1974 [3] estimated, that about1900, when whaling in Antarctica began, 190 million tonnes of krill had been consumed by whales. Todays reduced whale population consumes about 43 million tonnes. Therefore we could estimate a mathematical surplus of 147 million tonnes krill. However this mass is not swimming the oceans unutilized. Other krilleater use this resource. As already mentioned this component allowed for higher reproduction rates in seals and birds.  So it is not a good idea to observe only krill in managing its utilization. Antarctica too is home of diverse ecosystems and interactions, and a future development in fisheries has to take those into consideration if extensive changes in the biological systems are to be avoided in the future. In the case of krill we deal with one of the last few virgin stocks, and for the first time in history we would have the chance to place science before exploitation, in order to avoid the same errors as during the exploitation of many fish stocks and the whales. As paradox as it may sound: The whale yields of the prime years would have been sustained year after year until today without putting the stocks at risk if the different species would have been harvested synchronously and biologically balanced. It is well worthwhile to consider krill wisely: Taking the scientific knowledge of today it should be possible to harvest 70 million tonnes each year - that would be about a doubling of the current global yield of the oceans. In respect of the uncertainties of the production estimates, and also in order to prevent local disasterous effects, the members of the Antarctic Treaty (fig. 6.) call for international catch limits, allowing only slow expansions of krill fisheries with strict supervision of its effects.

Literature

[1] J. Marr, Discovery Rep. 32, 33 (1962). - [2] I. Everson: The living resources of the Southern Ocean. FAO. Rom 1977. [3] R. Laws, Proc. of the Third Symposium on Antarctic biology. Washington 1974. [4] G. Hempel, Verh. Dtsch. Zool. Ges. 1977. 67 - 85. Gustav Fischer Verlag. Stuttgart 1977. - [5] Naturw. Rdsch. 31, 244 (1978).


Dipl.-Biol. Dr. U. Kils (born 10th of Juli 1951) is with the Instituts für Meereskunde, Kiel. participant of the Deutsche Antarktis-Expedition 1977/78, Promotion about krill-behavior.

N. Klages, Diplomand am Institut für Meereskunde, Kiel, participant of the Deutsche Antarktis-Expedition 1977/78.


old address: Dr. U. Kils, Inst. f. Meereskunde, Düsternbrooker Weg 20, 2300 Kiel.



edited and served by uwe kils

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