Ecology of Krill

Ecology of the Antarctic krill, Euphausia superba

By Catherine Mattison

             Antarctic krill, Euphausia superba, are an important part of the polar food chain. Making up a large percentage of the Earth’s biomass, these tiny creatures provide food for much larger creatures such as seals, sea birds, and whales.   Since there is interest in developing krill as an efficient source of food for humans, it is important to understand how they survive in their natural habitat and what role they fill in the polar ecosystem. Studying the seasonal feeding behaviours of krill will help scientists see patterns that may allow for a greater development of the krill fishery. For instance, knowing the krill’s response to food, temperature, and time of day can help in harvesting them for commercial use. Although there is much that is known about krill, what remains to be seen is what effect harvest will have on native populations. Sudden drops in krill population sizes would have a dramatic impact on larger animal species, such as whales, fish, seals, and sea birds that are dependent on krill as a food source. Through public awareness, proper management, and fishermen education it is hoped that the possible ecological devistation caused by sudden decrease in krill can be prevented and avoided.
            First and foremost, what is a krill? Krill are tiny, shrimplike organisims that live in the icy water of the polar regions. There are several species of krill that live around the world (Nicol, 1999), but for the purposes of this discussion, we will refer to krill as the species Euphausia suberba, the Antarctic krill. This is because it happens to be the most predominant species, dominance being determined by amount of biomass. Krill happen to be amazingly abundant throughout the southern ocean. The total krill biomass probably outnumbers that of humans. (Kils, 1981) The tiny shrimps are an extremely important source of carbon for higher organisms, and without them there would be catastrophic changes in the food web. Whales, seals, penguins, fish, and sea birds would be forced to either relocate or find alternate food sources.
            The primary source of food for krill is phytoplankton. There are two primary types of pohytoplankton on which they feed; diatoms and cryptophytes. (Moline, et.al., in print) Krill have specialised appendages called filtering baskets to obtain their food. Using a filtering basket, an individual krill can filter particles as small as 1?m. (Daly and Macaulay, 1991) In response to algal blooms, krill may form a feeding swarm. Swarms migrate to a depth of about 100 m during the day, to avoid predators, and return at nightfall to the surface to feed. One reason for the krill’s vertical migration is that the microalgae on which they feed flows with the high density brine along the underside of the ice until it becomes part of the water column. (Stretch, 1988) The swarms may disperse, form large layers, vertically migrate, disperse at night, or exhibit a combination of these behaviours. (Daly and Macaulay, 1991) These swarms form in response to the krill’s high sensitivity to food. When the krill come into contact with downwelling streams containing microalgae, they go into a feeding frenzy. (Stretch, 1988)
            When the amount of a particular phytoplankton becomes limited, another type will emerge as the dominant species, and thus an interesting change in the residential composition of the Antarctic waters occurs. During the times when the krill's favourite food, diatoms, is lacking, it has been shown that another type of phytoplankton, known as the cryptophyte, becomes more prevalent in the water. (Loeb, et.al., 1997, Moline, et.al, in print) An increase in cryptophytes means that there will be more salps in the water and reduced numbers of krill.   When there is an increase in salps and a decrease in krill, this means that there will be lesser numbers of higher organisms such as whales, fish, seals, and penguins. These organisms are unable to utilise salps as a carbon source in the same way that they can use krill, and are adversely affected when there is a lack of their favourite food.
            The concentrations of Antarctic krill vary with season and food availability. In addition, the swarms themselves have their own special features. For example, each swarm is made up of one age group, that is, adults swimming with adults, and juveniles swimming with juveniles. The juveniles tend to stay near the underside of the ice, and they have also been observed to feed inside brine channels in decaying ice flows. (Daly and Macaulay, 1991) Adults swim mainly in the open, warmer water where there is more food, especially in the fall.   These are the locations where krill are most likely found however their locations may change with the seasons. For example, in the springtime both groups can be found in the open water. The location of krill is generally dependent on the age group.
             The seasonal feeding of Antarctic krill, Euphausia superba, is dependent upon the location and quantity of food. During the spring, the food supply is lowest under the ice, and highest in the open water. (Daly and Macaulay, 1991) The adult krill can be found feeding on the dense phytoplankton blooms in the open waters, while the smaller juvenile krill feed on the phytoplankton under the ice. The smaller, immature krill stay under the ice to avoid predators. During this time of the year, the days become longer with more light to produce greater concentrations of phytoplankton. In the summer months, the area which krill occupy extends to the area between the Antarctic convergence and the Antarctic coastline, (Nicol and Allison, 1997) almost an area of 35 million km squared. Because of the high abundance of phytoplankton, the krill can easily clear 100 cm² of algae from the ice in 5 minutes. (Nicol and Allison, 1997) The grazing behaviour that they use to clear the ice is defined as the behaviour that occurs when krill orient to the undersurface of ice to rake algal cells off the ice. (Stretch, et. al., 1988) As the light intensity increases, more algae can be found in open waters. In the fall, the concentration of adult krill is in the warmer, open waters, while Daly and Maculay (1991) found that the greatest densities of juveniles and immature adults were found under the ice. This shows clearly that the adult krill are responsible for most of the consumption of the phytoplankton. In all, the swarms consume 5-10% of primary production in the Antarctic. (Nicol and Allison, 1997) As the winter approaches, the feeding patterns change. Pakhomov, et. al. (1997) observed that krill regularly switch from herbivory to omnivority during the austral winter to meet their energy demands when the plankton concentrations are not sufficiently large enough to meet their needs. The diet of the krill may be supplemented with smaller animals, such as salps. (Kawaguchi and Takahshi, 1997) Gut contents of examined krill in fall and winter contained heterotrophic organisms and detritus in addition to phytoplankton. (Daly and Macaulay, 1991) Ice algal communities are also an important source of nutrients for the krill. Stretch, et. al. (1988) thought that this might help to explain how krill survive over the winters when stocks of phytoplankton are severely depleted. Even though food may not be readily available, Nicol and Allison (1997) discovered that adult krill could go for long periods without food by using their own lipids and proteins.   They undergo an amazing transformation where they moult and their body size decreases with the lack of food intake. (Nicol,1999) Both younger and mature groups manage to survive using what little food there is available.
            The species Euphausia superba has a unique method of survival. Their vertical migrations provide for a distribution so that there are enough krill for those animals that depend on them for their survival, but so that there are enough krill to reproduce so that the population size necessary for the surrounding ecosystem is maintained. The krill have adapted in a way so that they are able to get nutrients all year long in a region that has fluctuations in food production. Their specialised adaptation of raking the algae off of the underside of the ice cannot be equalled by any of the other nearby species. Another consideration is the feeding habits of the age groups. Because the mature adults feed in the open water sometimes, the younger krill are unable to compete, and are forced under the ice. Seasonably speaking this comes during the spring, when the phytoplankton blooms are developing. It is to the advantage of the adult krill to get as much food as they can in order to have a successful spawn. In addition, the separation of age groups prevents the krill from eating each other in times when algae and phytoplankton are scarce.
            The vertical migration is also an interesting form of behaviour. Since krill make up a large percentage of the Antarctic biomass, there are many larger animals such as whales, seals, and fish that forage for them. By retreating to depths of up to 100 m during the day, krill are able to hide from the active daytime hunters. When they return to the surface, the risk they face from predators is slightly reduced, and they are able to feed more safely.
            Euphausia superba is a highly interesting species that has a variety of survival tactics, ranging from their vertical migration to their change in winter diet to omnivoury from herbivory. They also change their location in response to the seasonal change in food. Without these specialised habits, the krill would have a reduced chance of survival and procreation in a harsh and unforgiving environment. This knowledge of krill behaviour will lead to a more profitable means of harvesting krill for application to human needs.
            With regard to harvest of krill, there has been little research in this area. Krill has been the largest fishery in the Southern Ocean for the last 25 years. (Nicol and Rockliffe, 1999) Russia, Poland, Ukraine, and Japan are all competitors in krill trawling, and it has been estimated that with today's technology, catches can average 30 tons per hour. (Christie, 1988)   Because of the feeding swarm behaviour and its unpredictibility, it is hard to determine exact population numbers. For this reason, there have been efforts made to restrict the catch in hopes of protecting the Antarctic community.
            Historically, krill have been fished commercially since the 70's. (Nicol, 1999) The animals were caught on large trawlers where they were processed aboard to maintain freshness. The six principle countries that fished krill were South Korea, Chile, Poland, Japan, Russia, and the Ukraine. Today, Japan, Russia, and Ukraine account for 96% of the total world catch.
            In 1981, it was recognised that the krill fishery could be easily exploited and thus potentially deadly to the Antarctic community. For this reason, the Convention for the Conservation of Antarctic Marine Living Resources was designed, signed and implemented. The purpose of this treaty was to begin to manage the fishery by limiting the total yearly catch, however it wasn't until 1991 when there was an actual limit set. The Commission of Antarctic Marine Living Resources set a limit of 1.5 million tons for the South Atlantic. This was a good step towards better management of the krill, but the best form of management comes from understanding the biology of the species, something that continues to be challenging due to the fact that krill are hard to keep in a laboratory setting. In addition a major problem exists-how will the quota be enforced? Once more is known about the living habits of krill, better ways of management can be decided.
            Population dynamics make for a difficult time in determining catch quotas for krill. Brierly and Reid (1999) reported that the population numbers fluctuate naturally every 3-4 years. Observed patters indicate that studies of krill predators can give vital clues to stock crashes. What they have seen is that the year before a population crash, the average length of the krill is unusually high. This indicates a lack of smaller-sized krill, and means that there are more larger-sized krill being consumed by predators. The reason for the lack of smaller krill has a lot to do with ice-coverage, which has a direct affect on the krill diet. (Loeb, et.al., 1997) When there is more ice-coverage, there will be more krill, and lack of it will lead to less krill and more salps. Krill require colder water with higher salinity in order for survival, however it is not necessarily the total dependence on these physical factors. Higher salinity and lower temperatures are optimal conditions for diatoms, as described by Moline, et. al. (in print) This may not seem like much, but a slight difference makes a tremendous change in the number of predator species.
            Larger predator species are dependant upon krill as a food source, and like any other creature, a change in a food source has a tremendous impact on their living habits. Christie (1998) reports that penguins are a good indicator of krill stock status. In 1986, the Chinstrap and Adelie penguins, who feed on the most heavily exploited krill stock, began to decline. In that year, commercial fishing vessels had stripped 400, 000 tons of krill from the same area. The penguins began to see population decreases of 10-20%. Since 1989, the King George Island penguin colonies have declined 40%. In 1994, there were massive seabird starvation reports off the coast of South Georgia, and as recently as this year, the rate of Adelie penguin chicks dying per day was as high as 50. These are some stunning statistics, but whether these declines are directly linked to the absence of krill is uncertain. Biologists feel that not enough is known about the life history and distribution to be sure.
            On a more positive note, it has been discovered that krill have a life span of several years as opposed to one, like the salp. The implication of this is that harvest rates will have the most impact on a single year class, instead of two or three, which would allow for a quicker recovery of the stock. This is not to say that more krill can potentially be harvested, but rather the numbers of reported catch may be slightly misleading, due to the fact that populations cannot be accurately measured.
             Overall, the krill is an amazing animal. Its high protein content makes it an excellent food source for many marine organisms as well as for humans. In addition, there are physical characteristics about the organism that may prove to be useful in the pharmecetical industry, such as the chitin in its shell as an anti-coagulent for human blood. Having many uses is good, but the popularity of the krill may lead to its ultimate demise of extinction from overfishing. For this reason, the stock must be protected through various means of management, something that is more widely known than ever before. The newness of the krill fishery will work in its favour because much is already known about other fish stocks and how to maintain sustainable fisheries where there can be a reasonable amount of harvest, but to no detriment of the total population. By understanding more of the biological and behavioural aspects of the krill, better management policies can be created and implementented. The one catch is that in order to be effective, things must change now before it is too late. There are too many species in the works that have been exploited beyond nature’s ability to repair. The krill is such a delicate animal that if not protected, would have tremendous impact on the Antarctic ecosystem. Eventually, this would trickle into the human world and affect us indirectly. Now is the time to take action.

References

Brierly, A. and K. Reid. (1999). Kingdom of the Krill. New Scientist, 17 April 1999.

Christie, A. (1998). License to Krill. Sea Shepherd Conservation Society. http://www.seashepherd.org/fs/fskrill1.html

Daly, K.L., Macaulay, M.C. (1991). Influence of physical and biological mesoscale dynamics on the seasonal distribution and behaviour of Euphausia superba in the Antarctic marginal ice zone. Marine Ecology Progress Series 79:37-66.

Kawaguchi, S. and Takahashi, Y. (1997). Antarctic krill (Euphausia superba dana) eat salps. (Abstract) Polar Biology 16(7):479-481.

Kils, U. (1981). Swimming behaviour, swimming perfomance and energy balance of Antarctic krill, Euphausia superba. BIOMASS scientific series 3, BIOMASS research series, Texas A&M University.

Loeb, V., Siegel, V., Holm-Hansen, O., Hewitt, R., Fraser, W. Trivelpiece, W., Trivelpiece, S. (1997). Effects of sea-ice extent and krill or salp dominance on the Antarctic Food Web. Nature 387: 897-900.

Moline, Mark A., et. al. (In Print). Potential impacts of environmental change on food web interactions in an Antarctic coastal region. Submitted to Limnology and Oceanography, 1999.

Nicol, S. and Allison, I. (1997). The Frozen Skin of the Southern Ocean. American Scientist. 85:426-439.

Nicol, S. (1999). Time to Krill? Australian Antarctic Division, Kingston, Tasmania.
http://www.antdiv.gov.au/science/bio/issues_krill/index.html

Nicol, S. and W. Rockliffe. (1999). Magicians of the Southern Ocean. Australian Antarctic Division, Kingston, Tasmania. http://www.antdiv.gov.au/resources/antarctic%5Finformation/krill%5Fprimary.html

Pakhomov, E.A. et. al. (1997). Energetics and feeding dynamics of Euphausia superba in the South Georgia region during the summer of 1994. Journal of Plankton Research 19:399-423.

Stretch, J.J. et. al. (1988). Foraging behaviour of Antarctic krill, Euphausia superba on sea ice microalgae. Marine Ecology Progress Series 44:131-139.