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.  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.  Presently krill are harvested for fish meal, however with breakthroughs in the understanding of krill ecology, it may be possible to profitably use krill for other purposes.  Utilising the known patterns of kill behaviour aquaculturalists will be able to increase the amount of krill harvested.
            The primary source of food for krill is phytoplankton.  They  use 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)  During the winter months, the amount of available phytoplankton is not as great as it is during the spring and summer months.  The lack of light at the polar ice cap causes reduced amounts of phytoplankton for krill, and thus they must find an alternate source of sustenance.  Current research has yielded a plethora of interesting information regarding the feeding habits and feeding locations of these animals, as well as information about the types of food that they eat at different times of the year.
             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 cm squared 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)  In addition, 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.   Younger krill need to maintain a daily intake to survive.   The effect of this overwintering strategy is that the krill seem to shrink in size.  Even so, both groups manage to survive using what little food there is available.
             In all, 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.  This is important in the competition for food because krill only have to compete with themselves for the phytoplankton.  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 omnivory 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.
 

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.

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

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.