the Antarctic krill, Euphausia superba
Antarctic krill, Euphausia superba, are an important part of the polar
food chain. Making up a large percentage of the Earthís 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 krillís 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
By Catherine Mattison
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
krillís 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 krillís 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
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
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
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 natureís 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
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