A world without krill? – could it be possible?
Krill could be one of the most important species on the planet and yet at only 2½ inches they are often overshadowed by their predators the great whales of the oceans.
So what are krill?
Everybody knows what krill are; the small crustacean found in the world’s oceans and there are 85 known species. Euphausia superba (Antarctic krill) are the most common krill species, found in the Antarctic waters. Antarctic krill are one of the most abundant animal species with 379,000,000 tonnes found in the southern ocean. Figure 1 shows how they can float in huge swarm densities, of which some have been measured at 2 million tonnes of krill spreading over more than 450 square kilometers. Even though krill are not one of the most popular organisms their feature part in the Disney film Happy Feet 2 has brought more attention and increased awareness of their importance in the ecosystem as can be seen in the video below.
Video clip from the film, Happy Feet 2, showing the lifestyle of krill; mainly their swarming behaviour and their role as prey for baleen whales.
The main reason for this swarming behaviour is as a defence mechanism from predators; because in a large swarm it is harder for predators to pick out individual krill. Krill have evolved rhythmic physiological and behavioral mechanisms to adapt to daily and seasonal changes. This is known as diurnal migrations. Spending the day in the deeper depths and then moving upwards in the water column to the surface waters at night ensures the minimal pressure from predators.
What is their role in the ecosystem?
Compared to other phytoplankton feeders in the ocean krill are irregular in the fact they are relatively large and significantly abundant. Even though they are what we would consider to be small in size, in a world of microscopic organisms they are relatively large and so this makes them more prone to predation. Because of this pretty much every other Antarctic animal that is larger than them is a predator including seals, fish, penguins, whales, seabirds.
Antarctic seas are highly productive and are one of the most productive ecosystems on the planet. The food web is shown to the right in figure 2 and although it looks very complex it is clear how central krill are in this environment. Antarctic krill are the major prey source for the majority of the Antarctic species.
Even though krill are heavily predated from the majority of Antarctica they are still greatly successful and this is because they are very efficient filter feeders. Their diet consists of microscopic phytoplankton which are largely abundant in the Antarctic waters due to the great upwellings of deep waters at the Antarctic convergence.
They are largely successful because of their ability to find food in several different types of habitats and efficiently exploit whatever food is available. In the winter months krill feed under the seasonal sea ice using their feeding appendages that are amended with scraping setae so they can eat the algae off the ice.
Krill for dinner
If asked what the role of krill was the majority would say that krill are food for the baleen whales of the worlds oceans.
The Baleen whales have an extremely effect method in catching the krill with their baleen plates playing an important of this technique. Baleen whales swim through the water with their mouths open taking in a large mouthful of sea water containing krill. The fringed baleen acts a sieve so when closing the mouth the excess water is forced out by the tongue, the baleen filter out this water and small plankton but keep and swallow the krill. This is seen in the video below. Different whales feed at different depths and are morphologically different in regards to their filters so there is not so much competition for the same sized krill.
The video below is from the BBC series Planet Earth and shows Humpback whales feeding on krill, showing the different techniques they use to capture their prey.
The decreasing krill..
There is evidence that krill stocks have decreased in Antarctica in recent years; which could also be linked to the drop in other Antarctic species that has been observed. The reason for this is most likely to be because of global warming resulting in rising temperatures which lead to a fall in the amount of sea ice in the winter months.
There is also increased human threats as new fishing technologies have allowed fishermen to capture even more krill than ever. Figure 4 demonstrates how these vessels use trawling nets to capture krill from the Antarctic waters.
Depleting krill numbers affect the predators as they cannot always move to another area, such as penguins as they are restricted to the waters closest to their nests when searching for food for the chicks.
There have been schemes and approaches set up for the conservation of krill including CCAMLR, The Commission for the Conservation of Antarctic Marine Living Resources. They focus on an ecosystem based approach to management focusing on the impact of fishing. An important component of this approach is the CCAMLR Ecosystem Monitoring Programme (CEMP), established in 1989 to detect changes, particularly with respect to krill-dependent predators, and to evaluate whether observed changes are due to krill fishing or environmental factors.
Would there be a world without krill? What would happen to the Antarctic ecosystem if the krill population were significantly diminished?
Without krill present, the species dependent on it would suffer and those populations then become susceptible to decline. Consequently it is important to understand the role of krill in the Antarctic ecosystem and how the population density can vary seasonally. Ultimately it is vital that krill are not overharvested to the point where the whole ecosystem is disrupted and destroyed.