Krill and Pelagic Food Webs

Although the term krill derives from the Norwegian "kril", used by whalers to denote small fish, it is now applied to crustaceans eaten by baleen whales in the Southern Ocean, and it is often reserved for the largely dominant species E. superba. This macroplanktonic species plays a central role in the seasonal pack-ice region of the Southern Ocean due to its abundance (probably one of the most abundant and successful animal species on Earth), large size (up to 6 cm long) and fresh weight of about 1 g. It largely affects the dynamics of pelagic food webs and biogeochemical fluxes of macro- and micronutrients, and therefore attracted much attention from biologists in the British Discovery expeditions (e.g. Fraser 1936; Marr 1962), the BIOMASS Programme (e.g. Miller and Hampton 1989; El-Sayed 1994), and in recent years (e.g. Knox 1994; Trathan et al. 1995; Hagen et al. 1996; Daly 1998; Perissinotto et al. 2000; Reid 2001). In spite of the large number of papers on krill biology, ecophysiology, behaviour and adaptations, its distribution, life cycle and biomass are still not fully understood. They generally grow to 5 cm in two years and have a lifespan of 5-8 years. While adults can tolerate starvation for over 200 days, the larvae can survive without food for only about 2 months. Thus, larvae usually overwinter beneath the ice, normally feeding off phytoplankton and small zooplankton filtered through a basket formed by fringed thoracic limbs adapted to capture algae scraped off the undersurface of the sea ice. Although sea ice allows the overwintering of adults and provides a nursery for larvae, it is still unclear whether the under-ice krill population represents a significant proportion of the total Southern Ocean population or whether it is only part of the more general ocean current-driven life cycle of krill (Hansom and Gordon 1998). It is still unknown to what extent krill populations take advantage of overwintering under the pack ice. For instance, there is evidence that, in summer and winter, krill in coastal waters near the South Shetland Islands (Ligowski 2000) mainly feed on benthic diatoms.

In general, E. superba shows a very patchy distribution. It mainly occurs in zones between the summer and winter limits of the pack ice, at a depth ranging from 20 to 150 m, where it takes advantage of phytoplankton blooms. It rises to the surface at night and sinks during the day, concentrating in swarms which may be very large (super-swarms may reach a density>1,000 g m-3, thickness of 100-200 m and length of up to several kilometres; Knox 1994). With respect to other zooplanktonic organisms, krill can swim at speeds greater than 10 cm s-1; the movement of individuals in swarms is coordinated and synchronised, and there is evidence that swarms can move against currents. However, E. superba adults are carried towards the APF in Antarctic Surface Water, where eggs released at depths of about 100 m begin to sink, hatch at around 1,000 m, and are carried back southwards together with young larvae in Circumpolar Deep Water. As larvae develop, they slowly ascend to the surface and begin a new cycle, allowing the distribution of E.

superba at different latitudes between the APF and the continent. Like assemblages of phytoplankton and of other Antarctic marine organisms, krill has a circumpolar distribution and a latitudinal zonation (Fig. 27). There is evidence that two main circumpolar currents, and the transport and collection of larvae and young adults in gyres and eddies provide a framework for high krill biomass in the Scotia Sea, around South Georgia Island, in the Bellingshausen Sea, and in several areas off the coast of East Antarctica, such as north of the Ross Sea, off Wilkes Land, Enderby Land and Queen Maud Land.

The short food chain based on krill represents the most outstanding product of a long evolutionary process in isolation, in a cold and nutrient-rich environment with a seasonal ice cover and light availability (Hempel 1985). While krill has a rather wide food spectrum, most of its consumers are specialised to feed on it alone. Like other highly productive pelagic areas which host short food chains with a large herbivore biomass (e.g. anchovies and sardines), in the marginal ice zone E. superba is the main species channelling organic matter from phytoplankton to cephalopods, fish, birds, seals and baleen whales. The giant squid (Mesonychoteuthis hamiltoni, over 5 m in length and 150 kg in weight) and other species of cephalopods are among the most important krill consumers, and they are in turn important in the diet of

Zooplankton Seabirds Seals Whales

Zooplankton Seabirds Seals Whales

Ice-free zone

60°S Seasonal sea-ice zone

7Q°S Permanent sea-ice zone

80°S Continental shelf

Fig. 27. Latitudinal distribution of some Antarctic marine organisms

Fig. 27. Latitudinal distribution of some Antarctic marine organisms

sperm whales. Thousands of squid beaks, many belonging to species rarely or never caught in nets, are sometimes found in the stomachs of these whales (Nemoto et al. 1988). Wandering albatrosses, Emperor penguins, seals and killer whales are among the other consumers of Antarctic squid.

There is still uncertainty about the total standing stocks of krill, and the most reliable estimates are probably those based on krill consumption by major predators. As a general indication, krill consumption by fish, seabirds, squid and seals has been estimated to be 15,30,40 and 130x 106 tonnes year-1 respectively (Knox 1994; tonnes, t). Current consumption by reduced whale stocks has been estimated to be about 40x106 1 year-1 (Armstrong and Siegfried 1991), i.e. about five times lower than the amount consumed before exploitation. However, it is wrong to suppose that the reduction in whale stocks has determined a "surplus" of krill, because there are no data on krill biomass during the period of whale exploitation and because other krill-con-suming populations have probably increased as a result of the reduced competition with whales. As suggested by Berkman (1992), pelagic ecosystems in the Southern Ocean appear to have already adjusted to a constant level of krill consumption (300-400x1061 year-1, which roughly corresponds to the total annual production; Knox 1994).

Owing to its wide distribution and abundance, since the 1960s Antarctic krill has attracted the interest of fishing fleets searching for new species as traditional ones are fished out. According to CCAMLR estimates, krill harvesting peaked at more than 500,000 t in 1981-1982, and during the last decade was about 100,000 t year-1 (Fig. 28). Overall, more than 5.74x1061 has been harvested to date. Krill fishing is causing much concern, and a better understanding of E. superba biology, distribution and swarm composition is essential for the proper management of fisheries. Although the present exploitation rate in relation to the total stock is probably not excessive, it cannot be excluded that localised stocks at the northern edge of the pack ice are already overexploited (Everson and Goss 1991). In addition to krill, squid are also fished commercially in some northern areas of the Southern Ocean, such as offshore Patagonia, the Falkland Islands and New Zealand. As in the case of krill exploitation, squid exploitation would pose problems for a number of predator species which, especially in the APF, depend almost exclusively on cephalopods for their breeding season diet.

Besides E. superba, the Southern Ocean contains other important herbivorous (e.g. protozoans, copepods, euphausiids, salps) and carnivorous (e.g. copepods, chaetognaths, pelagic polychaetes) zooplankters. These organisms are eaten by Parathemisto gaudichaudii and, in contrast to E. superba, they do not generally constitute a major food source for large predators.

In spite of its considerable age and size (about 10% of the world oceans), the Southern Ocean contains only 1% of fish species (about 21,700 species; Nelson 1984) inhabiting Earth's waters. The perciform suborder Notothe-nioidei, the dominant group of Antarctic fish fauna, contains 97 % endemic

Fig. 28. Catch of krill during the last decade. (Data from CCAMLR 2003)

Fig. 28. Catch of krill during the last decade. (Data from CCAMLR 2003)

species, suggesting that it evolved in isolation (Eastman 1993). Few species of Antarctic fish live in the upper 200 m of the water column. The Antarctic cod (Notothenia rossii) is a secondarily pelagic species which feeds on krill and has been overfished in recent years. The silver fish (Pleuragramma antarcticum) is one of the few neutrally buoyant species among notothe-nioids; adapted to temporary or constant life in pelagic water, it is one of the major Antarctic fish species in number and biomass, and is the prey of most of the larger Antarctic carnivores. Small myctophids (lantern fish) are among the most abundant mesopelagic fish in the Southern Ocean. They show diel migratory patterns and are among the most important krill predators. Their biomass is thought to exceed that of E. superba (Eastman 1993).

Of the 43 species of birds (mainly procellariforms and penguins) which breed south of the APF, only 12 breed on the Antarctic continent and, after a short breeding season, most of them disperse northwards (Knox 1994). As a result, the distribution of birds in the Southern Ocean shows a strong bias towards the sub-Antarctic islands. In general, birds feed on zooplankton (mainly krill) and larval fish; some species are tertiary feeders, also preying on squid and fish, and others, such as skuas and giant petrels, are predators on smaller seabirds and penguin chicks or on carcasses of larger birds and marine mammals. Like seabirds, several species of marine mammals in the Southern Ocean feed on krill and, as populations of previously hunted species of seals and cetaceans recover to pre-exploitation levels, the competition for krill increases. The number of crabeater seals (curiously named, given that their major prey is krill, and not crab) increased in the 1960s and 1970s, so that they became the most numerous pinniped species in the world; their number is probably decreasing at present. Baleen whales move into Antarctic waters in early summer and, after 3-4 months of intensive feeding on krill, they return to warm water to breed during the Antarctic winter. The minke whale is the most common cetacean in the Southern Ocean because it was exploited only at the end of commercial whaling; they were therefore not decimated like greater whales (e.g. humpback and blue whales were reduced to 3 and 5% respectively of their pre-exploitation numbers; Croxall 1992). Although whaling greatly perturbed the Southern Ocean ecosystem, it allowed food to distribute through the marine ecosystem to the benefit of competitors. Moreover, changes in the life cycle of animals in the krill system were probably determined by the greater availability of food. The growth rate and pregnancy rate of some species of baleen whales have increased, and the mean age at sexual maturity of crabeater seals and whales is decreasing (Knox 1994). This reveals the rather complex response of marine mammals to removal of competitors, and indicates that commercial fishing of krill and cephalopods may have unpredictable effects on krill-feeding organisms and top predators such as leopard seals and killer whales.

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