The word "krill" comes from the Norwegian meaning "young fish" but it is now used as the common term for the euphausiids, a family of pelagic marine crustaceans found throughout the oceans of the world. The term krill was probably first applied to the species of euphausiids found in stomachs of whales caught in the North Atlantic and, although there has been a recent trend to confine the term to apply to Antarctic krill (Euphausia superba), in this report we will use in its more generic form to apply to all species of euphausiids. A number of books and general articles have been written about the biology of euphausiids and the reader is referred to these for a detailed treatment of the subject (Mauchline 1980; Mauchline and Fisher 1969; Miller and Hampton 1989a).
There are 85 species of krill ranging in size from the smallest which are some millimetres long to the largest deep sea species which can reach 15cm in length (Baker et al. 1990). There are several features that distinguish the euphausiids from other crustaceans (Figure 1): the gills are exposed below the carapace unlike those of most other advanced crustaceans which are sheltered within it; there are luminous organs (photophores) at the base of the swimming legs, as well as pairs of photophores at the genital segment of the cephalothorax, near the mouthparts and in the eyestalks, which produce a blue light. The general body plan is, however, similar to many familiar crustaceans. The fused head and trunk - the cephalothorax - contains most of the internal organs - the digestive gland, stomach, heart, gonads and, externally, the sensory appendages - the two large eyes and two pairs of antennae. The limbs of the cephalothorax are modified into highly specialised feeding appendages; the nine mouthparts are modified for handling and grinding the food and the six to eight pairs of food-collecting limbs trap food particles from the water and move them to the mouth. The muscular abdomen has five pairs of swimming legs (pleopods) which move in a smooth paddling rhythm. Krill are heavier than water and stay afloat by swimming in bursts which are interspersed by short resting bouts (Kils 1979).
Some species of krill hold on to their eggs in a brood pouch until they have hatched but all species currently being commercially harvested spawn their eggs directly into the water where they develop independently. Krill pass through a planktonic phase when young but, as they grow, they become more able to move through their environment and to maintain themselves in particular areas. Most adult krill are referred to as micro-nekton which means that they are more independently mobile than the plankton which are drifting animals and plants at the mercy of the movements of the water. The term nekton embraces a wide diversity of animals from krill to whales.
Many species of krill are gregarious and are found for most of their lives in pelagic swarms or schools. It is this swarming habit that has made them attractive to commercial fisheries. The density of krill in swarms can be extremely high with biomasses of several 10s of kilogrammes and densities of over 1 million animals, per cubic metre of seawater (eg. Nicol et al. 1987). Swarms can cover large areas, particularly in the Antarctic where swarms of Antarctic krill have been measured covering an area of 450 km2 and have been estimated to contain over 2 million t of krill (Macauley et al. 1984). Most of the species currently harvested also form surface swarms and it is often this behaviour that has drawn attention to them as a harvestable resource.