Distribution

The circumpolar distribution of krill seems to be lopsided; there are large populations in the Atlantic Sector of the Antarctic, and thinner bands around the rest of the continent (Fig. 8.16). Whether these areas of high krill concentration represent separate stocks is not at present known. However, this is an issue of crucial importance to the way the krill resource is to be managed (Laws, 1985b).

Krill have a strongly aggregated distribution. The average krill patch size

Fig. 8.15. Krill (Euphausia superba) (Photo : Dr U. Kils).

appears to measure < 100-150 m horizontally and 40-50 m in depth. In February 1981, U.S. scientists on board the R.V. Mehrille north of Elephant Island, encountered a "super swarm" which occupied a surface area of 450 km2 and had an estimated biomass of more than two million tonnes (Macaulay et al., 1984).

It is well established that krill migrate vertically, but only on rare occasions has a recognizable pattern been discernible (Marr, 1962; Mohr, 1976; Fisher and Mohr, 1978; Kubota, 1981; Hampton, 1985). One can generalize that by day krill tend to be deep, while after dark they tend to congregate near the surface.

The relationship between the distribution of krill and their physical/chemical environment is one of the vexing problems facing krill ecologists. The southern circumpolar seas are often considered to be relatively uniform in thermal regimes and radiant energy; they are, however, seasonally variable. The physical oceano-graphic characteristics of the Southern Ocean have well known vertical and horizontal variabilities, but these and other environmental parameters have not been fully correlated with krill distribution. Interseasonal and interannual variations in krill populations also add to the list of unknowns. For instance, krill investigators reported a substantial decrease in the krill populations in 1983/84 austral summer (Heywood et al., 1985), coincident with an increase in the surface sea temperature. Some investigators speak of the possible influence of El NiƱo Southern Oscillation events.

Fig. 8.16. Krill distribution in the Southern Ocean (after El-Sayed, 1977).

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