Characteristic Properties and Dynamic Aspects of Krill Swarms from the Ross Sea

Abstract

In comparison with insect swarming, fish schooling and phytoplankton patches, studies on swarming behaviour of Antarctic krill, that take into account at least the ratio of random and non-random forces in terms of the density of organisms, do not exist. It is known that insects appear to disperse according to a reaction-diffusion model with a constant diffusion coefficient. The aim of this chapter is to verify if this mathematical approach, tested on insect swarming, can also be applied to krill swarms. Sixty-three swarms, registered without loss of information on the BioSonics hydroa-coustic system during the Italian expeditions to the Ross Sea in the years 1989–1990 and 1994 and identified as Euphausia superba from net hauls and visual inspection of their echograms, were used to verify this hypothesis. The selected swarms had a mean vertical extent around 32 m and differed in all other parameters. In particular, mean swarm density ranged from 34 to 4914 krill m⁻³. It was found that the mean vertical density distribution and the mean ratio of the diffusity and attractive forces of the krill swarms, whether considered by the density classes or all together, are consistent with insect model predictions. These results suggest that many dynamic aspects and physical characteristics of krill swarms can be similar to those of insect swarming. To proceed into krill swarming, however, other kinematic, biological and environmental data are required for a complete evaluation and an eventual improvement of the mathematical model.