Acoustic data analysis and models of krill spatial distribution

Acoustic data obtained on the 4-5 January 1987 aboard the R/V Professor Siedlecki were used in three descriptors of krill spatial aggregation: power spectra for krill biomass fluctuations in space, semivariogram (spatial autocorrelation of krill biomass) and the frequency distribution of krill biomass estimate. The wavenumber spectrum resembles a white noise at scales of 2-20 km, although at scales smaller than 1 km the spectrum appears to lose its power significantly. The semivariance of biomass does not vary significantly over most distances between points except for the distances smaller than 1 km. The computed frequency distribution of krill biomass is bimodal and appears to be the mixture of two lognormal distributions. These two distributions may correspond to the between and within patch biomass. These results of data analysis suggest that krill patch size or rather a basic swarm size should be smaller that 200 m, finest resolution of our data analyzed, and the real spatial distribution of krill should be the manifestation of the balance between the dispersion of the basic swarm units and long-range density-dependent attraction of the units. Simple dynamical and kinematical models can interpret the observed result.