Maximum entropy reconstructions of krill distribution and estimates of krill density from acoustic surveys at South Georgia, 1996-2000

This paper presents Maximum Entropy (MaxEnt) reconstructions of krill distribution and estimates of mean krill density within two survey boxes (dimensions 80 km x 100 km) north of South Georgia. The reconstructions were generated from line-transect acoustic survey data gathered in the boxes during austral summers from 1996 to 2000. Krill densities had previously been determined at approximately 0.5 km intervals along each of the ten 80 km transects in each box, providing about 1600 density estimates per box. The MaxEnt technique uses an iterative Bayesian approach to infer the most probable krill density for each of the 32 000 0.5 x 0.5 km cells in each box, taking explicit account of the spatial relationship between densities in the observed data. Despite some very large interannual and regional differences in mean krill density, the MaxEnt approach works well, providing plausible maps of krill distribution. The maps reveal some consistent 'hot spots' of krill distribution, knowledge of which could aid the understanding of mechanisms influencing krill distribution, and hence krill/predator interactions. The MaxEnt technique also yields mean krill densities for each survey, for which the confidence limits are often narrower than those determined from conventional statistical analyses.