Spatial organization of pelagic fish: echogram structure, spatio-temporal condition, and biomass in Senegalese waters

Understanding the spatial distribution of fish schools is expected to enable future improvement in the reliability of acoustic abundance estimates as well as catch data. The analysis of echograms will provide detailed morphological descriptions of fish aggregations together with characteristics of their habitat. The data set studied here is a series of comparable echograms taken during monitoring surveys and showing important fluctuations in biomass. Echoes are coded into different consistent morphological types. First, we study how the occurrence of these types varies with diel cycle, sea-bottom depth, and fish density by computing a chi-square table. Second, characteristics of inter-school spatial structure are analysed by computing indicator variograms. Third, location of high-density values within the areas where schools are present is studied by computing indicator cross-variograms. A descriptive model of habitat occupation is proposed where some parameters are related to biomass level and others are not.