Modelled day-night biases in spatial structure of jack mackerel (trachurus murphyi) in Chile

The jack mackerel population has been assessed directly by fisheries acoustics without consider the day-night fish school behavior. In this paper the day-night effect on spatial structure of Jack mackerel is analyzed from fisheries acoustics surveys. The diel behavior of fish schools was assessed by variograms in order to characterize the spatial structure of fish density. In general, variograms showed differences in the day-night spatial structure. The day variogram showed a higher nugget effect than night. The percentage of variance explained by the sampling design (sill/nugget) was 62.64 in the day variogram and 34.25% at night variogram and 32.70% in the day-night variogram. An intense schooling behavior during the day is likely to be responsible for this increased small-scale variability. The night variogram were characterized by a smaller range of autocorrelation than day variogram. These features may be caused by differences in schooling behavior which exists in jack mackerel with respect to day and night. On the day, schools of Jack mackerel were formed in compact higher densities that were distributed in a more extensive aggregation (variogram range 20.31 n.mi.), but occupying minor area of spatial distribution (30831.11 n.mi.2). While at night, these schools were joined to form large aggregations in extensive higher densities layers (variogram range 12.84 n.mi.), but occupying more area of spatial distribution (46615.07 n.mi.2). These differences in the form of school aggregation have strong implications for the estimation of biomass in the interpolation process by kriging.