MEPS 265:243-253 (2003)  -  doi:10.3354/meps265243

ABSTRACT: Acoustic data from 6 research surveys (4 in summer and 2 in winter) in the Aegean and Ionian Seas (eastern Mediterranean Sea) were analyzed to examine the effect of coastal topography on the spatial structure of populations of small pelagic fish using geostatistical techniques (omnidirectional and exhaustive variograms). The effect of topographic characteristics related to depth and the degree of land enclosure on the spatial structure of fish populations (as expressed by several parameters calculated from omnidirectional and exhaustive variograms) was studied by multiple regression analysis. The estimated autocorrelation range of omnidirectional variograms indicated that fish formed meso-scale to large-scale patches, but it was not significantly related to any of the area characteristics examined. The nugget effect was negatively correlated with mean bottom depth. Parameters related to anisotropy, calculated from the exhaustive variograms, did not show any consistent pattern related to the geographical characteristics of the area studied. Geometric descriptors of the exhaustive variograms provided a more detailed picture of the underlying spatial continuity, indicating the effect of the enclosure index (stronger in summer) and the size of the area (stronger in winter) on the spatial structure of fish populations. The organization of fish into clusters of schools was heterogeneous in closed sub-areas during both seasons and, for a given degree of enclosure, they were more homogeneous in small-sized sub-areas than in large sub-areas. These results suggest that the environmental spatial heterogeneity mainly affected the way schools were organized into aggregations rather than the maximum size of the area occupied by these aggregations.

KEY WORDS: Small pelagic fish · Spatial structure · Degree of enclosure · Exhaustive variograms