Abstract
Geostatistical techniques were applied and a series of spatial indicators were calculated (occupation, aggregation, location, dispersion, spatial autocorrelation and overlap) to characterize the spatial distributions of European anchovy and sardine during summer. Two ecosystems were compared for this purpose, both located in the Mediterranean Sea: the Strait of Sicily (upwelling area) and the North Aegean Sea (continental shelf area, influenced by freshwater). Although the biomass of anchovy and sardine presented high interannual variability in both areas, the location of the centres of gravity and the main spatial patches of their populations were very similar between years. The size of the patches representing the dominant part of the abundance (80%) was mostly ecosystem- and species-specific. Occupation (area of presence) appears to be shaped by the extent of suitable habitats in each ecosystem whereas aggregation patterns (how the populations are distributed within the area of presence) were species-specific and related to levels of population biomass. In the upwelling area, both species showed consistently higher occupation values compared to the continental shelf area. Certain characteristics of the spatial distribution of sardine (e.g. spreading area, overlapping with anchovy) differed substantially between the two ecosystems. Principal component analysis of geostatistical and spatial indicators revealed that biomass was significantly related to a suite of, rather than single, spatial indicators. At the spatial scale of our study, strong correlations emerged between biomass and the first principal component axis with highly positive loadings for occupation, aggregation and patchiness, independently of species and ecosystem. Overlapping between anchovy and sardine increased with the increase of sardine biomass but decreased with the increase of anchovy. This contrasting pattern was attributed to the location of the respective major patches combined with the specific occupation patterns of the two species. The potential use of spatial indices as auxiliary stock monitoring indicators is discussed.
Highlights
Living organisms tend to aggregate into patches and seldom distribute randomly or uniformly in space [1]
This study focused on the aggregation patterns of anchovy and sardine populations in two different ecosystems: an upwelling area, the Strait of Sicily and a continental shelf area, the North Aegean Sea
Biomass was generally higher in the North Aegean Sea compared to the Strait of Sicily, and in most cases, higher for anchovy than sardine
Summary
Living organisms tend to aggregate into patches and seldom distribute randomly or uniformly in space [1]. Small pelagic fish present aggregative behaviour at various scales, ranging from the individuals within schools to the spatial pattern of school patches or school clusters Aggregation patterns may be controlled by environmental conditions, the distribution of food resources, the presence of conspecifics and competing species as well as density dependent effects [6], [7], [8]. Understanding those mechanisms that determine fish spatial distribution is not trivial and can often be quite complex
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