Μελέτη της οριζόντιας κατανομής των μικρών πελαγικών ειδών Sardina pilchardus W. και Engraulis encrasicolus L. και των παραγόντων που την επηρεάζουν στις ελληνικές θάλασσες

Abstract

Acoustic data from six research surveys (four in summer and two in winter) in Aegean and Ionian Seas (eastern Mediterranean Sea) were used to illuminate the horizontal distribution of anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus). Geostatistical techniques (omnidirectional and exhaustive variograms) were applied to acoustic data in order to analyse and visualize the spatial structure of pelagic fish in each sub-area. The effect of certain topographic characteristics (related to bottom depth and the degree of land enclosure), on fish spatial structure (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, whereas the nugget effect was negatively correlated to 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. Geometric descriptors of the exhaustive variograms provided a more efficient picture of the underlying spatial continuity, indicating the effect of the enclosure index (stronger in summer) and the area size (stronger in winter) on the fish spatial structure. The spatial structures were more heterogeneous in closed than in open sub-areas during both seasons. These results indicated that the environmental spatial heterogeneity was mainly reflected to the internal organization (i.e. schools aggregations) of the fish spatial structures rather than the maximum size of the area occupied. In general, heterogeneity is amplified when local topography increases the environmental patchiness. The closed sub-areas show high heterogeneity due to the coastline effects, namely the higher extension of coastal bathymetric ranges and terrestrial influence. The extended coastline increases localities of exogenous nutrient enrichment and enhances spatial variation in the wind mixing effect and water column stratification. In the range of values used in the present study, the spatial structures of anchovy and sardine where more heterogeneous in small-sized sub-areas than in large sub-areas during the winter period. This is explainable in terms of the increase in fish density in small gulfs, along with the peculiarities of each species’ biology and the availability of the suitable fish habitats. The opposite was observed during the summer when were more homogeneous spatial structures were predicted in small-sized sub-areas than in large sub-areas, for a given degree of enclosure. This is explainable in terms of the total available area of the favorable fish habitat. In large seas, favorable habitats, which are generally localized systems (e.g. fronts, upwelling, gyres), might be large in extend to carry most part of the fish biomass closely associated. In a next step, acoustic data were combined with hydrological and biotic parameters with the aim of understanding the relationships between the spatial distribution of anchovy and sardine and environmental regimes. For this purpose two non-parametric statistical analysis were performed: Cumulative Distribution Functions (CDFs) and Generalized Additive Models (GAMs). Based on CDFs, anchovy was significantly associated to more productive areas during the summer. During the winter period a difference in habitat selection was observed between Central Aegean and Ionian Sea, which was related to the difference in the length frequency distribution of the population. Central Aegean Sea was dominated by anchovy of bigger length classes, which were associated to deeper waters and higher temperatures. Whereas the anchovy of smaller length classes that dominated Ionian Sea were related to more shallow waters, with higher productivity, lower temperatures and salinities. GAMs analysis further underlined CDFs results and revealed a consistent trend of increasing anchovy biomass towards areas of increased productivity (i.e. higher values of zooplankton or chlorophyll), independently to sampling periods and season. The depth of the upper mixed layer mainly (practically expressing the thermocline depth) and the temperature were the factors mostly affecting anchovy’s biomass during the summer. The relationship between these abiotic parameters with the fish biomass was practically associated to food availability as well as to the characteristics of those areas with increased productivity. During the winter period the differences observed between sub-areas was due to the differences in anchovy’s length frequency distribution. Similarly, sardine in Central Aegean Sea during the summer was significantly associated to the more productive inshore waters, whereas the opposite was observed in Ionian Sea. The depth of the upper mixed layer and mainly the temperature were the abiotic factors mostly affecting sardine’s biomass during the summer. The lack of a common trend between sub-areas was related to the differences in sardine’s length frequency distribution among sub-areas. During the winter, which consists the reproductive period for sardine in the Greek Seas, it was observed a consistent preference towards areas of high productivity such as river runoffs. In conclusion, productivity mainly, as well as temperature, thermocline depth and bottom depth consist the main environmental factors that define anchovy and sardine’s habitat in the Greek Seas. These environmental factors along with the complex coastal topography of the Greek Seas affected the organization of the spatial structures of the small pelagic fish. Eventually, the large-scale heterogeneity of the environment is reflected to the small- or meso-scale heterogeneity of the fish populations.