Παράγοντες που επηρεάζουν τη μεταβλητότητα των σχολείων των μικρών πελαγικών ψαριών και διερεύνηση του ρόλου τους στο οικοσύστημα με οικ...

Abstract

This thesis studies small pelagic fish schools’ variability, using hydroacoustic data collected during eight research cruises in the North Aegean Sea. Data analysis revealed that discrimination of schools belonging to different species was feasible and was even more satisfactory when restricting the time scale and the number of species considered. Schools of juvenile anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) differed from adult schools for both species. Concerning sardine, juvenile schools were smaller, presenting discrete bathymetric distribution compared to adult ones. A threshold length (10.7 cm) was estimated at which “juvenile school” structure changed to “adult school”. In contrast, concerning anchovy, a progressive change in most school descriptors was observed, following the increase in individuals’ length. Differences in internal school structure of juveniles and adults are a result of adaptations in different metabolic needs and predation pressure of the different size-classes. Moreover, spatial and seasonal differences were observed in schooling characteristics of both species, which were attributed to the discrete spatial hydrographic and topographic characteristics, to biotic features and to the physiology and size-structure of different populations. Concerning the diel variability in anchovy school structure, clear diurnal patterns in their vertical distribution and internal structure were revealed, which were mostly related to light intensity and were affected by bottom depth. Finally, in the framework of an ecosystem approach, an Ecopath mass-balance ecosystem model was built to study the ecological role of small pelagic fish in the N. Aegean Sea ecosystem. For the construction of the model, data from upgraded biomass estimations of small pelagic fish, based on aforementioned findings were used. Based on model results, anchovy and sardine were responsible for a high proportion of the system production and consumption and served as links between lower and higher trophic levels. The N. Aegean Sea food-web presented similarities with other Mediterranean ecosystems concerning their structure and fishing pressure, while the observed differences were mostly related to differences in productivity among regions.