Supervisors info:
1. Περσεφόνη Μεγαλοφώνου, Καθηγήτρια, Τομέας Ζωολογίας - Θαλάσσιας Βιολογίας, Τμήμα Βιολογίας, Σχολή Θετικών Επιστημών, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
2. Εμμανουήλ Δασενάκης, Ομότιμος Καθηγητής, Τομέας Ανόργανης Χημείας, Ανόργανης Χημικής Τεχνολογίας και Περιβαλλοντικής Χημείας (3ος Τομέας), Εργαστήριο Χημείας Περιβάλλοντος Τμήμα Χημείας , Σχολή Θετικών Επιστημών, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών
3. Ευγενία Λευκαδίτου, Ερευνήτρια Β’, Ινστιτούτο Θαλάσσιων Βιολογικών Πόρων & Εσωτερικών Υδάτων, Ελληνικό Κέντρο Θαλασσίων Ερευνών (ΕΛΚΕΘΕ), Αθήνα
Summary:
Knowledge of the diet of marine top predators, such as many of the chondrichthyans, is extremely important for understanding the food web and the functioning of marine ecosystems, so that the ecosystem approach to the management of marine fisheries resources can be applied in the future. In particular, the dietary changes of abundant chondrichthyans, such as the monkfish Raja clavata, largely reflect the temporal changes in the state of the marine biocommunities that constitute their critical habitats, and therefore can provide useful information on the effects of environmental change as well as of grinding pressure. Due to the biological characteristics of the species that make it vulnerable, as well as the increasingly widespread consumption of the species in Greece, the timely control of the concentration of heavy metals in its tissues is required, to prevent any effects on the populations of the crucian carp as well as consumers , which until now has hardly concerned the scientific community. In the context of this diploma thesis, the food ecology of the flatfish Raja clavata in Greek waters was studied for the first time, as well as the concentration of mercury in muscle, liver and gill tissues. The turbot samples used were caught by commercial bottom trawlers in the North Evoiko and Saronic Gulfs in the winter and spring of 2019. The diet study was based on 132 individuals (70 females and 62 males) with a total length of 37.9 to 88 cm (TL), of which 124 (93.3%) contained food. Both the qualitative and quantitative composition of the stomach contents were examined in relation to the sex, size and reproductive maturity of the falcon individuals, while any variations with the season/region of sampling were investigated. To determine the functional group to which the calcaneal belongs, the fractional level (TROPH) was calculated, per sex, for the size classes of immature and mature individuals. Thornback rays in the study areas feed mainly on Crustaceans, Osteichthyes and Cephalopods, while they occasionally consume Gastropods and Polychaetes. Crustaceans constituted the majority of the prey for both sexes (IRI%: 49.04 for males and 53.46 for females) followed by Osteichthyes, which presented a relatively higher percentage in the diet of male individuals. Crustaceans made up most of the prey in the youngest individuals, while Osteichthyes and Cephalopods increased in importance with the development of the thornback ray. Prey diversity increased with size. Larger prey species were found more frequently in the diet of larger individuals. Diet composition showed minor seasonal variations. The results of the quantitative analyzes indicate that Raja clavata is a generalized carnivorous predator, with a trophic level ranging between 3.65-3.99 and increasing with the size of individuals-, as has been observed in other Mediterranean and B .A Atlantic-. The determination of mercury concentration in muscle, liver and gill tissues was carried out in 95 subjects (52 males, 43 females). All the collected data were subjected to statistical processing with the SPSS program and the main statistically significant correlations were identified in relation to the mercury concentration in the tissues. In general, high levels of mercury were observed in the study species with higher concentrations of mercury in muscle tissues being more elevated in larger and more mature individuals. In Chondrichthyes, the accumulation of Hg in muscle increases more due to some special characteristics of it such as high trophic level, longevity and slow growth.
Keywords:
thornback ray, Raja clavata, food ecology, troph, mercury, bioaccumulation, heavy metals
