Supervisors info:
Μεγαλοφώνου Περσεφόνη (Καθηγήτρια Ιχθυολογίας, Τμήμα Βιολογίας, ΕΚΠΑ (Επιβλέπουσα)
Ζερβουδάκη Σουλτάνα (Ερευνήτρια Α΄, ΕΛ.ΚΕ.Θ.Ε.)
Ραΐτσος - Εξαρχόπουλος Διονύσιος (Επικ. Καθηγητής Θαλάσσιας Βιολογίας, Τμήμα Βιολογίας, ΕΚΠΑ
Summary:
This dissertation was carried out in the framework of the Interinstitutional Postgraduate Program: “Oceanography and Management of the Marine Environment” of the National and Kapodistrian University of Athens (NKUA) and the Hellenic Centre for Marine Research (HCMR). The aim of this study was to investigate the trophic and ecologic relationships between juveniles of two species of the genus Trachurus, namely T. mediterraneus (Mediterranean horse mackerel) and T. trachurus (Atlantic horse mackerel) with the Mediterranean jellyfish (Cotylorhiza tuberculata) in the wider area of the northern Euboean gulf. These species are economically important in world fisheries and stocks, so a better understanding of the ecological parameters that affect their behavior and the cross-species relationship that is created mainly in the early stages of their lives is of great importance.
In order to carry out this research, sampling was carried out in the Northern Gulf of Euboea, and specifically in the area of the settlements of Limni and Chronia on the island of Euboea, opposite the island and Gulf of Atalanti. Sampling was performed with a small boat, on August 2019. In the process, jellyfish together with their associated fish were collected on a boat using a hand net, with a narrow “eye” so that the fish could not escape when the jellyfish were taken out of the sea. The number of juvenile fish found in each jellyfish caught was then counted, as well as the size of the jellyfish bell itself. The juveniles were then stored in clear plastic containers and frozen at -20 °C in the lab until the laboratory analysis.
During the laboratory analysis, identification of the species, morphometric measurements (length, weight and lower jaw length) as well as stomach content analysis, were conducted. Correlations of morphometric characteristics were performed, while the Condition Factor, Vacuity Index and Fullness Index for each juvenile were calculated. The contents of the fish stomachs were analyzed and, aside from the organisms found, counted and divided into 3 major categories (Copepoda, Bivalvia, Gasteropoda), microplastics were also detected. The organisms not recognized due to them being partly or entirely digested, were categorized as “Unrecognizable”.
Regarding the results of the present study, the most widespread of the two species was T. trachurus with 115 individuals, out of a total of 171, while the remaining 56 belonged to the species T. mediterraneus, with the average value of their length being 19.46 mm, while separately for the two species T. trachurus and T. mediterraneus this value was 17.85 mm and 22.74 mm respectively. The average value of their weight was 0.142g, and separately for the two species T. trachurus and T. mediterraneus, this value was 0.106g and 0.215g respectively, and the average value of the lower jaw length was 2.64 mm for all juveniles, and separately for the two species T. trachurus and T. mediterraneus, this value was 2.49 mm and 2.95 mm respectively. The distribution of juveniles in terms of both their total length, weight and the lower jaw length was not normal, while the distribution of jellyfish size (bell diameter) was not normal either. The correlation of the fish weight with the fish total length followed the formula W = a * TLb, with a and b having values of a = 0,0000252 and b = 2,827773 in total for all of the samples. The total length with the lower jaw length were observed to have a linear correlation with each other, while there was no strong correlation between the number of juveniles caught with each jellyfish and the diameter of the jellyfish bell. The Condition Factor (K) of juveniles was also calculated and it was found that there was no correlation with either the length or the weight of the juveniles.
The weight of the stomachs was also measured, which presented no correlation with neither the fish weight nor the fish total length. The Vacuity (%VI) and Fullness (%Jr) indexes were calculated, with the former having a value for the total number of juveniles %VI = 38.02%. The Fullness Index also showed no correlation with the length or weight of the juveniles. In addition, analyses of the stomach content of juveniles were performed in terms of prey and it was found that these juveniles feed mainly on Copepods (Percentage of numerical abundance %Cn = 83.17%) and lastly, it should be mentioned that 8 microplastics were observed in the stomachs of 7 juveniles.
According to the results of this research, it was concluded that at least in the area of the Northern Gulf of Euboea, the association of juveniles of the genus Trachurus with the jellyfish C. tuberculata is mainly established to contribute to the commencing of the formation of the first schools of the specific genus, as a mean to help provide food to the fish (plankton), as well as shade, and finally to act as a form of protection from their predators.
