Influence of the rearing conditions on growth and food consumption of sea bream (Sparus aurata) during the early developmental stages

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Επίδραση των συνθηκών εκτροφής της τσιπούρας (Sparus aurata) κατά τα πρώτα αναπτυξιακά στάδια : μαθηματικές προσομοιώσεις
Influence of the rearing conditions on growth and food consumption of sea bream (Sparus aurata) during the early developmental stages

Παπανδρουλάκης, Νικόλαος

Κεντούρη, Μαρουδιώ

Η υδατοκαλλιέργεια , μία δραστηριότητα αιώνων , παρουσιάζει μια συνεχώς αυξανόμενη συμμετοχή στην παγκόσμια παραγωγή αλιευμάτων τα τελευταία χρόνια . Το 1997 το 25% περίπου της παγκόσμιας παραγωγής ψαριών προήλθε από την υδατοκαλλιέργεια ( Γαβρινιώτης , 1997). Σύμφωνα με τις διαγραφόμενες τάσεις κατανάλωσης σε παγκόσμιο επίπεδο , η παραγωγή της υδατοκαλλιέργειας αναμένεται να τριπλασιαστεί τα επόμενα 20 χρόνια προκειμένου να καλύψει τις συνεχώς αυξανόμενες απαιτήσεις της αγοράς . Στην Ελλάδα , η αλματώδης ανάπτυξη της καλλιέργειας θαλασσινών ψαριών τα τελευταία 10 περίπου χρόνια οδήγησε το συγκεκριμένο κλάδο της εγχώριας βιομηχανίας τροφίμων στις πρώτες θέσεις σε Ευρωπαϊκό επίπεδο . Όπως σε κάθε δραστηριότητα του πρωτογενούς τομέα παραγωγής , η ανταγωνιστικότητα , η αξιοπιστία και η αποδοτικότητα του κλάδου αυτού εξαρτώνται από την τεχνολογία που εφαρμόζεται και τη διαθέσιμη τεχνογνωσία ώστε να ικανοποιούνται οι βιολογικές απαιτήσεις των καλλιεργούμενων ειδών από τις συνθήκες εκτροφής . Σήμερα η παραγωγή της μεσογειακής ιχθυοκαλλιέργειας ανέρχεται σε 70.000 περίπου τόνους , και κυριαρχείται από δύο είδη , την τσιπούρα και το λαβράκι που αντιπροσωπεύουν το 90% της παραγωγής . Μετά από μία φάση ευφορίας και υψηλής κερδοφορίας στις αρχές της δεκαετίας του 90, η Μεσογειακή ιχθυοκαλλιέργεια πέρασε σε μία δύσκολη περίοδο καθώς η αύξηση της παραγωγής δεν ακολουθήθηκε από αντίστοιχη αύξηση της κατανάλωσης (Stefanis and Divanach, 1993- Divanach, et al ., 1998). H προσπάθεια που καταβάλλεται σήμερα από τον κλάδο ιχθυοκαλλιεργειών αφορά τόσο στη μείωση του κόστους παραγωγής με την ανάπτυξη νέων τεχνολογιών , όσο και στη διαφοροποίηση του προσφερόμενου προϊόντος με την εισαγωγή νέων ειδών για εκτροφή (Kentouri, et al ., 1995- Papandroulakis, et al ., 1997) με στόχο την διατήρηση της ανταγωνιστικότητας και αποδοτικότητας του κλάδου . Παρά τη συνεχή αύξηση της παραγωγής τσιπούρας και λαβρακιού σε βιομηχανική κλίμακα , η μελέτη των ειδών αυτών συνεχίζει να παρουσιάζει ενδιαφέρον καθώς παραμένουν άγνωστα πολλά στοιχεία της βιολογίας των ειδών και η εκτροφή τους δεν είναι πλήρως ελεγχόμενη . Επιπλέον , η μελέτη της τσιπούρας οδηγεί σε αποτελέσματα που μπορούν να χρησιμοποιηθούν για την εισαγωγή στην ιχθυοκαλλιέργεια νέων ειδών της ίδιας οικογένειας όπως το φαγκρί και η συναγρίδα . (EL)
Food consumption and assimilation efficiency of sea bream (Sparus aurata) larvae, during the early developmental stages, was studied using both an experimental and a theoretical approach. The first approach was based on the experimental estimation of the food consumption rate of the larvae, and its changes during the first 20 days after hatching. The factors related to changes in food availability affecting consumption rate such as, the duration of the daily photophase, the presence of phytoplankton during rearing and food frequency distribution were studied. Four daily photophases (8, 12, 18 and 24 hours) were tested in larval rearing experiments, under intensive conditions, which were performed in either the presence (pseudo-green water technology) or absence (clear water technology) of phytoplankton. Frequency of food distribution was tested on populations reared with the clear water technology under continuous illumination, for which food distribution was elongated to 14 hours daily (instead of 10 hours). In addition, the influence of the above factors on growth and survival of the reared populations were studied. The methodology for estimating the daily food consumption of the population was based on frequent monitoring of the changes of the concentration of zooplankton in the rearing tanks. Individual consumption was determined after estimating the daily population of fish larvae. Consumption rate was expressed in carbon units using measurements of the carbon equivalent of the plankton organisms as well as of the fish larvae. Also, formulas were developed relating larvae total length to wet weight and body carbon. The results show that both daily food consumption and consumption rate depend on rearing conditions and the duration of photophase. For individuals reared in the presence of phytoplankton, daily consumption increases with the duration of photophase while, when rearing without phytoplankton, results are reversed. Daily consumption of larvae reared with the clear water technology is from 3 to 10 times higher than the consumption level for individuals reared with the pseudo-green water technology. Food consumption rate is about 0,4 d-1, mg of carbon per mg body carbon, during the first period of exogenous feeding (4th to 8th days post-hatching) independently of the rearing conditions. As larvae grow, this rate increases, reaching a plateau the value of which varies according to the rearing conditions. For larvae reared in the presence of phytoplankton or with more frequent distribution of food, consumption rate stabilizes at about 0,7 d-1, independently of the duration of photophase. In the absence of phytoplankton, the food consumption rate is higher and inversely proportional to the duration of photophase (from 1,0 to 5,0 d-1, for individuals reared with daily light regime of 24 and 8 hours respectively). The result is related to the efficiency of larvae to assimilate food, which in turn depends on the rearing conditions. Food assimilation efficiency varies during development. It is reduced during periods where individuals adapt either to exogenous diet or to new diet (transition from rotifer to Artemia). Maximum assimilation efficiency, about 17% as mean value during the whole experimental period, was observed for individuals reared using the pseudo-green water technology and photophase of 18 or 24 hours. Similar results were observed for individuals reared with more frequent distribution of food. Larvae reared in the presence of phytoplankton but with 8 or 12 hours photophase exhibited lower assimilation efficiency (8.5 and 10% respectively). However, individuals reared using the clear water technology presented lower food assimilation efficiency with values from 2 to 8%, for short and long photophase respectively. Observed differences are due to variations in zooplankton quality and in rearing medium, factors that are affected in the presence phytoplankton. The presence of phytoplankton in the rearing medium resulted in stability of microflora in the tanks. Microflora in tanks is directly related to the one in larvae gut that has proved to influence larvae efficiency to assimilate food. Direct action of phytoplankton on digestion has been also reported during the rearing of other species. Additionally, phytoplankton presence during rearing resulted in the preservation of rotifers both in terms of population size and also in terms of their energetic value. In rearing with the clear water technology, the carbon content of the rotifers presented a reduction of 30% after 24 hours. The role of phytoplankton in the preservation, both quantitative and qualitative, of zooplankton was further supported by the results obtained during the experimental rearing with the extended food delivery. Concerning growth and survival of larval populations, the results of the rearing with the pseudo-green technology were better compared to those obtained with the clear water technology. The relative difference in growth after 20 days of rearing reached 25% regarding larvae total length, while for wet weight the relative difference reached up to 100%. Survival of the populations in presence of phytoplankton was 2 to 10 times higher than in its absence. In addition to the experimental approach a theoretical simulation of the early developmental stages of sea bream under rearing conditions was done in two ways. The first one utilizes differential equations to develop mechanistic models at the population and individual level in order to study growth and food consumption of sea bream larvae as well as the interactions between plankton and larvae populations. The second simulation uses principles of the fuzzy set theory in order to develop a model of the changes of feeding requirements of a reared population. At the population level, the study was based on the interactions between plankton and larvae populations during rearing. Models, which include simple predator - prey interactions presented limited capacity to describe the development of the system. Comparison between experimental results and the solutions of model equations showed similar evolution for populations reared with phytoplankton presence. However, the model could not describe the development of populations reared in the absence of phytoplankton. After revising the interactions between populations, and in particular the ones between phytoplankton and fish larvae, the solutions of the new system of equations described accurately growth and consumption of reared sea bream larvae. The revisions concerned the influence of phytoplankton on mortality rate and on food assimilation efficiency of the larvae. The predictions of the revised model presented a relative difference of about 10% to experimental values for the biomass change of population reared with photophase 18 and 24 hours, independently of the rearing technology. For short photophase (8 and 12 hours), relative differences were higher although the model predicts the trend of biomass change. Concerning food consumption, the model predicted, with a relative difference of about 6%, the experimental values of the total food consumption during the rearing period for populations reared with the pseudo-green water technology and long photophase (18 and 24 hours). For short photophase, the model predicted the general trend of food consumption change but the relative difference with the experimental values of the total consumption was high. For rearing with the clear water technology (and photophase 12, 18 and 24 hours), predicted total consumption presented relative difference of 45% compared to the experimental values. The model however did not satisfactorily predict the trend of the changes in consumption. In contrast, for rearing with 8 hours photophase the trend of change in consumption was well predicted by the model but not the actual total consumption. The population bioenergetic model developed predicts the trend of growth and consumption evolution of sea bream larvae under different rearing conditions. Approximations used, although not precise, are correct and represent an attempt to understand the mechanisms involved in the evolution of the system. For the study of the bioenergetic changes in individual base, a Dynamic Energy Budget Model, which describes the biological mechanisms involved in food consumption and the utilization of the assimilated energy was developed. The model describes energy flow according to the following scheme. Consumed food is partly assimilated, and then the individual, after covering its maintenance cost, utilizes the rest. Available energy is used for growth / reproduction while a fixed part is stored and utilized when necessary. The individual utilizes a fixed fraction of the available energy for growth only when stored energy recovers its previous maximum value. Two developmental stages were determined in order to describe a sea bream larvae: i. the autotrophic stage (when individual neither feeds nor reproduces but utilizes endogenous resources) and, ii. the juvenile stage (when individual feeds but does not reproduce). Food consumption and digestion were described based on physiological and behavioral characteristics of the individuals. The model includes the possibility of simultaneous consumption of two types of preys by the individual. The model in its final form includes 3 variables and 24 parameters, the values of which were determined from bibliographical references and the experimental data from this study. Although there is a relatively large number of parameters, the solutions of the model are sensitive only to changes of those determining the food assimilation efficiency and the rate of energy utilization for maintenance. The model predicts with high accuracy (the relative difference to the experimental values are between 1 to 7%) the growth, in terms of total length, of an individual reared with the pseudo-green water technology independently of the duration of photophase. Concerning total consumption during the rearing, the model predicts about 40% relative difference to the experimental values. Relative difference decreases to 15-20% for the period after the 10th day post hatching, showing the inaccuracy of the model during the first period of exogenous feeding. Despite the differences, the model exhibits interesting characteristics related to the predicted growth for relatively long periods. The model can also serve as a base for further study. The lack of exact knowledge of all biological mechanisms did not allow the precise mathematical description of larval growth and energy requirements. A different approach, however, allows the utilization of the results obtained during the present study in production level. Using principles of the fuzzy set theory, a methodology was developed in order to control the evolution of the rearing and in particular the feeding requirements of the reared population. The objective of the development was the integration of a controller in already existing feeding-systems, which will adapt the quantities of food delivered daily according to the needs of the reared population. The control process was based on the identification of the current situation of the population and the implementation of such actions that would allow the optimization of its development. Analysis was performed through a "knowledge base" that included rules in the "if-then" form. The rule base was formed based on the results of the present study as well as on existing experience of the development of the rearing system. Theoretical inquiry into the controller's action showed that it simulates with high accuracy the changes of the larval feeding requirements. Application of the controller allows the partial automation of the rearing during larval stages. (EN)

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