Objective
The overall aim of the project is to provide a scientific and technical basis for effective and reliable reproduction of the common dentex (Dentex dentex), a prime new species for intensive fish farming. The main objectives of the project are: a) to provide basic knowledge on the reproductive biology of this species, b) to investigate external factors likely to be used for manipulating the spawning period, c) to develop an appropriate and cost-effective methodology for synchronization of spawning and increasing yields and quality of gametes, and d) to support the development of a new candidate species for aquaculture.
Data have already been obtained on the timing of sex differentiation, sexuality pattern, and gametogenetic cyclicity.
Sex differentiation in D. Dentex, occurs between 5 and 12 months of age. Among the 323 individuals studied so far, no bisexual gonads (apart from one male with a few scattered previtellogenic ovocytes in its testes) nor any other suggestion of sexual inversion were found. The relative frequency of both sexes (sum of all studied cohorts) did not differ significant from 50%, although variations as a function of the age and the cohort of the fish were observed. Five stages of maturity have been determined for females (F1: beginning of ovogenesis, F2: previtellogenesis, F3: endogenous vitellogenesis, F4: exogenous vitellogenesis, F5: final maturation) and six for males (M1: beginning of spermatogenesis, M2: full spermatogenesis, M3: completed spermatogenesis, M4: full maturity, M5: post-spawning, M6: beginning of the second spermatogenesis). From the end of March to the end of May, about 100 % of the males, older than 1 year, are mature and give sperm (stage M4), whatever they are 21-23, 33-35 or 46-47 months old. Young males, 10-12 months old, develop testes, but these are small (GSI lower than 0.1 %) and do not produce sperm. The females, older than 1 year, are mature nearly simultaneously with the males. However, the percentage of mature females is a function of the age: 30-70 % at 21-23 months, 50-80 % at 34-35 months and 70-100 % at 46-47 months. The spawning period (Crete, 35?N) take place in spring (end of March until the end of May-start of June) at a temperature range of 15.4?C to 21.4?C.
Photoperiod appears to be the main environmental factor controlling spawning time. Fish exposed to a 12 month-long seasonal photoperiod cycle advanced by 3 months in relation to the natural cycle (Group A) and at an almost constant water temperature (18.5-20.5?C) spawned almost three months in advance than controls (Group B). Duration of the spawning period for Group A fish was 65-80 days (January 6 to April 3, 1997) and for controls 66-72 days (March 26 to June 5, 1997). No difference in the average number of released eggs, fecundity, proportion of viable/non viable eggs, and semen characteristics was found among the experimental groups. Concerning melatonin, results indicate a secretion profile closely resemble those in other fish species studied to date, with levels rise immediately at the onset of darkness and fall to basal at lights on.
Seasonal fluctuations and maturity stage-depended differences were found in almost all of the estimated serum components. Amongst 180 males, only three of them showed detectable serum vitellogenin levels. No vitellogenin was detected in undifferentiated fish and in females at stage F1. In females, vitellogenin was mainly detectable in stage F4 and F5. Mean levels increased from stage F3 (52.6 ± 22.7 µg/ml) to F5 (1124.2 ± 170.3 µg/ml) whatever the age. In males the maximal plasma concentrations of the androgens 11-ketotestosterone (11-KT) and testosterone (T) were on 3.61 ± 1.19 ng/ml and 4.04 ± 1.19 ng/ml respectively. In females maximal plasma concentrations of T and in 17?-oestradiol (E2) were 1.21 ± 0.28 ng/ml and 0.75 ± 0.16 ng/ml respectively. Concerning the seasonal cycle of T and E2 in females and T and 11-KT in males, levels were undetectable or near the detection limits of the assay from June to December with levels rising to a peak during March or April (spawning period). In females serum thyroid hormones (T3, T4), total proteins (TP), total lipids (TL), cholesterol (Chol) and triglycerides (Trig) displayed minimum values in immature (F2, F3) and maximum in mature fish (F4, F5). In males no significant differences were observed in T3 serum levels, among the maturity stages, while the rest of the determined components showed maximal values in stage M4 (Trig in M3). Glucose, in both sexes, did not differ significantly among maturity stages.
The energy content of the female gonads was positively correlated with the maturity stage; having a maximum value at stage F5. The energy density of female gonads varies between about 22 (F2) and 27 kJg-1 (dw) (F5). The energy density of the male gonads had no clear correlation with the maturity stage or season. The energy densities of liver and muscle varied greatly between the individuals. There seems to be slight decrease in the energy density prior to spawning period. Total energy (TE) contents of the liver increased with increasing gonadosomatic index until it decreased significantly some months before the spawning period. TE of the gonads follows the development of GSI. Lipid density (LD) of liver and gonad exceed that of muscle. Glycogen densities (GD) of muscle and gonads are extremely low if compared to the GD of liver. In liver the GD seems to follow the reproduction cycle of D. dentex. The decrease both in the energy density of muscle and liver and total energy content of the liver may indicate the transfer of energy from somatic tissue to female gonads during the period of fast gonadal development of female fish. Statistical analysis on the correlation between energy densities, total energy contents, lipid density and glycogen density is in progress. Analysis of the fatty acids content of gonads, liver and muscles are in progress. Preliminary data showed that the main polyunsaturated fatty acids (PUFA) of the samples analysed this far are docosahexaenoic (DHA, 22:6n-3) and eicosapentaenoic (EPA, 20:5n-3) acids. The main n-6 PUFA were linoleic (18:2n-6) and arachidonic (20:4n-6) acids in TG and PL fraction, respectively. Palmitic (16:0) and oleic (18:1n-9) acids are the main fatty acids of the saturated FA and monoenes, respectively.
Study on the identification of MIS is in progress. First results indicate that D. Dentex ovary has the potential for producing both 17,20-dihydroxy-4-pregnen-3-one and 17,20,21-trihydroxy-4-pregnen-3-one, the second being hardly detected. Steroid levels in water were comparatively higher than in plasmas. The trend for each steroid for a particular treatment group was similar whether it was the free, sulphated or glucuronidated steroid. In addition amounts of steroids within a group followed a similar trend, with levels of 5?-pregnane-3a,17,20?-triol (5? 3a)> 17,20?-dihydroxy-4-pregnen-3-one (17,20?-P) > 17,20?,21-trihydroxy-4-pregnen-3-one (17,20?, 21-P). In all groups regardless of treatment amounts of steroid followed the general trend glucuronidated > sulphated > free.
Discussion
The present study describes for the first time details on the reproductive biology of D. Dentex reared in intensive culture conditions. Results indicate that, although some (young?) specimens could have bisexual gonads and although the sex ratio could in some cohort differ significantly from 50 % due to some undetermined reasons, common dentex is a gonochoric fish. Both sexes attained full maturity at their second year of life (21-23 month-old), however the percentage of mature females is a function of the age. Spawning period take place in spring (end of March to beginning of June) and photoperiod manipulations can lead to out-of-season egg production. Spawning can be also induced by using GnRHa implants (in the form of EVAC pellets or microsperes) with results similar to those reported for another farmed species the common sea bream Pagrus pagrus (Fostier et al., 1996). First data on the identification of MIS showed that probable 17,20?-P or 17,20?,21-P is the MIS in D. Dentex and an intensive metabolism seems to occur, thus MIS production could be transient. It is expected that completion of all running experiments during the third year of the project will provide a basic understanding of the reproductive biology of the common dentex and will support the development of a new candidate species for aquaculture.
Acknowledments
We express our gratitude to Mrs. M. Kagara, Mrs. S. Keravec, Mrs. H. Kononen and Mr. S. Kukkonen for technical assistance; sincere thanks to Mr. C. Thomadakis and Mr. C. Aggelakis for valuable efforts in fish husbandry. We are indebted to the staff of Aquaculture Department of IMBC for their collaboration in carrying out this project.
References
Bauchot, M.L. and Hureau, J.C. 1986. In: Whitehead, P.J.P. M.L. Bauchot, J.C. Hureau, J. Nielsen, E. Tortonese, ed. Fishes of the North-eastern Atlantic, and the Mediterranean 2, UNESCO, Paris, 883-907.
Bibiloni, G., Cladera, A., Gomila, M.A. Massuti, S. and Zaratiegui, I., 1993. Contribution to the growing of Dentex dentex. In: Carillo M., L. Dahle, J. Morales, P. Sorgeloos, N. Svennevig, J. Wyban, eds. From Discovery to Commercialization. Abstracts of contributions presented at the International Conference World Aquaculture '93, Torremolinos, Spain. May 26-28. EAS Spec. Publ., 19, Oostende, Belgium.
D' Ancona, U., 1950. II differenziamento delle gonade e l' inversione sessvale degli sparidi. Arch. Ocean. Limnol. Fasc II-III, 97-163.
Efthimiou, S., Divanach, P. and Rosenthal, H., 1994. Growth, food conversion and agonistic behaviour in common dentex (Dentex dentex) juvenils fed on pelleted moist and dry diets. Aquat. Living Resour., 7, 267-275.
Fostier, A., Pavlidis, M., Kokokiris, L., Canario, A., Le Menn, F., Le Bail, P.Y Mourot, B., Breton, B., Divanach, P., and Kentouri, M. Sexual cycles and growth performance in the common sea-bream, Pagrus pagrus, in rearing conditions: Application to spawners breeding and spawning induction. Final Report. EC Contract NR AIR 2-CT93-1589.
Franicevic, V., 1991. Preliminary results on the larval rearing of common dentex, Dentex dentex (Linnaeus, 1758). In "Aquaculture and the Environment", Aquaculture Europe '91, Dublin (Eire), 10-12 June 1991. Specific Publications of European Aquaculture Society, 14, pp. 106-107.
Glamuzina, B., Dujakovic, J. and Katavic, I., 1989. Preliminary studies on reproduction and larval rearing of common dentex, Dentex dentex (Linnaeus 1758). Aquaculture, 77, 75-84.
Kentouri, M., Koumoundouros, G., Divanach, P. and Sterioti, A., 1992. Developpement embryonnaire du Pagre, Pagrus pagrus et du Dente, Dentex dentex en Crete. Rapp. Commun. Int. Mer Medit., 33, 384.
Morales-Nin, B. and Moranta, J., 1997. Life history and fishery of the common dentex (Dentex dentex) in Mallorca (Balearic Islands, western Mediterranean. Fisheries Research, 30, 67-76.
D. dentex is a highly valued table fish in the Mediterranean region and elsewhere in the tropics and the continuously increasing catches suggest a high and unsatisfied demand for this sparid fish. Rearing experiments with juvenils have identified D. Dentex as highly suitable for intensive aquaculture (Bibiloni et al., 1993; Efthimiou et al., 1994). However, studies on this species are sparse (Bauchot and Hureau, 1986; Franicevic, 1991; Kentouri et al., 1992; Morales-Nin and Moranta, 1997) and basic knowledge of the reproductive biology under intensive farmed conditions is extremely limited and conflicted. D' Ancona (1950) concluded that D. dentex is a gonochoristic sparid. However, Bauchot and Hureau (1986) and Glamuzina et al. (1989) stated that D. dentex was a protandric hermaphrodite, with sex reversal being unlikely to be obligatory. A recently field study (Morales-Nin and Moranta 1997) supports the initial information of D. Dentex as gonocoristic species.
The project comprises four major tasks: (1) morphological, histological, endocrine and metabolic study of female and male sexual cycles in fish aged 3 months to 47 months (which covers three spawning seasons; (2) investigation of the role of melatonin, in conveying important information about diurnal and annual changes in daylength and investigation of the role of photo- and thermo period on the control of reproduction for out-of-season egg production; (3) spawning induction treatment and identification of the steroids synthesized by the gonads, as well as identification and measurement of the maturation-inducing steroid(s) or their metabolites in the water as a candidate marker for monitoring spawning success; (4) investigation on the importance of the sex ratio and interaction within sexes on the efficiency of spawning. Tasks are supported by quantitative and qualitative analysis of released eggs and sperm.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- agricultural sciences agriculture, forestry, and fisheries fisheries
- natural sciences biological sciences biochemistry biomolecules proteins
- natural sciences biological sciences biochemistry biomolecules lipids
- natural sciences biological sciences zoology ichthyology
- natural sciences biological sciences reproductive biology
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