Introduction
Greater amberjack (Seriola dumerili) is a cosmopolitan species of great interest to the aquaculture sector due to its excellent flesh quality, worldwide market availability and high consumer acceptability (Nakada, 2000). Its rapid growth (i.e., short time to market size) and large size makes this species very suitable for product diversification and development of value added products. Even though farming in the Mediterranean started in the 90s with wild-caught juveniles, production is still low accounting ~2 t in 2012. Bottlenecks for the incorporation of greater amberjack in the EU aquaculture industry include lack of reliable reproduction and production of adequate numbers of juveniles while there is insufficient knowledge on the nutrition and the pathology of the species. In the frame of the FP7-funded DIVERSIFY project major topics related to greater amberjack aquaculture are being studied, aiming to develop appropriate rearing methods for all stages of the production process. During the first years of the project, advances were made in the reproduction and the nutrition while work has already started in the husbandry and the pathology of the species.
Results
In the area of Reproduction, significant progress has been made this first year of the project. A large number (approximately 140 individuals) of wild-caught sexually mature fish has been acquired, resulting in the establishment of six different broodstocks, two of which are being kept in cages. Excellent progress has been achieved towards the development of an optimized spawning induction protocol for captive greater amberjack, with both natural and hormone induced spawnings providing large numbers of eggs of good quality (>75% mean hatching). In addition, it was shown that it was possible to induce maturation and collect fertilized eggs of high quality from stocks maintained in sea cages. The sampling of wild and captive-reared specimens in different gametogenesis phases started in order to describe the reproductive cycle and detect possible dysfunctions occurring in captivity. Finally for the development of an optimized spawning induction protocol for F1 greater amberjack in the eastern Atlantic, induced spawns from groups formed with females and males F1 individuals were obtained, although eggs were not fertilized yet.
The objectives of the first year in Nutrition have been focusing mostly on the definition of the requirements for the most relevant nutrients during first feeding regimes. Greater amberjack enrichment products were improved. Five levels of the essential docosahexaenoic acid (DHA) were tested for Artemia enrichment. The lowest DHA content lead to poor survival and growth (total length and body weight), the higher (1-2% DHA) resulted in improved performance while excess levels of DHA reduced growth. Furthermore, results demonstrated that adequate levels of DHA in Artemia (1-2%) prevented bone malformations. Different sources and levels of LC-PUFA rich lipids were tested for rotifers enrichment. The results indicated that rotifer enrichment using marine lecithin was the best in terms of LC-PUFA levels compared to the lipid composition of wild fish eggs. Additionally, experimental broodstock diets have been formulated and will be tested in the next period.
A series of preliminary trials were performed in order to establish the appropriate methodologies for semi-intensive and intensive larval rearing. Based on these trials, experiments were performed to determine the most effective larval stocking density. Eggs, from a natural spawning, were stocked at densities of 25, 50 and 75 eggs l-1 in 2,000 l tanks, testing each density in triplicate. Although, severe cannibalism and size dispersion were observed from 10-15 dph, resulting in poor larval survival after 15 dph, the 50 eggs l-1 treatment resulted in significantly (P<0.05) larger larvae (5.471± 0.64 mm total length at 15 dph) than the other treatments.
Finally for the health issues of the species, attempts have been made to isolate pathogens from cultured greater amberjack. Greater amberjack bacteria isolated from skin ulcers have been identified as belonging to Vibrio of the harveyi clade and Staphylococcus epidermidis. Gill parasites were identified as the monogenean Zeuxapta seriolae, with cysts attributed to eggs of digenean parasites of the Paradeontacylix genus. A skin monogenean was also identified as Neobenedenia melleni that belongs to the Benedenia genus (Capsalidae). In addition, attempts to isolate the aetiological agent of Epitheliocystis in greater amberjack larvae have been made during larval rearing trials. An experimental rearing was performed and samples were taken. Following DNA extraction, samples were subjected to PCR using universal chlamydia primers but also specific primers for bacteria associated with previous outbreaks of Epitheliocystis disease, however no Epitheliocystis disease was recorded nor any microbial agent related to the disease was detected. Finally, primers have been designed to the key immune genes to be cloned from greater amberjack. Preliminary PCRs have been carried out using the samples collected and promising products have been obtained.
Discussion
DIVERSIFY is expected to advance the current knowledge beyond the state-of the art and have a significant impact on the greater amberjack aquaculture. The diverse and complementary nature of the work planned will allow a number of key basic questions of various fields such as reproduction, development, growth, nutrition, adaptation and immunity to be addressed.
The results will provide both basic knowledge on the biological processes involved in the rearing of the species and also applied knowledge on the development of species-specific husbandry practices.
Acknowledgments
This project has received funding from the European Union's Seventh Framework Programme for research, technological development and demonstration (KBBE-2013-07 single stage, GA 603121, DIVERSIFY).
References
Nakada, M. 2000. Yellowtail and related species culture. Stickney, R.R. (Ed.), Encyclopedia of Aquaculture. Wiley, London, pp. 1007-1036