Greece holds the second highest global sea bass production total of 48,000 tons, in comparison with the Turkish highest production amount of 51,600 tons, in 2013.
In Greece, as in other Mediterranean countries, mariculture is a dynamic aquaculture sector with an also intensive development of hatchery tanks and cage culture in Spain, Italy and France.
Hatchery production figures establish Greece as the most significant provider with a production total of 192,000 thousand sea bass juveniles in 2013, which are not only used nationally, but also exported to other Mediterranean countries.
European sea bass was the target fish species of this study, looking at disease outbreaks through an epizootiological survey with emphasis on the causal pathogen agents and their geographical distribution, for a long period from 1998-2013.
Similar epizootiological surveys have been performed in other Mediterranean or European countries, especially in France and Spain. This study produced clear results for the most important risk factors in important aquaculture locations in Greece.
The most important mariculture locations of this survey along the Greek coastline consist of four Mediterranean fish culture areas, with approval from Greek Government Authorities, as follows:
- Thesprotia Sagiada and Mitikas gulf – Ionian Island coasts (Ionian Sea);
- West – Central – East Amvrakikos gulf (Ionian Sea);
- Maliacosgulf –– Biotia – Evoia (North Evoic gulf) (Central Aegean Sea);
- Argolicos gulf – (South Aegean Sea).
A database of 152 cases of severe bacterial diseases in sea bass was formulated. 134 of these cases concerned vibriosis and photobacteriosis outbreaks, caused by pathogenic bacteria.
The other 18 cases concerned motile aeromonas septicemia and tenacibaculosis (formerly marine flexibacteriosis) caused by Aeromonas hydrophila, A. sobria, Aeromonas spp. and Tenacibaculum maritimum (formerly Flexibacter maritimus).
The 152 cases were classified into groups according to:
- temperature as either low (11-19°C) and high (20- 28°C) values;
- seasonality as winter (December – February), spring (March – May), summer (June - August) and autumn (September – November);
- the above four areas of Greek coastline;
- average body weight, separated into classes of 0.1-5 g (immature innate non-specific immunity and almost absent specific acquired immunity class of larvae and juveniles), 6-80 g (mature innate and immature acquired immunity class of elder juveniles), 81-150 g (mature innate non-specific and mature acquired specific immunity class of young fish), 151-400 g (mature innate non-specific and mature acquired specific immunity of adult fish);
- case-year from the period (1998-2013).
A Principal Component Analysis of how these different factors affected disease outbreaks pointed out three principal components with the following ranking order:
- temperature and seasonality,
- group area and average body weight, and
- case year.
In a following statistical Logistic Procedure, the two first principal components were considered. The results of the logistic analysis showed the temperature - seasonality as the most important factor affecting disease outbreaks, and the only statistically significant epizootiological risk factor (p<0.05).
A statistical Contrast Test with temperature as the only variable (low compared with high temperature values), also showed the significant effect of this risk factor (p<0.05). Statistical analysis was performed with SAS.
In this survey, the sea bass classical vibriosis outbreaks from Listonella anguillarum appeared in a wide temperature value range (12-260°C). However, the acute classical vibriosis affected larvae and juveniles mainly at high temperatures, while the chronic or asymptomatic form appeared in young and adult fish at low temperatures.
The chronic form skin ulcerative and visceral or intestinal of classical vibriosis has been reported only for adult fish at low temperatures in Greek coastline areas.
Larsen reported that L. anguillarum can survive, not only in winter but also in summer, reaching a maximum population number. It has been suggested that vibriosis appears when the water temperature exceeds 10-120°C and occurs more often in areas where overcrowding exists and environmental equilibrium is fragile.
Outbreaks from Vibrio harveyi were recorded for the acute vibriosis form in juveniles at 190°C in May with high losses and for the sub-acute intestinal and visceral vibriosis in adult fish, at 220°C in August.
Incidences from V. harveyi have also been reported for sea bass, at similar temperature values, in North Ionian Sea (South Italy) [12]. Contrary to the above, vibriosis from Vibrio alginolyticus and V. splendidus sero-type II outbreaks were recorded at low temperatures 15-170°C (November-February) in both juveniles and adults, causing higher mortalities in the former.
Similarly, cases from V. alginolyticus have been reported for sea bass at low temperature values in Adriatic Sea (North Italy) and North Ionian Sea (South Italy). The photobacteriosis outbreaks from Photobacterium damselae subspecies pisci-cida (former Pasteurella piscicida) emerged at temperature values 19-24.5°C (April–September).
At this temperature range, cases from sea bass juveniles and young fish with acute septicemia epizootics in Amvrakikos and Argolikos gulf have been reported. In addition, chronic visceral or asymptomatic photobacteriosis cases have been referred to adult fish in Mediterranean coasts of Turkey, Egypt, Italy and Spain.
Other researchers have identified photobacteriosis epizootics mainly in summer, after heavy rain falls in brackish water coast areas, with high losses.
The motile Photobacterium damselae subspecies damselae, in our survey, was found in sea bass at a temperature value range 19-24.5 °C (May – August).
Specifically, chronic or asymptomatic photobacteriosis cases appeared in adults at 19°C, while acute septicemic photobacteriosis outbreaks with high losses in juvenile and young fish at 21-24.50°C, particularly under stress conditions caused by cages transportation.
This newer motile subspecies has been detected in Italy and Spain coasts from sea bass at high temperature values.
All the above data for the principal epizootiological risk factor, temperature and seasonality, effect on vibriosis and photobacteriosis, for European sea bass, may be utilized to an innovative marine biosecurity programme.
It can be applied especially for the stenothermal vibria like V. harveyi, V. alginolyticus and V. splendidus serotype ΙΙ.
For the eurythermal vibrio L. anguillarum a preventive medicine procedure has recently been applied in Greece for sea bass, including administration of probiotics and immunostimulants to larvae (0.1 g), bath vaccinations of L. anguillarum serotypes O1 and O2 to 2/5 g and intraperitoneal vaccination to 25g juveniles.
A bivalent bacterin vaccine for the protection from both classical vibriosis and photobacteriosis has also been applied, particularly in areas with higher water temperatures.
A further research is needed for vaccination procedures concerning vibriosis from the rest vibria. In these vaccination programs, the role of temperature seasonality should be taken into consideration.
In addition, the rearing temperature adjustment at adverse levels for each stenothermal Vibrio sp. can be applied where this is feasible, such as in recirculating aquaculture systems.
Specifically, the up-to-date data point out optimum temperatures low (e.g. <190°C) for V. harveyi and high (e.g. >170°C) for V. alginolyticus and V. splendidus serotype ΙΙ. It seems that rearing of sea bass at a temperature about 18°C prevents main health problems related to Vibrio pathogens.
Therefore, under such preventive veterinary medicine biosecurity conditions, we can reinforce the sustainability and profitability through continual epizootiological survey and strict surveillance.
August 2015
