Immersion or injection?
Practical considerations of vaccination strategies

Vaccinating fish at on-growing cage sites in Greece is an important management decision.

Dr. PANOS VARVARIGOS explains the practical considerations when planning to vaccinate sea-bass or sea-bream at the on-growing sites.

|| Introduction || General principles || Immersion (dip) || Intraperitoneal injection || Costing immersion ||
Costing injection || Immersion vs. injection costs || Vaccination strategies || Constraints || Evaluation ||

Author: Dr. Panos Varvarigos
Freelance Veterinarian – Fish Pathologist, Athens, Greece.


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Vaccines are marketed in Greece to protect against serious bacterial systemic diseases of farmed sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus). They comprise formalin killed bacterins or oil adjuvanted products. Products that may be administered orally by mixing with the fish feed are also gradually appearing on the market. The available vaccines may offer protection against several pathogenic serotypes of Vibrio anguillarum causing vibriosis to sea bass, and/or Photobacterium damsela subsp. piscicida (Pasteurella piscicida) causing pasteurellosis (or pseudotuberculosis) to both sea bass and bream.

The different products that are marketed at present are either monovalent, that is, offering protection against one bacterial strain or multivalent, that is, protecting against more than one bacterial strains. The application methods and the vaccination schemes that are suggested by the vaccine distributors differ, but all employ either the immersion of the fish in groups in a suitable vaccinal dilution and/or the intraperitoneal injection of individual fish with a specific dose of undiluted vaccine. Oral vaccination is by far less common since oral vaccines have yet to realise consistent good results.

The practical needs in terms of implements, workforce and time, differ according to fish size, the type of holding unit (net pen, tank, raceway) and the application method employed. Injection vaccination on the cages at sea is considerably more time consuming and labour intensive, since the fish have to be completely anaesthetised and treated individually. On the other hand, immersion vaccination of the fish proceeds fast as groups of fish are added simultaneously to a vaccine dilution. The vaccination schemes depend on the epizootiology of the site and the duration of the production cycle compared to the expected period of immunity cover as well as on the availability of time and labour.



General principles for vaccination

Vaccination is recommended only to healthy fish, which are not under any form of stress. Vaccination is not indicated when fish are suffering a disease outbreak or have been through recent severe handling or other environmental stress.

The fish should be deprived of food prior to vaccination in order to have empty gastrointestinal tract at vaccination. The smaller the fish size is and the higher the water temperature the smaller the required fasting interval. Fasted fish suffer less handling stress and respond better to anaesthetics.

Vaccination must be performed in a disease free environment and precede exposure to disease or transfer to a disease prone site by 12 days when water temperatures range around 17oC (³ 200 degree-days). Smaller intervals are required for the establishment of immunity at higher water temperatures.

At dip vaccination, the temperature difference between vaccinal dilution and holding water should not exceed 2oC.



Immersion (dip) vaccination

Large groups of fish are cut off from the rest in a cage and enclosed in a tarpaulin where a slight dose of diluted anaesthetic is added to sedate them.

Air or oxygen is continuously pumped in to avoid anoxia.

The proper vaccine quantity is calculated according to the estimated biomass of the fish to be vaccinated (usually for every 100kg of fish one litre of vaccine is required). The vaccine is diluted with sea-water in a suitable receptacle (the common dilution rate is 1:10, that is, 9lt of water added for each 1lt of vaccine). Oxygen may advantageously be trickled through the vaccine dilution to reduce stress.

The sedated fish are netted out of the tarpaulin in lots of approximately 0.5kg, avoiding overcrowding or crushing. Holding water is drained from the fish, which are then placed in the vaccine dilution where they are allowed to swim for a certain minimum time, usually ³ 30 seconds.

Common practice is to place the netted fish from the tarpaulin in a perforated plastic bowl within the dilution container. When immersion time is up, the fish are withdrawn from the vaccine dilution and released in their holding facility.


Groups of caged fish are cut-off in a tarpaulin where oxygenation is provided and a light dilution of anaesthetic is added for sedation. The fish are netted out in small groups, drained for a few seconds from sea water and immersed in the dilution of vaccine. Usually a perforated plastic container is used to hold the fish in the vaccinal dilution.


The sedated fish remain in the perforated bowls for at least 30sec. Then the bowl is up-lifted from the vaccine dilution and is left to drain for a few seconds prior to transferring the vaccinated fish to their cage or raceway.


After drainage, the vaccinated fish are carefully released into the receptor raceway or cage, situated next to where the immersion takes place.



Intraperitoneal injection

When the sea bass are sufficiently large to be individually handled (>10g average weight) the vaccine may be safely administered undiluted by injecting them intraperitoneally using injection guns. Groups of fish are cut off from the rest in a cage and enclosed in a tarpaulin as for immersion, but because the fish are larger, a smaller number of them is enclosed. Extra care is also required when supplying the anaesthetic since the larger the fish the grater the risk of self-injury due to stress reactions.

Subsequent to sedation in the tarpaulin, small groups of fish are netted out and placed in a container with a higher dose of anaesthetic dilution until completely immobile.

The anaesthetised fish are then taken to a "vaccination table" where they are handled individually. The vaccination table consists of one or more troughs filled with water where the immobile fish are presented to the operators floating belly up.


A certain dose of vaccine (usually 0.1ml to 0.2ml) is injected in the abdominal area of each fish held with the ventral side up and the head away from the operator΄s body. The needle is inserted into the peritoneal cavity at a 45o angle. Needle lengths range from 3mm to 6mm depending to fish size at injection. Shorter needles for small fish require short bevel lengths and suitable gauge calibre. Thus, proper needle selection has to be seriously considered. Automatic injection guns accepting luer lock hypodermic needles are used for this purpose.

Subsequent to injection the fish are released into their holding unit where they recover from anaesthesia in a few minutes.

Usually, the vaccination table is constructed in a way that allows simultaneous easy grading of the injected fish into size groups. The fish are released according to size class into distinct channels. Water is pumped from the sea into the channels and flushes the fish along tubes leading them to different cages.

The table, pumps, tubing and all other implements are often installed on a floating platform.



Costing immersion (dip) vaccination

Appreciating the cost of immersion vaccination is relatively easy, since all necessary implements are cheap and either used for other tasks and hence, readily available, or can be quickly made in-house. For example, ordinary household plastic bowls can be purchased from super markets and perforated with an electric drill on farm.

The major cost items comprise consumables, such as vaccines, anaesthetics, fuel, oxygen, etc. Labour costs may not be included in the calculation when the vaccinators are drawn from the regular farm workforce. Fish losses due to handling stress or mishaps are often negligible when immersion vaccination is performed methodically and not in a haphazard manner. Anyway, the fish are still relatively small and of low value.



Costing injection vaccination

Apart from the consumable items, which coincide with those of the immersion administration (vaccines, anaesthetics, fuel, oxygen), injection vaccination requires considerable investment in infrastructure. A spacious, steady working platform (raft) is paramount. The vaccination table has to be ergonomically designed and made according to its expected use, that is, not only for injecting fish but also for grading and counting. A powerful enough water pump is important in order to supply plenty running water to the grading channels. Injection guns do not comprise a major cost element but need to be meticulously maintained after use in order to last. 5 to 6 “guns” plus at least one spare have to be available for each vaccination crew of 8 people.

On large-scale fish farming operations it has been tried to adjust the automatic injection machines that are used for salmon vaccination in Northern Europe. Success with sea bass has been limited however, because it is a scaly fish with less skin mucus and far more vulnerable than salmon to handling stress and injury. Hence, injection vaccination of sea bass remains a labour intensive operation and often, casual labour is employed. Labour constitutes a serious cost element when budgeting for vaccination costs.

In addition, fish losses due to handling stress and trauma should be accounted for. Not only can they reach 1% of injected fish or more, but also the fish are larger and hence of greater value.



Immersion vs. injection vaccination costs


Immersion (dip)

Intraperitoneal injection









Oxygen (if from cylinder)

Oxygen (if from cylinder)


Fuel (for air pump)

Fuel (for air & water pumps)


Ancillary items (tubes, bowls, handling nets, ropes, weights, air stones, valves)

Ancillary items (as for dip)



Hypodermic needles


Labour force

3 to 4 workers per cage

6 to 8 workers per cage



@ 100 kg of fish / 30 min

@ 3000 fish / 60 min


Labour (man-hours per 100,000 fish)

@ 20 (5hrs x 4 people) (fish at 5-10g)

@ 230 (3.000 fish/hr x 7 people) (fish ³ 25g)


Depreciation of durable items




Air pump (if not O2 cylinders)

Vaccination table


Oxygen & sea temperature meter

Air pump (if not O2 cylinders)



Oxygen/sea temperature meter



Water pump



Injection syringes “guns



Automatic counters (optional)



Floating platform


Expected fish losses due to handling stress, injury or operator errors

Negligible (0.05%) (small fish of low value)

0.2% up to 1% of fish (but large fish of considerable value)



Vaccination strategies for sea bass and sea bream

The vaccination strategies for sea bass and sea bream depend on the combination of the following factors:

There is an obvious correlation between the lasting result of vaccination and the size/age of fish at vaccination (development of anosopoietic tissues).

Vaccination strategies for sea bass against vibriosis

Sea bass frequently suffers vibriosis outbreaks at any stage during their grow-out period. On the other hand, pasteurellosis may become a problem for bass mainly during the first summer, or until the fish reach about 70g of body weight.

Immunity lasts longer the older/heavier the fish are. Immersion vaccination of small bass, 1.5g to 2g average weight, against vibriosis should provide effective cover for about six months, whereas when the fish are larger at vaccination, between 10g to 20g average weight immunity against vibriosis should last for a whole year.

Therefore, when the production cycles are short, that is, bass is grown to market size in 16 to 18 months, then two vaccinations by immersion would suffice to protect the stocks against vibriosis. That is, fry are immersion vaccinated at 1.5g to 2g and a repeat vaccination, also by immersion, is performed when the fish reach about 15g to 20g of average weight.

When the production cycle of bass until market size is considerably longer, or in the cases where the fish are to be marketed at much larger sizes than the usual 350g, injection vaccination is necessary.

It may be combined with counting and grading of the fish by size into distinct groups. In such cases, the vaccination plan consists of an immersion application when the fish are about 2g to 8g and a second application by injection when the fish obtain an average weight of between 60g and 100g. Since injection vaccination is inherently a precisely dosing technique applicable to larger fish with mature anosopoietic system, immunity is expected to last for more than a year post injection. Thus, this plan ensures immunity cover over longer production cycles, even up to 24 months. However, vaccination schemes involving larger fish have been compromised after the emergence of large, open-sea circular cages in modern fish farming holding hundreds of thousands of fish.

After the fish are transferred into these cages (often 120m circumference) vaccine administration becomes a practically difficult process. It is excessively hard for the operators and stressful to the fish to administer vaccines when the fish are held in such large confinements, unless “helper cages” are used to break down the population increasing the time and labour needed. Therefore, the fish have to be injected early and as soon as they reach a size suitable to be handled and injected individually.

Currently, many farms growing millions of fish in large cages inject their sea bass fry when only around 10-15g. Thus, vaccine administration is performed once during the production cycle. Due to labour scarcity, fish growers start injecting their fish as soon as they reach “handling size” and continue injecting for months utilising vaccination crews as the young fish come out of the smaller holding units and prior to their transfer into the large unmanageable cages. Timing is crucial for such long lasting work.

Vaccination strategies for sea bream against pasteurellosis

Sea bream is a far more resistant fish to environmental stress and suffers from bacterial diseases mainly when young. As it grows its resistance to bacterial infections strengthens thus, sea bream is perceived as a "safe" fish to grow.

Sea bream naturally resists vibriosis, but suffers from acute pasteurellosis with very high mortality frequently when very young (0.1g to 1.5g) and still in the hatchery. Pasteurellosis outbreaks in hatcheries usually decimate stocks. Therefore, measures to protect it from the disease have to be taken early. Later, when the bream exceed 4g of weight, they become capable of resisting acute infections but remain considerably vulnerable until about 8-12g of weight. Later during production, bream may also suffer from chronic pasteurellosis with moderate losses. Antibiotic treatments are effective against such relatively mild outbreaks.

In order to effectively protect bream, it would necessitate vaccination of fry against pasteurellosis in the hatcheries when very small (0.1g to 0.5g). Unfortunately, efforts to immuno-stimulate and vaccinate bream at such an early stage of its life have been unsuccessful. Trials have shown that such young bream are incapable of effectively utilising the vaccine antigens, thus, casting doubts on whether vaccination of bream would be beneficial in hatchery practice.

The merits of bream vaccination against pasteurellosis for the on-growers are debatable, unless bream could be effectively vaccinated by immersion in a monovalent vaccine dilution either prior to delivery or shortly after delivery at the on-growing sites when the fish are still between 1g and 2g. A six-month immunity cover (protection from mid-May through to mid-October) would suffice.



Practical constraints

The ideal for fish farmers would be to effectively protect their stocks against the major diseases throughout the production cycle, quickly and with the minimum of cost and effort. Reality, however, is far from this ideal. Vaccinating fish is not only hard work but a delicate operation too. Harried manoeuvring of cage nets, overcrowding of fish, and careless application of anaesthetics, insufficient air or oxygen supply in the tarpaulin, rough handling of fish at injection or when netting and draining from water are some of the many causes of stress and injury which may kill fish.

Time and labour are always scarce resources on any farm. Therefore, careful job planning is important especially on large farms. Very often there are conflicts with other crucial operations, such as harvesting or net changing, which divert the necessary labour and weather conditions may be unfavourable at times. Hence, very often, decisions to vaccinate remain wishful thinking.

Money is yet another scarce resource no matter how large or profitable a fish farm may be. Vaccination is like insurance. It requires paying for vaccines and equipment and working hard in advance in view to protecting the fish from future disease outbreaks which are likely to occur but not absolutely certain. So, it is tempting to redirect funds to other investments in the hope that the coming production year is going to be fortuitous.

It is not surprising therefore, why the Greek farmers who have adopted some form of vaccination strategy for their fish have done so subsequent to repeated, devastating disease occurrences.

Nevertheless, it is certain that there is going to be no profitable future for the intensive marine farming of bass and bream in Greece unless vaccinations are widely adopted and the administration methodologies and technologies continuously upgraded in accordance with the evolving farming environment.



Vaccination evaluation

The result of vaccination, be it a laboratory or field trial or a commercial application, is evaluated according to the "Relative Percent Survival" formula or RPS.

RPS = 1- (vaccinated fish mortality % / non-vaccinated mortality %) x 100 %

RPS expresses the percentage of fish, which would have died from the disease if not protected against it. That is, the proportion of fish saved due to vaccination. An economically acceptable, successful vaccination should exceed a RPS value of 70%, meaning that in case of a disease outbreak the unvaccinated fish against this particular disease would suffer at least a three-fold mortality loss than the vaccinates.



Author: Dr. Panos Varvarigos
Freelance Veterinarian – Fish Pathologist, Athens, Greece.


Reproduction of this website (or parts of it) is illegal and strictly forbidden.
No rights can be derived from this website.


Every effort has been made to ensure that the information is accurate until the date of last editing. It is based upon the accumulated personal experience of applied veterinary work. The author cannot take responsibility for incorrect interpretation or any resulting consequences. The contents may be used as an educational guide and are definitely not meant to become a stand-alone diagnostic tool or operations manual.


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