Biosecurity is often the most cost-effective disease insurance on a fish farm. Many serious outbreaks are not introduced via water or wild fish but through purchases of eggs, fry, juveniles, live feed, equipment or people moving between sites. Once an infection is established it can be very hard and expensive to eliminate. Modern aquaculture therefore aims to prevent pathogen introduction rather than treat after the fact.
Secure purchases of eggs, fry and fish
- Buy eggs, fry and fish only from reputable suppliers with documented health status.
- Always request health certificates, disease history and information on any ongoing disease programmes.
- Apparently cheap or unknown suppliers can become very expensive if they introduce disease.
- Even clinically healthy fish can carry bacteria, viruses or parasites without visible symptoms.
- Eggs are often safer than live fish, but eggs can still transmit some diseases if biosecurity fails.
- Egg disinfection with approved methods is used routinely in many hatcheries to reduce pathogen pressure.
- Transport water from external suppliers should never be mixed into production systems.
Quarantine
- Newly purchased animals should be held in separate quarantine before introduction into main production.
- Quarantine duration is adapted to species, age and regional rules.
- Use separate nets, hoses, boots and tools for the quarantine area.
- Staff should work from youngest and healthiest groups toward older or potentially infected groups — never the reverse.
- Observation of appetite, behaviour, mortality and water quality is especially important during quarantine.
Zoning and hygiene
- Divide the facility into clean and less clean zones.
- Restrict visitors and document who has been on site.
- Use footbaths, hand washing and protective clothing where appropriate.
- Avoid moving equipment between different departments.
- Disinfect nets, grading equipment, tanks and tools regularly.
- Remove organic material before disinfection — disinfectants work much worse on dirty surfaces.
Feed as a biosecurity factor
Many farmers focus on pathogens but underestimate the importance of feed. Feed directly affects the fish’s immune system, gut health and resistance to infection.
- Use feed from established manufacturers with a traceable raw-material chain.
- Store feed dry, cool and protected from pests.
- Old or poorly stored feed can lose nutritional value and oxidise.
- Overfeeding worsens water quality and creates better conditions for opportunistic bacteria.
- High feed quality is often one of the most effective ways to reduce disease risk.
UV treatment
UV is one of the most important biosecurity barriers in modern RAS and flow-through systems.
- UV inactivates bacteria, viruses, parasites and algal cells by damaging their DNA and RNA.
- UV adds no chemicals to the water.
- Effect depends on UV dose, transmittance, hydraulics and lamp maintenance.
- High turbidity or high TSS levels reduce the effect sharply.
- UV is often used on incoming water, after mechanical filtration or as a complementary barrier in RAS.
- Many hatcheries regard UV as one of the most important protections against pathogen introduction.
Ozone treatment
Ozone is one of the strongest oxidants used in aquaculture. Used correctly it can improve water quality substantially; used incorrectly it can damage fish very quickly.
- Ozone inactivates many bacteria, viruses, fungi and parasites.
- Oxidises organic matter and improves water clarity.
- Reduces colour, odour and some dissolved organic substances.
- Can improve UV transmittance by reducing organic fouling.
- Often used together with protein skimmers and contact reactors in marine RAS.
Ozone risks in seawater
Seawater requires special caution. Seawater contains high levels of bromide ions (Br⁻). When ozone reacts with bromide, oxidation products form, including hypobromous acid (HOBr) and bromate (BrO₃⁻). These brominated oxidants can be far more persistent than ozone itself.
- Fish can be damaged even when free ozone can no longer be measured.
- Residual oxidants can irritate gills and affect osmoregulation.
- Marine species are often sensitive to residual oxidants.
- De-ozonation and sufficient contact time are critical parts of system design.
- ORP should be monitored continuously but can never alone guarantee safe operation.
- Overdosing can cause acute gill damage, stress and mortality.
Advanced Oxidation Processes (AOP)
AOP often combines UV with oxidants such as ozone or hydrogen peroxide. The aim is to form highly reactive hydroxyl radicals (•OH) that can break down substances otherwise hard to remove.
- Can reduce some organic micropollutants.
- Can reduce colour and odour problems.
- Can improve water quality in systems with low water exchange.
- Used mainly in specialised applications and advanced RAS facilities.
- Requires careful process control and competent system design.
Vaccination
- Vaccination is used routinely in several parts of commercial fish farming.
- Especially common in salmon farming.
- Can substantially reduce antibiotic need and disease losses.
- Used where licensed vaccines and relevant disease indications exist.
The biosecurity chain
No single measure stops all diseases. Successful biosecurity is built from several independent barriers:
- Healthy broodstock.
- Certified eggs and fry.
- Quarantine.
- Good hygiene discipline.
- Zoning.
- UV treatment.
- Controlled ozone use.
- High feed quality.
- Daily monitoring.
- Rapid response to deviations.
Rule of thumb: Many of aquaculture’s worst disease outbreaks start with a single introduced fish, one egg lot or one piece of equipment. A consistent biosecurity culture is therefore as important as biofilters, oxygenation and water quality for producing healthy fish.
See also