Fish diseases

In aquaculture it is much better to concentrate on disease prevention rather than cure. Not only can significant cost reductions be achieved by excluding expensive treatments but also, very often, maximised fish growth and performance can be achieved by eliminating operations that stress the live stock. Stressed animals are much more sensitive to infection.

Heavy reliance on medicines as a strategy for the long term eradication of disease can, in fact, add problems to farm management. Increased costs, the development of resistance of disease organisms to treatment, the persistence of the medicinal agent in the environment -with unknown future consequences - are all matters that concern the farm manager. As legislation concerning acceptable Minimum Residue Levels (MRL) of specific compounds is imposed, the availability of medicines to the commercial animal production sector is becoming more limited. All actions that can help the farm maintain its activity while reducing medicine use are to be appreciated.

Fish stress leads to poor feed conversion, bad growth, and may ultimately increase susceptibility to diseases. Treatments and treatment regimes such as chemical baths can often add further stress to the problems they are supposed to alleviate. While fish may still demonstrate active feeding activity while stressed, the efficiency of conversion can drop dramatically.

A management plan should be developed to diminish stress and used to ensure that normal farm practices (grading, net changing, tank cleaning etc.) produce a situation for stock that is as stress free as possible.

There are viable methods of control which can be undertaken.

• Use only vaccinated fish.
• Source and purchase fry or juveniles from hatcheries with certified disease-free status.
• Keep year classes separate.
• Avoid farm practices that give potential for cross-infection and ensure that adequate disinfection is done when and where necessary e.g. from one farm to another; site to site/between year classes.

Other techniques such as ‘single bay management' and fallow periods are useful in helping the farmer to break disease cycles and reduce the abundance of some external parasites in the farming area.

Questions

Do you take samples of fish organs (check list below) on a regular basis for the assessment of disease status by a fish diseases laboratory?

• Liver
• Heart
• Spleen
• Gills
• Kidney
• Skin tissue

Do you examine your stock on a regular basis for signs of:

• Unusual behaviour (feeding, swimming etc.)?
• External lesions or marks?
• External or gill parasites?
• Internal gut parasites (flukes etc.)?

Would further training in any of these areas be an advantage?

Danger Point

Close monitoring of disease and mortality incidence, combined with food conversion ratios, can give indications that the farm management plan needs revising.

If the company has no long-term strategy for disease prevention and control within its overall financial/operational framework stock can be wiped out by repeated virulent attacks.

There are several viable preventive measures which can be undertaken by the company, notably in the establishment of regular and reliable measurement of disease incidence.

Training checklist

See checklist for training provision in fish disease and disease prevention.

Fish Diseases and Disease Prevention

• Stress and its role in fish stock health management
• Sampling methods and monitoring for fish diseases
• Disease diagnostic procedures
• Treatments and treatment regimes for disease (e.g. use of antibiotics, dosage and application)
• Sea lice or other external parasitic treatments
• Vaccination of fish
• Methods to prevent and reduce the spread of disease on your farm
• Marine hatchery practices in the prevention of disease