General Advice

Health and Diseases

Pathogens or disease-causing organisms are rarely absent from marine and fresh-water aquaculture systems. Healthy fish in the wild and in aquaculture systems can continuously harbour potential pathogens without suffering any ill effects; however, intensive and semi-intensive aquaculture systems in particular create ideal environments for serious disease outbreaks. The fish in these systems are unnaturally crowded, so the transmission distance for pathogens is short, the contact frequency high and the water thermally stable and rich in nutrients.

Diseases can arise not only as a result of pathogenic organisms, but also be caused by poor nutrition, bad management or unsatisfactory water quality. Stressed fish are highly susceptible to disease and optimum conditions should be maintained in high-density systems to minimise stress factors. The primary causes of diseases in many marine and fresh-water finfish aquaculture systems are protozoan and metazoan organisms, bacterial infections and nutritional deficiencies.

It is important to note that many chemicals used to treat fish may be toxic to humans and the aquaculturist should observe all necessary safety precautions during their administration. Codes of practice have been established to voluntarily control the use of chemical substances and therapeutics in aquaculture and the use of many antibiotics is strictly controlled.

Disease Management

The control of diseases in aquaculture should concentrate on prevention or prophylaxis rather than chemical treatment, which should only be used as a last resort. Disease control depends on three main interrelated factors, namely:i. correct diagnosis;

ii. the inclusion of suitable preventative methods in the management programme; and
iii. correct treatment.

The correct diagnosis of a disease includes developing an understanding of the life cycle and ecology of the pathogen or pathogens responsible. Preventative or prophylactic methods of disease control include the maintenance of good water quality, reduction of stress-causing factors (such as low dissolved oxygen concentrations, high ammonia concentrations, temperature extremes and the build-up of waste products), adequate nutrition, development of disease-resistant stocks, chemical prophylaxis and environmental manipulation of the systems.

Methods used to treat diseases by way of chemotherapy are provided in specialised texts. Chemicals that are added to culture systems for disease control can reduce or eliminate pathogens, but can also have harmful or residual effects on the fish. There are restrictions on chemicals which can be used for disease control in fish being raised for food for human consumption. Many chemicals are carcinogenic or may cause other damage to people who handle the compounds. In general, the widespread use of chemicals is discouraged.

Disease prevention by environmental manipulation of culture systems provides the best form of disease control. Environmental management includes the reduction or elimination of stressors and other factors that result in lethal or sub-lethal effects and predispose the fish to disease.

Disease Monitoring

Daily observations of the fish in each tank in the hatchery and growout farm, or in each sea cage, are vitally important. Changes in colour, feeding or swimming behaviour may indicate incipient disease. More advanced signs include flashing, excessive jumping or mucous secretion by stressed fish, faster and erratic swimming behaviour, gaping mouths and flared opercula.

Particularly during the early development stages of an aquaculture project, a few fish should be anaesthetised weekly and gill scrapings or biopsies and skin scrapings examined as wet mounts under a suitable microscope.

A system should be established for measuring and recording growth and FCR to provide indications of poor nutrition or water-quality conditions. Periodically, vitamin C and E levels in diets and fish tissues should be monitored and histological examinations made to detect dietary myopathies, toxic or fatty liver changes and other disease.

Prophylactic treatments can be applied regularly if required; however, the regular use of antibiotics is a practice that should be avoided: the frequent use of antibiotics and other substances can lead to the development of resistant strains of pathogens.

Fish-health laboratories should be used to confirm diagnoses and to determine likely causes of disease and methods for their control or eradication.