Disease is an inevitable part of aquaculture production. Some pathogens are always present in farmed stock and only cause disease when the right conditions occur – such as when animals are stressed or when environmental conditions are suitable. The impact of these pathogens can be managed with good hygiene and husbandry practices. Other pathogens can be very damaging even under ideal husbandry conditions – these should be excluded from your farm wherever possible. 

According to FAO publication The State of World Fisheries and Aquaculture, constraints in dealing with aquaculture diseases include: 

• limitations in diagnostic techniques 

• the existence of cryptic pathogens and benign organisms that may become pathogenic when introduced to new hosts and new environments 

• limitations in control options for aquatic animal diseases 

• the occurrence of multifactorial disease syndromes and frequent subclinical infections 

• the undomesticated status of most farmed aquatic species 

• the paucity of information on the health status of aquatic animals

In Norway, 15-20 per cent of the number of salmon and trout in sea farming have been lost. In terms of weight, this corresponds to 6-9 per cent. Fish mortality and other losses in production are important for how efficiently resources are used and for the economy in the industry. The losses in production relate to fish death in cages, fish escapes, and some can disappear in other ways. Fish that die in cages constitute the largest loss in production. 

In Scottish aquaculture, the most commonly reported causes of death were diseases. Other frequent killers were “treatment losses” from misusing chemicals or de-licing machines, “handling” errors, algal blooms, jellyfish, seals, “poor water quality” and oxygen shortages.