Farmed and wild fish, like all animals, are susceptible to bacterial, viral and parasitic infections. Healthy farmed fish are advantageous to both aquaculturists and natural resource managers. Fish farmers depend on high survival rates and marketing healthy fish in order to keep businesses operating.
However, disease is a fact of life in all animal populations and production systems. When finfish aquaculture operations are in the marine environment, water moves freely between farms and the ocean. Risks include the amplification and transmission of disease from farmed to wild fish, and the introduction of nonnative pathogens and parasites when fish are transported.
Fish diseases occur naturally in the wild, but their effects often go unnoticed because dead fish quickly become prey. Disease events can occur in fish farms because 1) fish are reared at higher densities than nature, increasing contact between fish; 2) infected fish are not removed as promptly from the farm as they would be by natural predators; 3) farmed fish are more closely and easily observed than wild fish. Thus pathogens that normally exist in low numbers and do not cause disease in the wild may result in disease in farmed fish.
Management and Remedies
Over the last two decades a number of measures have been developed to effectively control bacterial diseases in farmed fish. These include vaccines, probiotics, limiting culture density, high-quality diets and when appropriate, judicious use of antibiotics. Antibiotics are considered a method of last resort, and their use has decreased drastically both in the United State and worldwide.
The management of viruses is focused on monitoring for diseases and maintaining culture conditions that enable healthy fish to resist disease through good nutrition, genetics and low stress environments. When viral diseases are discovered, fish are removed from the affected farms. Vaccines for viral diseases are currently being developed, but remain experimental.
Successful measures to control parasites on farms include therapeutants, fallowing farm sites, and pest management strategies. Cleaner wrasses and other species have also been used commercially with success to reduce parasites .
Preventing Disease Transfer
Most states have comprehensive aquatic animal health regulations to prevent the introduction of diseased animals into the state, and manage disease if it occurs. Effective programs include routine health exams and inspections, accurate record-keeping by the farmer, biosecurity measures for farm sites, preventative medicines like vaccines and probiotics.
Principles to prevent and treat health risks are specified by the World Organization for Animal Health and the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service.