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Diseases of Fish, Crustaceans & Molluscs

Like all animals, wild and farmed fish and shellfish experience disease problems. These may be broadly classified into infectious and non-infectious diseases. However, it is not always easy to make a strict division between these as non-infectious causes can stress fish and render them more susceptible to disease agents such as bacteria and viruses.

Fish and shellfish, unlike mammals, are cold-blooded animals and all their physiological processes, including their ability to mount an immune response to pathogens, are greatly influenced by water temperature. Similarly, the pathogens which affect fish are also influenced by water temperatures which means that diseases and diseases outbreaks are often seasonal in their occurrence which varies between diseases, some occurring during the colder winter months, with others predominantly occurring during the warm summer months.

Some fish and shellfish diseases of particular significance in Scotland are described in this section.

Reporting Skin Damage in Wild Atlantic Salmon - 17 June 2019

Scottish Government’s Fish Health Inspectorate (FHI) has been responding to reports of wild adult salmon displaying skin damage such as reddening (petechial haemorrhaging) around the fins and belly (ventral surface), inflamed (swollen/red) vent and associated fungal infection

Further information will be provided when the laboratory results are available from fish sampled across a number of Scottish rivers.

In the meantime, observations of adult salmon demonstrating clinical signs of infection or damage should be notified to the local District Salmon Fishery Board (DSFB) and the FHI.

Considerations are:

  • Wild adult Atlantic salmon returning to rivers to spawn can naturally present with some physical damage due to a number of environmental factors;

  • There is no general requirement at this stage to remove fish with damage for disease control purposes;

  • Fish should not be removed from rivers where retention is prohibited, for example under the conservation of salmon regulations, except under the appropriate licence;

  • Moribund or lethargic fish should be targeted where sampling is considered appropriate;

  • FHI sampling will be prioritised on moribund fish that can be maintained alive (in keep nets or suitably bio-secure tank facilities);

  • Moribund fish that cannot be maintained alive should have details recorded and photographs taken, where possible, before being returned to rivers. Details should be sent to local DSFBs and FHI;

  • If local wild fishery interests determine that moribund fish are not to be returned to the river, they should be percussion stunned or suitably dispatched and maintained in a refrigerator at 4°C, until a determination on sampling is undertaken;

  • Good biosecurity practice should be followed when handling affected fish with hands, clothing and equipment being suitably cleaned and disinfected, where appropriate.