Fish diseases: Why prevention is better than a cure

An image of fish to illustrate the concept of fish diseases.

As fishing industries such as the £1bn Scottish salmon farming industry are being affected by the growth of diseases, focus is now turning from the treatment of fish diseases to their prevention.

IN 2016, the Food and Agriculture Organisation of the United Nations (FAO) reported that the European Union (EU) is representative of the largest single market for fish imports. However, this could be at risk as mortality in fish is being affected by the growth of fish disease, which has been linked to a variety of causes. During 2018, the five main environmental challenges effecting aquaculture farming were known as being:

  • Pollution;
  • Escaped fish;
  • Access to feed;
  • Predators; and
  • Fish diseases (including parasites and chemicals).

For fish diseases to spread, the FOA (2018) deems the following as the three main factors needed:

  • A susceptible host;
  • A viable transmission pathway;and
  • Environmental conditions.

The combination of all three factors can result in either increased virulence of the pathogen, decreased resistance of the host, or conditions for the pathogen to replicate to overwhelming numbers. Once a pathogen or disease agent is introduced and becomes established in the natural environment, there is then a very low possibility that it can be eradicated.

The natural environment

In terms of the natural environment, interacting factors such as droughts, pollution and predators, are resulting in low grade infections developing into a fatal fish disease. One of the main causing factors of these is climate change; resulting in things such as eroded river banks and flooding, resulting in an increase in droughts.

It has been suggested by researchers that between the combination of fish being stranded as a result of droughts and water temperatures rising, fish are being put under stress which is resulting in them clustering in refuge areas where the water temperature is higher, and the oxygen levels are declining. In addition to this, sudden water movements or internal circulation can bring anoxic water, areas of water depleted in oxygen, to the surface, or trigger toxic algal blooms. The combination of these is then resulting in diseases being spread progressively.

The increased globalisation of vessel movements is also contributing to the spread of diseases in fish. As vessels become more global and frequent, there has been an increase in the amount of new diseases being brought into the environment as a result of infected marine life, such as oysters, being transported on the sides and bottoms of the vessels.

A threat to the £1bn Scottish salmon industry

Farmed salmon is one of Scotland’s biggest food exports and is estimated to be worth more than £1bn (~€1.14bn) every year. However, in 2017, around 125,000 salmon died as a result of a disease bacterium breakout, Pasteurella Skyensis, on two farms on the isle of Lewis. Scotland has now launched a major research initiative to develop a better understanding and prevention of diseases affecting farmed salmon, such as sea lice, which has been reported to of currently cost the industry over €45bn, and gill health conditions.

Prevention at the SAIC

While there are currently vaccines available for farmed salmon with furunculosis, which is a form of skin infection, for instance, the Scottish Agriculture Innovation centre (SAIC), a £3.5m programme, is focusing on the prevention of diseases in fish, rather on treatments. This will involve an increase in monitoring water quality and disease outbreaks.

SAIC’s focus is primarily on the following two main areas:

  • Examining factors which cause gill damage to occur, including water quality and the farming practices/equipment used; and
  • Analysing the genetic characteristics of salmon to understand why some are more susceptible to diseases than others.

The focus on gill health stems from the importance that this can have on the vertebrate’s overall health and wellbeing. SAIC’s focus is representative of previous studies which have suggested that the three main ways to manage the risk of diseases in fish are prevention, mitigation, and coping, with the focus being on prevention and health management practices.

The European Union Reference Laboratory

The European union reference laboratory (EURL) for fish and crustacean diseases is funded by the European commission and is primarily concerned with harmonising diagnostic procedures for notifiable fish and crustacean diseases in Europe. EURL focuses on the importance of preventing the occurrence of diseases amongst aquatics, as outbreaks could potentially result in both a loss of marine life and a loss of revenue for the industries involved. Therefore, the main purpose of EURL is to ensure the quality of diagnostics of fish and crustacean diseases in Member States and to harmonise the procedures and methodologies applied.

The diseases mentioned in the EURL Council Directive are the main diseases focused on for this programme. The Directive states that there are two main categories of disease: ‘exotic’ and ‘non-exotic’. Exotic diseases include Epizootic Haematopoietic Necrosis which is a systemic iridovirids infection that primarily affects rainbow trout and catfish. Whereas, non-exotic diseases includes VHS (viral haemorrhagic septicaemia) and ISA (infectious salmon anaemia).

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