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How to Achieve Good Health and Welfare of Farmed Fish

30 January 2014
Fish Vet Group

ANALYSIS - Maintaining good health and welfare of fish is vital to a sustainable aquaculture sector. Dr Peter Southgate, Fish Vet Group, speaks to TheFishSite Editor, Lucy Towers, about how farmers can achieve good health and welfare management and what diseases are currently causing problems around the world.

How important is welfare to successful fish management?

Welfare is fundamental to all aspects of fish management and has a profound effect on the health, survival, productivity and final product quality. Every management procedure we impose on our fish stocks from the egg to the final harvest will have an impact on the welfare of the fish by causing stress or even damage.

The stress response in fish, as in other animals, is a protective mechanism which allows the fish to cope with changes imposed on it. Stress itself does not necessarily have a negative effect on fish production, it is usually only when there is continuous or repeated stress that this becomes damaging to the fish and increases the risk of clinical disease, poor growth and poor survival which is then reflected in increased mortalities and poorer productivity in terms of increased disease incidence, reduced growth performance and poorer food conversion. By ensuring the best welfare at all stages of the production cycle, stress and damage are minimised resulting in greater chances of successful fish management and productivity.

Knowing how important welfare is, how can farmers implement good welfare practice?

Farmers need to know where impacts on fish welfare occur. These may be the obvious management procedures such as transport, grading, crowding and other handling activities, but a good stockman will also be aware of more subtle stresses to the fish such as noise, light, vibration, human activities and also small changes in water quality such as a drop in dissolved oxygen levels at night and have procedures and practices in place that mitigate against these stressors.

By using best practice in terms of the ‘standard’ management procedures such as low impact passive grading rather than mechanical grading and ensuring very gentle crowding techniques with optimum water quality then stress and damage to the fish are minimised resulting in better welfare.

Farmers must have empathy with their fish and understand where welfare may be impacted, carrying out a welfare risk assessment throughout the production cycle to identify these risks and put procedures in place to help alleviate negative impacts on welfare. A good stockman will be able to recognise welfare indicators in terms of fish appearance and behaviour to give early warning of any problems developing and be able to act rapidly to protect their stocks - good observation is absolutely vital for this and stocks must be closely monitored at all times during any management procedure. Due to the nature of aquaculture, this may need to be done remotely by camera observation to observe how fish are behaving e.g. during transportation.

Crowding pre-harvest is a good example where close monitoring is vital to ensure that any signs of stress or poor welfare are picked up instantly and attended too. Overcrowded fish will quickly show signs of stress in increased activity at the surface of the water, escape response, signs of distress, skin colour changes etc. Good practice should ensure that these signs are not seen, but also your observation will pick up early warning signs and corrective action can be taken instantly.

What can farmers do to make sure they have good health/ biosecurity on their farms?

Disease prevention is a top priority when it comes to maintaining a healthy stock. Good health starts with ensuring that only healthy/disease free fish are stocked into a clean environment. This often means carrying out health checks prior to stocking fish or shellfish onto the site using competent trained personnel or buying fish which come from stock which have been certified as free from specific diseases.

Biosecurity on the farm is equally important and, although 100 per cent biosecurity is difficult to achieve in aquaculture, due to the fact that most farms rely on natural water sources – rivers, ponds, seas, it is important that effort is made to ensure that no pathogens are introduced into the farm from vehicles, visitors, staff, equipment, etc. If possible, incoming water is treated for pathogens using ultra-violet radiation or ozone – this is usually only practical in hatcheries or recirculation systems where a relatively small volume of water is being treated, but particularly important in hatcheries where juvenile fish and larval shellfish are particularly susceptible to disease.

Farm biosecurity audits and risk-assessments should be made on a regular basis and appropriate control measures put in place at each risk point. Effective hygiene and disinfection procedures must be developed at each point ensuring that the appropriate disinfectant at the required concentration and contact time is being used.

Appropriate hygienic protective clothing should be available for staff and visitors and visitor control should include written assurance that they have not had any recent contact with other potentially infected animals. Appropriate predator and pest control must be in place as these can also present a biosecurity risk. All elements of the biosecurity programme must be adequately monitored and documented, e.g. replenishment of disinfectant foot baths on a regular basis.

A second tier of disease prevention is the use of vaccines to protect fish against specific pathogens. Many vaccines are now available against common bacterial and viral pathogens and are a vital tool in maintaining healthy stocks. Vaccines, however, should never be seen as an alternative to good management and appropriate biosecurity procedures.

Many fish and shellfish diseases are triggered by stress and poor environment and good management, described above, combined with an optimum aquatic environment and a high nutritional status are all necessary if good health is to be maintained. Many disease organisms are ubiquitous in the aquatic environment and it is impossible to eliminate them from farms. Only by ensuring optimum welfare, vaccination status and environment can the farmer ensure that his fish will be in robust enough condition to defend themselves against these pathogens.

Breaking the pathogenic cycle where management procedures reduce the level of environmental pathogens is important. This includes never stocking more than one generation of fish/shellfish in the same environment – this ensures that pathogens are not passed from one generation to another. And it ensures that there is a break between harvesting and re-stocking the farm, i.e. fallowing the site for a minimum period to reduce the pathogenic reservoir in the environment or moving the farm to a different location.

All these factors will help to reduce the pathogenic challenge to the fish stocks.

What diseases are currently causing problems for fish farmers?

Despite good biosecurity, vaccination status and excellent husbandry/management of the fish, disease outbreaks do still occur. This is partly due to the fact that many disease organisms are present in the aquatic environment and sometimes conditions favour the pathogen so that the pathogenic overcomes the fish’s ability to resist the pathogen and clinical disease results.

Currently there are a number of disease organisms that are causing clinical disease in farmed stocks and this list is a small representation of some of the most significant. Many other pathogens may cause clinical disease and present challenges to the fish veterinarian because there are very few available effective treatments for many of these diseases:

Sea lice – These crustacean ectoparasites continue to be a major threat to the Atlantic salmon industry. Despite massive investment in control measures lice represent a major cost to the salmon farmer in terms of stress, damage, manpower and costs of control measures.

Amoebic gill disease (AGD) – Over the last few years AGD has been identified as a major disease of the gills in salmon causing severe gill pathology and fish mortalities. Control is difficult and time-consuming.

Protozoan ecto-parasites – These are commonly found in freshwater production throughout the world and cause damage to the gills and skin of a variety of species of fish. There is a wide range of these parasites, but they include very common species such as Trichodina, Ichthiobodo and Ichthyophthirius (white spot disease). Some of these are also problematic in marine situations, e.g. Trichodina in turbot.

Nodovirus – A serious virus of the nervous system of marine fish, causing particular problems in hatcheries in juvenile fish, but it can affect all stages of fish. At this time, no treatment or vaccine is available.

Shrimp virus disease – Many shrimp viruses have been found to cause significant diseases. Despite attempts at biosecurity and other control procedures, many of these diseases have spread throughout the shrimp-producing countries and have resulted in severe economic losses.

Shrimp Early Mortality Syndrome (EMS) or Acute Hepatopancreatic Necrosis Syndrome – This has recently been causing major problems with small shrimp newly stocked into growout ponds. The cause has still to be absolutely defined, but recent findings suggest the cause to be a bacterium - Vibrio parahaemolyticus - which carries a phage causing it to produce a toxin deadly to the shrimp.

Freshwater fungusSaprolegnia is a type of water mould that is ubiquitous in the freshwater environment; it can cause serious problems, particularly in juvenile fish and can be difficult to control once established. This is another pathogen that infects a wide range of fish species.

Edwardsiella – A bacterial infection causing significant disease in Pangasius catfish in Viet Nam. A recently introduced vaccine may help to control this.

Streptococcus and Francisella – As a bacterial infection of tilapia and other farmed species, control at this time is quite difficult, although Streptococcus vaccines are now available.

In all disease circumstances, it is necessary to have rapid and accurate diagnosis so control measures and treatments can be identified and implemented as rapidly as possible. Appropriately trained individuals must be in place to recognise the early indicators of disease so the appropriate investigation and analysis may take place.

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