Proactive management reduces health risks in trout farming
This article was featured in Eurofish Magazine 4 2023.
As in almost all aquaculture systems, there is also a latent threat of disease outbreaks in trout farms, which brings with it considerable risks for fish populations and thus threaten the economic success of the company. To prevent such problems, welfare-oriented animal husbandry and proactive health management play a vital role. Good professional practice and compliance with legal duty of monitoring are essential.
A World Bank-commissioned study in 2014 put the annual economic losses in global aquaculture due to disease outbreaks at around USD6 billion. An extremely cautious and certainly far too low estimate, especially since the “farm gate value” of world aquaculture has increased since 2014 from USD160 billion to USD282 billion (2020) according to FAO. This figure overlooks the financial losses and reductions in growth from parasites like sea lice, something that salmon and salmon trout farms have to spend hundreds of millions of dollars every year to control. Disease outbreaks reduce animal welfare, and the necessary treatments bring about enormous costs. They represent a constant challenge for aquaculture operations, for which there is no easy solution in most cases. For fish farmers, farm managers, and fish veterinarians, preventing disease outbreaks is crucial for the success and sustainability of their operations. It is important to develop effective management strategies and implement tools that can help mitigate the risks of diseases and limit their consequences. Disease prevention and management are critical for the success of all types of aquaculture, including trout farming.
Almost all living beings are naturally susceptible to parasite infestation, pathogenic germs, and fungal infections. In aquaculture, this is particularly true for fish that are reared in intensive farming environments. Under such conditions, the high stocking densities can increase stress and exacerbate the spread of diseases. That is why proactive health management is particularly important in intensive breeding. The water quality and the health status of the fish must be constantly monitored. Quarantine for newly arriving stocking fish would be an effective measure to prevent the introduction of diseases into aquaculture farms, however, it may not always be possible due to space limitations.
Tighter regulation becomes increasingly restrictive in aquaculture
In Europe at least, the aquaculture sector is strictly regulated by law, and many management decisions do not solely lie in the hands of fish farmers. There are detailed regulations for environmental, water and consumer protection, and animal health is monitored by regular veterinary checks. The abundance of rules and regulations in aquaculture can help to ensure high production standards. However, for fish farmers who are less legally savvy, it can be difficult to keep track of the regulations. To make matters worse, some data are collected multiple times and by different administrative levels. Aquaculture operations are subject to approval and require a permit under water law. They are also subject to accounting obligations resulting from various laws and regulations. These include food, animal health and pharmaceutical legislation. Compliance with all of these obligations is monitored as part of official operational controls.
An important focus of the inspections is the health and welfare of the fish. Trout can be affected by a number of bacterial and viral pathogens and parasites, creating particular vulnerabilities in both freshwater and marine environments where salmon trout, like salmon, are reared in net pens. Sea lice, for example, have become a major problem for marine salmonid cultures. These parasitic copepods eat the skin and blood of salmon and trout, causing lesions that can lead to secondary infections. The producing countries have now switched to far-reaching health management measures, since direct control measures are expensive, time-consuming and rarely lead to lasting success. Both national and regional biosecurity plans aim to reduce the sea lice problem. These measures include, for example, the spatial and temporal separation of the fish stock in individual areas, more effective controls of transporting the equipment and personnel within and between the farms, the thorough disinfection of the equipment used and the observance of rest periods between restocking of the net enclosures. In combination with drug-based sea lice control campaigns, which are subject to strict regulations and are usually carried out by veterinarians, the problem should be limited and, if possible, suppressed. However, biosecurity plans also prevent the spread of many other diseases.
Fish are exposed to many hazards in open systems
The risk of disease outbreaks is particularly high in open waters because the fish are constantly exposed to potential external pressure from pathogens. It is difficult to control sudden changes in water quality that affect the resistance of the fish to disease. The cause of a disease outbreak is often not a virus, a bacterium, a parasite or a fungus alone, but is usually a co-infection in combination with environmental stressors. Infectious diseases often break out when the fish are under constant stress due to inadequate environmental conditions, suboptimal husbandry practices and poor hygiene. This weakens their immune system and makes them susceptible to pathogenic germs. The combination of virulent pathogens, a vulnerable host animal, and adverse environmental conditions creates ideal conditions for diseases to spread rapidly if they are not caught early and treated. Unlike in open waters, the water quality can be influenced to a limited extent in many trout farms and thus diseases can be prevented. In this context, the temperature and the oxygen content in the water are of particular importance. These master factors must always play a central role in preventive biosecurity and
health management.
Examples from trout farms in Baden-Württemberg show how important a good oxygen supply is. By aerating the fish tanks with pure oxygen, not only could the water volume be used optimally, the water quality improved, the stocking densities increased and thus the profitability of the farms increased, but the health and resistance of the trout to the Aeromonas and Pseudomonas bacteria was also significantly strengthened. Since then, furunculosis has almost completely disappeared.
Preventive vaccinations strengthen the immune system
Vaccines, which stimulate a strong protective immune response against certain pathogens, are also gaining importance as a management tool in the toolbox of disease prevention. Such vaccines already play a certain role in marine salmon trout farming, but they are still largely irrelevant in freshwater. This is probably also due to the fact that there are hardly any commercial vaccines available for trout farming. In addition, they are sometimes controversial in the industry and not approved everywhere. Nevertheless, immunization by vaccination could be an important and promising future preventive approach that belongs in the “first line of defence” of disease management. In addition, vaccinations are better than reacting to the outbreak of a disease with chemicals or even antibiotics.
We’ve come a long way with the use of functional feeds and some alternative products that boost the immune system of fish whilst improving their resistance to stress and disease. The range of products in this area are now so diverse and extensive that it is often difficult to separate scientifically proven and effective remedies from dubious alternatives that probably only serve to increase the bank accounts of their providers. Since there are undeniable links between chronic stress, weakened immunity and increased susceptibility to disease, a lucrative market is opening up for products that are designed to elicit physiological responses that could improve the overall health status of fish. Probiotics that are added to the feed that reduce bacterial loads or increase host resistance are widely used. Prebiotics in the feed promote a healthy microbial flora and so-called postbiotics, which contain antimicrobial peptides or polysaccharides, act against bacterial pathogens. Various herbs and spices and their derivatives are said to have similar health-promoting properties. But how do you properly assess the health and welfare of trout? Are there any objective criteria that provide reliable information on this subject? Absolutely – and for free even! For example, a 130-page manual in English and Norwegian has been available to download on the Nofima website since 2020. This handbook allows trout farmers to assess the welfare of their fish using “indicator toolboxes”.
EU animal health law regulates the obligations of companies
On April 21, 2021, Regulation EU 2016/429 (Animal Health Law) came into force. This replaced the Directive 2006/88/EC, which previously regulated the obligations of European trout farmers. The new regulation aims to harmonize the principles of disease monitoring and control throughout Europe and to make them clearer, streamline content and clarify ambiguities. The new Animal Health Law, which regulates almost everything from the export of horses, goats and deer to the border crossing of carrier pigeons, has been ambitiously adopted, but this resulted in a bureaucratic monster that is difficult to understand and leaves many questions in practice. Producers and exporters of “aquatic animals”, which include crustaceans and molluscs in addition to fish, have to tediously search text after text just find the legal requirements and obligations that apply to them. Luckily, relatively little has changed for fish and trout farmers. Most farms, from the hatchery to the fattening facility, must continue to meet the high standards of biosecurity that have existed up to now. All activities of parties involved must be aimed at preventing the spread of fish diseases at the national and international level.
The obligation to keep records of incoming and outgoing fish stocks remains in place, as does the obligation to report animal transports. For the transport of fish within the EU, TRACES certificates must be issued by the competent veterinary authorities. The electronic database system TRACES (Trade Control and Expert System) was set up throughout the whole of the EU on the basis of the Decision 2003/623/EC42. The obligation to notify and report particularly dangerous fish diseases, which form the basis for government preventive and control measures, also remains in place. In the case of trout and other salmonids, this applies in particular to listeriosis and salmonellosis, ISA, EHN, IHN, and VHS. Registered and licensed aquaculture companies are required to keep records of unusually high mortality rates in trout stocks. If an increased mortality rate is found that cannot be clearly attributed to certain husbandry or transport conditions, this must be reported to the competent authority immediately.
Acting quickly in the event of disease reduces consequential damage
Despite all caution, disease outbreaks can occur even in well-managed trout farms. Early recognition of symptoms and rapid action is important to prevent damage to fish stocks and limit economic losses. In order to be able to initiate the necessary measures, every fish farmer should have a basic knowledge of the most common diseases, how to identify them and immediately treat them. An extensive arsenal of clearly illustrated specialist literature is also available to support them, showing the characteristic features and damage symptoms of the diseases. As a result, in cases of an outbreak of any disease, initial assumptions of its nature can be drawn, its precise determination by a specialist veterinarian be completed and the necessary therapeutic treatments initiated. Prescribing and use of medication is strictly regulated in almost all trout farming countries and is also usually carried out under the supervision of a veterinarian. In principle, only drugs that are approved for the disease to be treated may be used in EU countries. All these measures ensure that treatments are effective and safe for the fish, the environment, and consumers. In order to be able to guarantee consumer safety, the prescribed waiting times between treatment and later harvesting for commercial use must be observed. Every single step in this package of measures must be meticulously documented. Obligations to provide evidence of the use of pharmaceuticals for edible fish result from the Drugs Act, the Food Hygiene Ordinance and the water law permit. Fish farmers must provide evidence of the purchase of veterinary prescription medicines (e.g. prescription, invoice or delivery note) and must record their use. At the same time, the veterinarian must also provide corresponding evidence of their prescriptions as a sort of cross-check.
Even RAS are not immune to diseases
The manufacture, use, and monitoring of veterinary medicinal products is regulated by the EU Code for Animal Medicines, which is supplemented by additional regulations in member states. However, in aquaculture there is a limited number of medicines that are specifically approved for use in fish. The limited number of approved medicines for use in fish farming has been a cause for concern, with critics warning of a “therapy emergency” in the event of a disease outbreak. The use of disinfectants in aquaculture can be comparably complicated due to the limited number of approved disinfectants. Some active substances, such as malachite green, copper sulphate, or potassium permanganate, are no longer permitted in the EU. Even the use of lime for pond disinfection requires prior approval and must be used in prescribed concentrations to prevent it from entering other bodies of water.
Finally, a few words about the situation in recirculation systems, or RAS. As such systems are essentially isolated from the outside world, operators and the general public frequently assume that RAS are disease-resistant and safe. However, practice indicates that this is not always the case. In fact, circulatory systems have particularly favourable conditions for the emergence of illnesses and the multiplication of opportunistic microbes. High stocking densities, along with poor water quality, can promote the emergence of non-infectious illnesses. Ammonia, nitrite, carbon dioxide, suspended solids, and ozone levels in circulation systems have already resulted in fish losses. Pathogen-related illnesses have also been observed in RAS rainbow trout. Bacterial (bacterial gill disease, furunculosis, bacterial kidney disease, fin rot) and viral (IPN, VHS, IHN) infections are common. Occasional reports of parasites (Gyrodactylus, Chilodonella, Trichodina, Epistylis, Trichophrya, Ichthyophthirius) and fungi (Saprolegnia) have been recorded. The cause of these diseases is often the introduction of already infected stocking material and poor staff hygiene. Chemotherapy is extremely difficult in circulatory systems because there is always a risk of affecting the bacteria in the biofilter. Consequently, effective health management is critical to the success of these facilities and all other trout farms.
Manfred Klinkhardt