Council Directive 98/58/EC concerning the protection of animals kept for farming purposes lays down minimum standards for the protection of animals bred or kept for farming purposes, including fish. Following a request from the European Commission, the AHAW Panel was asked to deliver a Scientific Opinion on the animal welfare aspects of husbandry systems for farmed trout. The Scientific Opinion was adopted on 11th September 2008. The scientific opinion focus on the two main species farmed as "trout' in Europe, these are the rainbow trout, Oncorhynchus mykiss (Walbaum 1792); and the brown trout; Salmo trutta L 1758. Trout are produced across Europe for various purposes and in a wide variety of production systems however there has been no systematic survey of production systems and husbandry procedures at a European level. From the data presented in the scientific report, several factors affecting trout welfare were identified: abiotic and biotic factors, feed and feeding, husbandry, genetic, disease and disease control measures. A risk assessment approach was carried out to obtain a risk ranking for these groups of factors, estimate which hazards are more important for each life stage and enable a comparison of the different production systems. Due to the limited amount of quantitative data related to production systems and effects of potential hazards on trout welfare, the risk assessment was mainly based on expert opinion. Water quality is an important factor when considering trout welfare; however water quality is the result of a complex of interacting factors. Also the absolute level or the rate of change at which any particular abiotic factor exceeds the adaptive capacity of the fish is not easily predicted since it is co-dependent on: size of the fish, previous experience, health status and other abiotic factors. Trout have the capacity to adapt to a range of external environmental factors. Once their adaptive capacity is exceeded they may suffer from physiological or pathological disturbances. Where available tolerance levels for the various abiotic factors were indicated. Potential welfare effects caused by interaction of individuals of the same or different species were also considered. Predation is a significant welfare issue for farmed trout in many systems however there is no systematic data available on the scale of the problem. Effective and legal alternative predator control strategies are required. Intra-specific aggression can cause poor welfare, causing for example fin damage and reduced access to food. Stocking density is relevant to welfare but its effects are mediated through other variables such as water quality and fish behaviour. Consequently it is difficult to set clear guidelines for both maximum and minimum stocking densities that would safeguard welfare. Instead the monitoring of the fish condition should be regarded as a preferred option. Farmed trout are almost exclusively fed on commercial feed and problems may occur through changes in formulations or poor storage. While there are advantages and disadvantages of various feeding methods related to growth there is no clear indication of the relative benefits or disadvantages for fish welfare. Trout as poikilotherms do not have an energy demanding fixed temperature to maintain and will naturally undergo periods of inappetance. The length of time that food can reasonably be withheld for husbandry reasons without affecting welfare is related to size, lipid reserves, life stage and temperature and it is not possible to specify a simple maximum acceptable duration for food deprivation. Husbandry and management are central to maintaining the health, welfare and productivity of farmed fish. Fish are handled for a variety of purposes during the production cycle, however, much of this is standard husbandry practice and has not been the subject of scientific publications. It was recommended that the frequency and duration of handling events should be minimised and fish should be exposed to air for a short time as possible. A substantial proportion of the welfare experience of farmed fish is related to the systems in which they are grown. As the sophistication of the infrastructure and loading of the system increases (biomass per unit volume) so the system becomes more susceptible to acute failures. Thus effective backup systems, a higher level of contingency planning and staff training are required. It was concluded that genetic selection for resistance to endemic diseases constitutes a benefit in the context of welfare though selective breeding may modify other desirable traits unless carefully managed. Poorly structured breeding programmes have the risk of inbreeding with associated poor reproductive performance and egg survival, loss of genetic variation and development of undesirable physiological side effects such as deformities. No evidence was found to suggest that, when rainbow trout are reared in good environmental conditions triploids do not have similar survival and growth, (and in some cases better growth), than diploids. The reduction in aggression associated with all-female production coupled with triploidy can constitute a benefit in the context of welfare. As with any form of intensive livestock production, health and diseases are a major welfare issue for the trout industry in Europe. There have, however, been very few attempts to collect systematic data across the whole industry except for notifiable diseases. Endemic diseases related to management practices are often of greater welfare significance than the currently notifiable diseases. A major welfare issue is the lack of available veterinary medicines. Vaccines have produced major welfare benefits for the industry; however, they are currently only available for a small number of diseases. Furthermore the administration methods and inherent toxicity of some therapeutants and vaccines can in itself lead to adverse effects on welfare. In conclusion of the risk assessment no major differences concerning overall welfare risk between the different production systems used for each life stage were found. However, different production systems of the same life stage can differ for specific risks, as a result of the different conditions. Measures to improve welfare should be adapted to different production systems and take into consideration the specific requirements of each life stage. A minority opinion was received based on the view that the accepted Report and adopted Opinion are incomplete and that in order to answer the mandate from the European Commission, the general chapters on the welfare, biological functioning and farming of fish should be included.
