Welfare of farmed seabass and gilthead seabream

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 seabass (Dicentrarchus labrax) and seabream (Sparus aurata). This Opinion was later published by the European Food Standards Agency.

From the available data, factors affecting farmed seabass and seabream welfare were identified which led to conclusions in the Scientific Opinion. These factors are grouped as: abiotic and biotic factors and behavioural interactions, food and feeding, husbandry and management, genetic selection and the impact of disease and disease control measures. A risk assessment was carried out to obtain a ranking of risk and compare the production systems.

Sea basss and sea bream are eurythermal and euryhaline species, tolerating wide range of temperature and salinity variations. Rapid and elevated changes of temperature close to the thermal limits are more likely to lead to poor welfare. Seabass and seabream are tolerant species capable of coping with large ranges of dissolved O2 concentrations through physiological adjustments. In cages, however dissolved O2 is a limiting factor at high temperatures.

O2 saturation in outlet water should be monitored daily and should be maintained above 40 per cent saturation. Further studies are recommended on the combined effects of high O2 and CO2 levels on different stages. Seabass and seabream can be considered tolerant to pH variations.

There is an increased risk of poor welfare at pH values below 6.0 and above 8.5. The daily monitoring of the water pH in recirculated (RC) and ongrowing flow-through (FT) systems are recommended. More studies are needed to evaluate the combined effects of low pH and elevated CO2 concentrations. Super saturation is a rare but serious cause of loss in farmed fish, with serious welfare implications when it occurs. CO2 concentration depends on pH, temperature and salinity of the water as well as the respiration of the fish and other organisms. Its management is complex in RC systems and can become a welfare issue.

Studies on the CO2 tolerance and possible welfare implications are recommended. High stocking densities and insufficient water flow may result in build up of ammonia in the water. Sub-lethal concentrations of ammonia can damage the gills and also impair immune function leading to increased susceptibility to infectious disease but further research is necessary to determine potential welfare effects of long term exposure to low levels of unionized ammonia nitrogen. Photoperiod is an important factor conditioning larvae growth and development and also the induction of spawning.

The welfare consequences of artificial photoperiod, if any, are not fully understood. Ammonia and other metabolites may cause poor welfare where inadequate water flows occur. There is, however, very limited information about flow rate requirements in tank systems.

Stocking density can affect welfare because of its consequences on fish social interactions and water quality. Stocking density per se (biomass/volume) cannot be used as a good indicator to predict welfare. Intra-specific aggression (including cannibalism) in post larvae can be problematic and avoidance by maintaining grading and adequate stocking densities and water flow is necessary. At the post larval stage, husbandry parameters, such as adequate stocking density, water flow and feed access should be maintained to avoid intra-specific aggression.

While predation damage and the presence and predation activities are undoubtedly a significant welfare issue for seabass and seabream in certain systems, there is no systematic data available on the scale of the problem. Clear guidance based on scientific evidence on the issue of predator control should be developed and provided to the fish farming industry. Larval first feeding is a very sensitive stage where both high quality and abundant live feeds has to be provided to the fish in order to obviate a welfare compromise. Post larvae and ongrowing stages are less sensitive to feeding strategy providing that even access to feed is allowed to all fish in order to avoid aggression.

At the larval stage, inadequate size and quantities of live feed in the diet can cause empty gut, metabolic stress, impaired growth and at worse fasting leading to death. Fish are exposed to various husbandry stressors during all stages of the life cycle in intensive culture conditions that can lead to injury, stress, increased disease susceptibility and impaired performance. Proper equipment, handling and anaesthetic protocols are important to minimize stress and physical damage associated with handling procedures. Assessment of seabream and seabass sex and sexual maturation by urogenital catheterization biopsy is an invasive stressful procedure that may threaten fish health and reproductive performance.

Viral Nervous Necrosis is an important disease for seabass production with major implications in fish behaviour and welfare as no commercial vaccines are available and no treatment is possible. Monogenan parasites, winter syndrome, vibriosis and pasteurellosis are common problems in most farms and can become a significant welfare problem if not effectively controlled. Availability of veterinary medical products for seabream and seabass is very limited. Vaccines have made a significant contribution to controlling serious infectious diseases; however further research is recommended.

The risk assessment outcomes showed that in the majority of farms monitoring of health and production management is carried out to a high standard, with the possible exception of handling procedures. Hazards when they occur are generally quickly detected and corrected. Poorly formulated feed and poor storage, which may cause low level chronic effect and may go undetected was highly scored hazard for a number of life stages and across production systems (except extensive).

The failure of fish to adapt to feed distribution modes and feed storage conditions during the summer months are important hazards in some stages and are open to improved management. The lack of availability of authorised anaesthetics for use in broodstock was an important hazard. Disturbance to the fish due to routine management is inevitable to a degree but management practices should be implemented that minimise the effect on fish.

There were no significant differences between larve, juveniles and ongrowers in flowthrough tanks compared with recirculation systems. The main hazards were associated with management (e.g.handling, disturbance, poor tank hygiene), feed and disease. In extensive systems predation and water temperature were important hazards for ongrowers and juveniles. The impact of infectious and non-infectious diseases in flowthrough, recirculated and also in extensive systems is an important hazard.

Further Reading

- You can view the full report by clicking here.

January 2009

the Fish Site Editor

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