Animal Welfare Aspects Of Husbandry Systems For Farmed Fish: Carp Scientific Opinion Of The Panel On Animal Health And Welfare

Following a request from European Commission, the Panel on Animal Health and Welfare was asked to deliver a scientific opinion on animal welfare aspects of husbandry systems for farmed fish. Council Directive 98/58/EC concerning the protection of animals kept for farming purposes lays down minimum standards for the protection of animals, including fish. The Scientific Opinion on welfare of carp was adopted on 22 October 2008.The common carp (Cyprinus carpio, Linnaeus 1758), which is by far the species farmed in largest numbers, is the scope of this opinion. Common carp is a domesticated species that has well adapted to the husbandry systems within which it is reared.In Europe, only two major production systems (monoculture of carp and polyculture of carp) are commonly practised. There are numerous combinations of polyculture with common carp production. Since the species involved are all cyprinids and occupy only slightly different ecological niches in the pond system, they were subjected to the same risk and hazard assessments as carp in monoculture. Consequently, for the purpose of risk assessment, polyculture was considered as an intensive monoculture production system.The various life stages of carp considered are: fertilised eggs and embryos, yolk sac fry, free swimming fry, nursed fry, fingerling, overwintering carp, on-growers, marketable fish, and broodstock.A review of environmental conditions and factors that were identified as possibly affecting the welfare of common carp, Cyprinus carpio, at different life stages has been conducted. These factors are grouped as: abiotic and biotic factors (including behavioural interactions), food and feeding, husbandry and management, genetics, and the impact of disease and disease control measures. It is however important to realise that the environmental conditions are always defined by a range of inter-related factors. While each specific variable is described separately, there are very few occasions in reality where only a single factor is involved in any fish welfare issue relating to environmental conditions. For this reason, only ranges of acceptable levels for the various factors can be given and these should always be considered in the context of the other variables involved.A semi-quantitative risk assessment was carried out to obtain a ranking of risk for different life stages under different production systems.Light levels applied to embryos should be kept low to avoid over-stimulation leading to exhaustion of endogenous reserves.The anatomy of carp may render them particularly vulnerable to excessive noise and vibration.There are no water flow requirements under carp farming conditions (except for eggs). Cessation of water flow helps hatching. Excessive water flows may cause crowding of fry and wasted use of energy reserves leading to starvation. Too low water flow was found in the risk assessment to be a more highly ranked hazard (causing hypoxia) in early life stages. Carp are remarkably tolerant to low oxygen levels, except in the first few months of the life cycle when fry are more sensitive. Occasionaly severe oxygen depletion associated with collapse of algal blooms in late summer at high temperatures can affect welfare. However, the results of the risk assessment found low oxygen to be the most important or second most important hazard for all life stages. Fry is a life stage particularly sensitive to low but also high O-2 levels. The risk assessment scoring found low oxygen to be an important cause of mortality at all life stages; but low oxygen levels are most important in the early stages.Eggs exposed to levels of gas supersaturation can lead to long term effects on surviving individuals. Supersaturation of dissolved gases may lead to gas bubble formation inside the larvae causing disruption of swimming activity.Carp can survive across a wide temperature range by adjustment of their level of activity. Carp larvae are very susceptible to temperature fluctuations: excessively high temperatures lead to increased rates of body deformities. Rapid temperature changes rather than absolute limits are normally a significant welfare issue for carp fry. Rapid temperature change was found in the risk assessment to be an important hazard for yolk and nursed fry and fingerlings.During incubation high water pH can lead to incomplete embryonic development and deformities and was found in the risk assessment to be an important hazard. Carp larvae are particularly sensitive to acid stress. In later life stages, water pH per se has little importance in carp husbandry, it is only important in terms of its influence on other abiotic hazards (e.g. ammonia) should they be present.Carp are not affected by suspended solid levels except at the very high concentrations that may occur during harvesting. High suspended solids can suffocate eggs and embryos (it was the third highest ranked hazard for this life stage) and the water supply to the hatchery water should be filtered to limit suspended solid and predatory invertebrates.Unionised ammonia can be a serious welfare issue under conditions of intensive photosynthesis such as following collapse of algal blooms. This was reflected in the risk assessment tables where unionised ammonia was highly ranked in nearly all life stages. A great care and understanding is required in levels of fertiliser in ponds to avoid this hazard.In the present culture systems in Europe, overwintering ponds do not present a particular welfare risk, compared with other life stages, although it is important that these overwintering ponds are sufficiently deep for the coldest weather. Catch pits in harvest ponds should be carefully designed to minimise the suspended solids problems. For larvae, bad design of the tank would prevent the swim bladder filling with air resulting in incomplete development of the swim bladder, impaired swimming ability, poor health and slow growth rates.Unless ponds are managed carefully, anoxic layers producing toxic metabolites can develop. Algal blooms can be a significant welfare risk because of their effect on pH, because the algae may themselves be toxic, and because the collapse of algal blooms may cause severe and acute anoxia. The results of the risk assessment indicated that algal blooms were most important during the on-growing stage.Predation is a very serious welfare issue particularly significant in the farm pond environment where cormorants in particular can induce significant stress to the fish manifested by behavioural modifications and associated reduction in feeding, and the birds may also inflict injury.Whilst carp generally feed on natural feed, above certain densities they should receive supplementation with a correctly balanced feed for a fish that already feed on natural feed. Stocking fry into ponds with insufficient supplies of suitable food will result in low survival rates, poor growth and significant deformities. Food supply is the main factor governing appropriate stocking density. This was reflected in the risk assessment results: a shortage of natural food was the highest ranked hazard for the free and nursed fry stage.Disease is one of the major welfare issues in all fish husbandry. In the case of carp it is particularly related to environment. Most carp pathogens are facultative pathogens present in the environment. Those diseases with chronic, often sub-clinical effects are often of greatest welfare significance. In more acute conditions, control or treatment measures may have significant welfare impact in their own right. Good husbandry should be the major means of disease control. Currently very few of the treatments which are effective can be used due to the fact that they are not licensed. The risk assessment scored three selected diseases. These hazards affected mainly the earlier life stages (fry) and their importance varied between production systems.