The welfare of cultured fish is of increasing concern for all stakeholders in the aquaculture sector. This rising interest is responsible for growing regulation at international level , as well as for the appearance of welfare standards in certification schemes . Concern for the welfare of cultured fish arises in part from ethical considerations based, for example, on the moral importance of caring for captive animals. Over and above this, good welfare potentially benefits all parties in fish culture. From an economic perspective, up to a point and in many cases, good production and good welfare go hand in hand, with poor welfare often impacting production-related traits. In addition, in some markets, there are premiums to be charged for fish cultured under welfare assurance schemes. Additionally, ensuring good welfare in farmed fish is an important aspect of job satisfaction for fish-farm workers. Finally , poor welfare often increases the adverse impacts of aquaculture on the environment, such as when appetite is suppressed in stressed fish which increases feed wastage into and pollution of the water surrounding farm cages.
Climate change is underway and is already having an impact in aquaculture, being set to alter profoundly the activity of fish farming, especially in open systems exposed to weather and climate such as pond and cage farming . Increasing our knowledge on cultured finfish biology and their diversity, as well as on welfare science will help us cope with the inevitable consequences of climate change.
The term ‘finfish’ (‘fish’ in what follows) refers to a great variety of animals that have little in common, except they are vertebrates that live in water, have fins and mostly respire using gills . More than 34,000 species of fish are recognized; less than 1000 are cartilaginous fish (elasmobranchs), the remainder being bony fish. The great diversity among teleosts in physiology, behaviour and ecology influences how they cope with environmental challenges. Additionally, t here are also striking differences within a given species, associated with life history stage and among populations adapted to different habitats. In terms of coping style (or personality traits), for example, proactive and reactive animals flourish in different environments in the wild , proactive individuals being favoured when resources are abundant and predictable, population densities are high and predation is low, while reactive animals do best in the opposite conditions. This variability in personality traits is well documented to occur in farming systems and has clear implications for how well farmed fish adapt to aquaculture conditions, present and future.
To protect the well-being of farmed fish, much effort has gone into developing systems for improving fish health, directed at pathogen-induced diseases and other threats to health, such as malnutrition and injury. This is entirely appropriate because ill health is usually a powerful cause and indicator of poor welfare. This is not just because disease generates poor health and mortality, but also because survivors may experience welfare problems after the disease has passed. However, there are some interesting layers of complexity because 1) poor general welfare often makes fish more susceptible to disease, 2) disease treatments may also compromise welfare, 3) good health does not necessarily equate to good welfare and 4) poor health does not always equate to poor welfare.
Protection of fish welfare makes it imperative that climate changes are anticipated, and that coordinated action is taken before it becomes impossible to ensure that pond or cage-farmed fish have the environmental conditions they need for good welfare. It is more than likely that technological advances will allow not only a better understanding of farmed fish biology and their responses to farming contexts, but also the development of more adequate and effective welfare measures and their monitoring. This should include disease prevention (involving new diseases, emerging from climate change), precision fish farming, analysis of big data and more knowledge on positive welfare, for example.
Climate change is also likely to influence the range of species and strains of fish farmed in cages, in favour of those that are more tolerant of high temperatures and of environmental fluctuations. The welfare imperative here is to ensure that enough is known about the biology of the species and strains concerned to allow their welfare needs to be met, before intensive farming of such fish gets under way and lessons have to be learned by trial and error.
Besides the expected steady increases of temperature, fish in exposed farming systems will also experience an increasing number of acute events such as storms, algal blooms and heatwaves, which can only have adverse effects on their welfare (Table I) . Little can be done to protect fish against such events, except making good use of available computational tools to predict these accurately and to have systems ready and in place to protect them.
In conclusion, measures to mitigate the effects of climate change could include, for example, upgrades in cage design, making the stronger and submersible; preference for resistant fish strains and species; relocation of fish farms; and improvement of fish monitoring systems and weather forecasting. All these responses will be challenging, but the fact that production and economic goals will be pulling in the same direction as the demands of protecting welfare is one reason for optimism.