In aquaculture systems, the function-based approaches, including the physiological responses to stress, tend to dictate fish welfare. Alternatively, the five freedoms approach provides valuable guidance in animal welfare (FAWC, 2010) and encompasses all potential stressors fish can be exposed to in aquaculture systems. Animals should be free from hunger and thirst (osmotic regulation in case of fish), environmental challenges (proper water quality, appropriate temperature ranges according to the species, etc.), from pain, injury and disease, from behavioural restriction (including lack of space) and from mental suffering.

Stressors are common in the daily life of farmed fish and during their development. From egg to adults, fish are under different environmental, physical and social challenges that can trigger a stress response.  A stress response is nothing other than an adaptive strategy for coping with a perceived threat to homeostasis; that is the stable equilibrium the internal body environment wants to maintain. Animals can suffer two types of stress, acute and chronic, and both will lead to different stress responses with more or less impact on the life cycle of the fish in the aquaculture rearing system. When allostatic overload occurs, this can lead to pathophysiology and death if not corrected properly.

Over the life cycle of farmed fish, a series of stress-related issues may occur that can be of a welfare concern. Each life stage has different susceptibilities to stressors and identifying and correcting these stressors will improve the welfare of the fish being reared. Not only different stages should be considered but also different rearing systems: from controlled RAS (Recirculation Aquaculture Systems) to flow through or open sea cages, all provide us with different challenging conditions to deal with.

A list of potential farmed fish risk factors and stressors during the whole life cycle of farmed fish have been identified and matched with the farming events potentially leading to poor welfare from broodstock (if applicable) to eggs up to adult fish. Main Key factors will be identified and revised like feeding quality and delivery methods, water quality and flow, fish stocking density, transport methods and a number of transport events during the fish life cycle and main management practices and interventions like grading or vaccination up to slaughter.

If farmers want to detect welfare problems they must have good feasible operational welfare indicators (OWI).

Ideally, Operational welfare indicators (OWI) should follow a series of criteria.

Operational Indicator criteria:

• Measurable: in a quantitative way preferably, mainly by SCORING SYSTEM
• Comparable between species and different production systems
• Non-invasive
• Easy to evaluate by farm staff and non-disruptive of normal husbandry procedures
• Meaningful for all systems

We suggest a list of welfare indicators based on individual and groups of fish for the fish farmers to assess the impact of the stressors on their fish and be able to respond on time. The OWI based on individual fish will include physical health assessments and physiological parameters. The group-based OWI are mainly behavioural measures and basic environmental parameters like water quality, temperature and other like predators or enclosure design and substrate access.

Each of the risk factors and stressors during the life cycle of the farmed fish can be assessed by a welfare indicator closely related to the original stressor. For example, acute or unexpected changes in water temperature (the stressor) can lead to changes in the appetite level of the farmed fish. Monitoring and evaluation of the feeding responses is a common welfare indicator potentially related to an environmental stressor. Real-time monitoring of the fish and the environmental parameters is key for the identification of environmental-related welfare issues.

Table 5. Some OWI that could be considered for the sustainability indexes

The main challenges facing aquaculture today are still based on improving best management practices (BMPs), the importance of the training of the staff by continuous professional development courses (CPDs) available and the improvement of the monitoring systems both for the aquatic environment and the behaviour, health and welfare of the fish. A systematic integrated welfare assessment encompassing all the different aspects of the fish farming and life cycle up to slaughter should be implemented in all aquaculture systems to better understand the basic needs of fish, to reduce the stressors and stress responses and improve productivity and quality of the final product. The development of precision fish farming (PFF) with the possibility of system modelling and machine learning is a step forward to aquaculture systems and the solution for many welfare stress-related issues.

Figure 11. OWI table for salmon (from Noble et al 2018, fishwell, EU project)