Redox Balance Affects Fish Welfare

*Sergio Sánchez-Nuño, Teresa Carbonell and Antoni Ibarz Valls*

## **Abstract**

Aquaculture is a growing industry that is increasingly providing a sizable proportion of fishery products for human consumption. Thus, in the last years, several efforts are made in improving fish welfare. As well as in the rest of vertebrates, fish welfare is sensible to a balanced redox status. Numerous inputs like diet and environmental factors could alter this balance. In this sense, the last feeding strategies are focused on developing a more sustainable aquaculture, trying to maintain a redox balance. On the other hand, under culture conditions, animals cannot migrate to more favourable conditions, and environmental stress is one of the most relevant inputs that could compromise redox balance. This chapter is focused on the review of last works in redox balance analysis in Mediterranean aquaculture species and is organized as follows: (1) redox reactions on poikilotherms versus homeotherms; (2) effect of feeding strategies and environmental stress in fish redox balance; and (3) wide vision in fish redox balance.

**Keywords:** antioxidant enzymes, glutathione redox cycle, lipoperoxidation, protein oxidation, Mediterranean aquaculture

## **1. Introduction**

In the recent decades, the increase in demand of fish products has boosted the development of aquaculture, being today responsible of the major supply of fish for human consumption. Currently, aquaculture is growing faster than other major food production sectors. In 2016, 88%, more than 151 million tons (t) of the fish produced was destined for human consumption (FAO). Thus, the potential of oceans and inland waters could be fundamental for human nutrition in the coming years, due to the expected demographic increase.

Classically, the vast majorly of aquaculture research have been focused on growth studies, feed efficiency, larval maturation or fillet quality. However, as in all intensive animal exploitation, fish culture presents different alterations and pathologies affecting fish welfare, causing great economic losses to the aquaculture sector. Rearing conditions in sea cages expose animals to several stressful inputs like crowding, feed competence, predators and environmental parameters (tides, salinity, temperature, etc.). These stressful conditions affect fish welfare, being more susceptible to infections or even triggering mortality [1, 2], causing important economic loss. To evaluate the individualized effect of each of the inputs and also to test new feeds, aquaculture research applies indoor models to evaluate the cellular and physiological responses. Therefore, a growing interest about fish welfare exists, being the oxidative stress and the fish redox status a new focus of research.

Like to the studies in mammals' species, the most of fish approaches on redox aquaculture subjects were focused in liver as the main physiological organ. Liver shows powerful enzymatic antioxidant machinery, it is involved on glutathione synthesis and it is a main target of reactive oxygen species (ROS). However, digestive tract, white and red muscle even plasma are of a novel targets for redox studies in fish.

Despite the bibliography reporting redox markers in the last years are increasing, there are only few works tackling on the redox balance and the consequences of its alteration in fish welfare. This chapter aims to evaluate the most relevant and recent studies, and the advances of oxidative status, antioxidant defences and global welfare in fish. Along the chapter we will be focused on the studies which report dietary effects and environmental challenges on fish culture.
