**3.2 Redox balance in response to environmental stress conditions**

Another of the most relevant aspects in animal culture is the exposure to continuous environmental stressors and, in the most cases, fish are challenged to various types of abiotic or biotic stressors simultaneously. Whereas in the wildlife, fish have the freedom to migrate to locations where environmental conditions are within their tolerance range or to escape from disfavour ones, in culture conditions their confinement makes the escape impossible and entails the need to face up stressors with physiological responses. The classical abiotic and biotic stressors for fish culture are infections, parasites, changes in water salinity, exposure to dissolved heavy metals, the decrease in oxygen availability, the food access limitation, human handling, higher densities and mainly, at temperate latitudes, the natural and seasonal variations in water temperature. From the last two decades, the implications of physiological redox balance to face up culture conditions stressors are of the main interest for scientists and farmers. Some of the most relevant works are summarized in **Table 2** and discussed below.



#### **Table 2.**

*Last decade works related to environmental stress in Mediterranean teleost fish.*

## *3.2.1 Food deprivation*

Food deprivation in wild fish, is a common fact and the physiological impact tends to be less aggressive than in higher vertebrates. Starvation reduces energy metabolism and cellular activity, resulting to lower oxygen consumption which can lead to oxidative stress caused by hypoxia [44]. Due to the economical repercussion for the industry, fasting is widely described for the vast majority of fish, classically focusing in metabolism and growth. However, few studies approach to redox balance and on the consequences of an unbalanced oxidative context. For instance, long-term starvation (1–2 months) in sea bass, *Dicentrarchus labrax* altered the redox balance in red muscle, white muscle, intestine and liver. Moreover, refeeding period has been demonstrated as a crucial and additional stressful period [44]. Alterations in redox balance seem to be tissue dependent, but triggering LPO in liver. However, food deprivation and refeeding enhanced antioxidant enzyme activities in intestine, and decreased SOD activity in red and white muscle, and CAT and GPX in white muscle.

#### *3.2.2 Salinity*

In seawater, salinity variations influence physiological processes and condition fish species abundance, being in consequence, one of the most determinant environmental stressors. Some species are tolerant with variable salinities, euryhaline species, and could be related with its migratory behaviour. Thus, the physiological adaptative strategies to cope with the changing environmental salinities were very recurrent on fish biology studies. Recent studies suggested that changes in salinity may also induce oxidative stress compromising antioxidant defences. However, it becomes difficult to extrapolate a solution under culture conditions after evaluating the response to different stressors independently. For that reason, the current trend in salinity studies is to combine different stressors to evaluate the joint response. For instance, [45] studied the combined effects on the sea bass redox balance of the salinity and the High Environmental Ammonia (HEA) The results evidenced the antioxidant defences strength in this specie in low-saline seawaters (up to 10 ppt) remaining unaltered even with increased HEA levels. However, it seems to be a limit at hypo-saline environment (2.5 ppt) in combination with HEA exposure where antioxidant defence were compromised.

#### *3.2.3 Hypoxia*

As it was referred above hypoxia is one of the main factors resulting in an oxidative attack in fish as in mammals. However, the mechanisms of hypoxia-induced

**101**

temperatures LPO was higher.

*Redox Balance Affects Fish Welfare*

temperatures [48].

*3.2.4 Temperature*

*DOI: http://dx.doi.org/10.5772/intechopen.89842*

oxidative stress remain still unclear. One of the hypotheses supports that a reduction in the mitochondrial electron-transport chain efficiency may contribute to ROS generation [46]. Sustained severe hypoxic conditions could trigger fish mortality. As an example, [47] demonstrated in *Sparus aurata* an increase of LPO, increased CAT activity and reduced GR and GPX hepatic enzymes in response to environmental O2 fluctuations after an hypoxia exposure. All these effects seem to be mitigated by the dietary supplementation of seaweeds, suggesting its protective role against oxidative stress. The principal causes of hypoxia in the sea are crowding and the increase of water temperature, being the climate change one of the main effectors. Because climate change is currently a trending topic, some researchers have focused their efforts on finding new strategies to mitigate the effects of high

One of the most relevant environmental inputs is thermal variations, being a challenge for poikilothermic animals. When the seawater temperature escapes from the limits of intraspecific tolerance, wild animals can respond in different ways, being the physiological escape (migration) one of the most common responses [2]. Under culture conditions, animals are not able to migrate and are obligated to face these temperatures, forcing an adaptation and implying physiological changes. Focusing on the physiological 'symptoms', loss of appetite would be the first response to stress due to low temperatures [49]. Due to the productive interest and the evidences of the increasingly extreme seasonal temperatures, great efforts have been devoted to improving animal welfare during the thermal changes, being the influence on metabolism and redox balance analysis key topics. In last decade, several studies have introduced novel techniques in fish to evaluate the redox balance. Ibarz et al. [50] approached proteomics to evaluate the cold exposure in sea bream liver. Their results demonstrated that after 10 days at 8°C LPO increased by 50% and antioxidant proteins such as betaine-homocysteine-methyl transferase (BHMT, related with glutathione synthesis), GST and CAT were downregulated, suggesting that this species are very sensitive by low temperatures. The warming response was also evaluated after 10 days at 24 and 28°C, evidencing that growing temperatures also increase LPO and stimulates CAT and GR activities in several tissues like heart, muscle (white and red) and liver [48]. In addition to acute exposure to thermal stress, some studies have focused on the medium-long-term effect of temperature challenge, trying to understand the physiological response. In this way, Sánchez-Nuño et al. [21] described the redox balance behaviour (in liver and in plasma) after 50 days of cold exposure at 14°C. Cold exposure compromised antioxidant enzyme activities mainly CAT and GR, which subsequently affected the glutathione redox cycle and caused an acute reduction in total hepatic glutathione levels. During temperature recovery, antioxidant enzymatic machinery was gradually restored but the glutathione redox cycle was not recovered. Despite field studies are not very common, it was evaluated the effect of seasonal temperature fluctuation in heart, liver and muscle to understand the adapted physiological state, including redox balance [51]. Results evidenced clear seasonal metabolic patterns involving oxidative stress during summer as well as winter, but more prominent during warming because of the increased aerobic metabolism. During cold acclimatization and under increased

By its way, [52] evaluated in sea bass white muscle the LPO and CAT activity when increasing temperature from 16–18, 24 and 28°C and maintained during 15 and 30 days, describing that temperature rise increase LPO and CAT activity. The exposure time conditioned the response, evidencing an acclimation after 30 days at

### *Redox Balance Affects Fish Welfare DOI: http://dx.doi.org/10.5772/intechopen.89842*

oxidative stress remain still unclear. One of the hypotheses supports that a reduction in the mitochondrial electron-transport chain efficiency may contribute to ROS generation [46]. Sustained severe hypoxic conditions could trigger fish mortality. As an example, [47] demonstrated in *Sparus aurata* an increase of LPO, increased CAT activity and reduced GR and GPX hepatic enzymes in response to environmental O2 fluctuations after an hypoxia exposure. All these effects seem to be mitigated by the dietary supplementation of seaweeds, suggesting its protective role against oxidative stress. The principal causes of hypoxia in the sea are crowding and the increase of water temperature, being the climate change one of the main effectors. Because climate change is currently a trending topic, some researchers have focused their efforts on finding new strategies to mitigate the effects of high temperatures [48].
