by Iris Kröger, Anne Möddel, Dr Eckel Animal Nutrition GmbH & Co. KG
Hypoxia is a critical condition caused by insufficient oxygen saturation in aquatic environments and is particularly common in intensive aquaculture systems. The main short and long-term effects of hypoxia include increased mortality, reduced feed intake, inefficient metabolic changes and limited performance.
Furthermore, hypoxia is associated with oxidative stress and inflammation. This represents a serious risk to optimal performance. So to the tackle performance loss associated with hypoxia effectively, the fish farmers' first goal should be to ensure that there are optimal oxygen levels in the water.
However, due to various external influences, oxygen fluctuations can still occur. In addition to good farm and water management, modern feeding strategies including flavonoid-rich plant compounds have the potential to improve fish tolerance against hypoxic stress.
Hypoxic stress – The performance killer
To survive hypoxic water conditions, fish produce a coordinated response of behavioural, physiological and metabolic changes that contributes to severe performance loss following hypoxia.
For example, fish that cannot accommodate their oxygen requirements with behavioural (e.g., hyperventilation) and physiological changes (for example, bradycardia), reduce their energy requirements. This will also reduce their oxygen requirements as 95 percent of the oxygen consumed by fish is used to produce the energy provider ATP (Adenosine Triphosphate).
Therefore, one possibility to reduce oxygen requirements by up to 70 percent is decreasing the metabolic rate (van Waversveld et al. 1989; van Ginneken et al. 1997). However, this will manifest in the form of reduced performance due to the lower activity level, feed intake and reproduction rate of the fish.
Switching from aerobic to anaerobic metabolism can reduce oxygen requirements further. However anaerobic pathways that produce ATP are 15 times less efficient than aerobic pathways and will further reduce fish performance.
Despite mechanisms that reduce the ATP and energy requirements of fish in order to deal with low oxygen levels, recent research has revealed that hypoxia increases oxidative stress and the expression of inflammatory genes (Zhao et al., 2020). The close relationship between oxidative stress, inflammatory reactions and high-energy requirements of an activated immune system, leads into a vicious cycle that results in dramatic loss in performance during and after hypoxic stress.
If adaptation to hypoxic stress fails, hypoxia even increases the mortality rate of fish with severe consequences on animal welfare and farm profitability.
Hypoxia limits performance in fish due to a number of potential complications including the metabolic suppression to reduce oxygen requirements (-30 to -70 percent), the anaerobic rather than aerobic metabolic pathways (15 times less efficient), increased energy requirements due to oxidative stress and inflammation and increased mortality if adaptation to hypoxic stress fails.
Hypoxic stress – A herbal solution for everything
Apart from management measures that reduce the risk of hypoxic water conditions, innovative feeding strategies with phytogenic feed additives possess a great deal of potential for improving the tolerance of fish to hypoxia: plant flavonoids in the diet can help fish cells adapt to hypoxic stress by reducing inflammatory reactions (Xia et al., 2020), for example.
The phytogenic feed additive Anta®Ox FlavoSyn recently showed its anti-oxidative capacity for reducing inflammation by which it supports the performance in farm animals (Gessner et al., 2008; Shata et al., 2019). Moreover, Anta®Ox FlavoSyn increased the survival rate and body weight after triggering the immune response in aquaculture (Niyamosatha et al., 2015). Since it improves animals' tolerance to oxidative and inflammatory stress, the study proposed that Anta®Ox FlavoSyn could help reduce the performance-limiting effects of hypoxic stress in fish.
To test this hypothesis a study with 640 goldfish was conducted. The fish were split into groups of 40 animals and kept in 16 glass tanks (38 l) for five weeks. The total fish mass was 2770g corresponding to a stocking density of 4.56kg/m³. To induce hypoxic stress in the fish the water in the tanks was not oxygenated for four days during the first week of the experiment.
Throughout the trial, the fish received two different diets: while the control group received no feed additive, the diet of the treatment group contained 400mg/kg Anta®Ox FlavoSyn (Dr Eckel Animal Nutrition GmbH & Co KG). The body weight of the fish was determined at the beginning and end of the experiment. Results showed that hypoxic stress reduced body weight in the control group by 17 percent five weeks after experiencing hypoxic stress conditions.
In the group fed Anta®Ox FlavoSyn, body weight was only reduced by four percent. This emphasises both the huge impact of hypoxic stress, even weeks after experiencing the hypoxic situation, and the importance of protecting fish against hypoxic stress in order to achieve optimal growth and development of fish.
The test results also demonstrate that Anta®Ox FlavoSyn is an effective tool to assist fish producers in stabilising performance even in stress-inducing conditions including hypoxic stress.
Improves hypoxic stress tolerance
Mechanisms to cope with a low oxygen supply negatively affect the performance, health and welfare of the fish. The tested flavonoid combination Anta®Ox FlavoSyn improves the tolerance to hypoxic stress by preventing oxidative and inflammatory reactions. This reduces performance loss associated with hypoxic stress and helps to maintain optimal profitability even under challenging conditions. Due to adaptive mechanisms and oxidative and inflammatory reactions, hypoxic stress in fish leads to performance loss that can last for weeks after hypoxia.
