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As with all living animals, fish require energy, and that energy is provided by the oxidation of the organic components in feed. Fish require energy to carry out their daily activities, such as breathing and swimming, and to transform, restore, and grow their body tissues. The energy requirements of fish depend on their physiological state and on the environmental conditions. In general fish make a more efficient use of the energy ingested compared to terrestrial mammals, due to the following reasons:
Aquatic species are poikilotherms, which means that their body temperature is the same as the surrounding water, so they do not need to spend energy heating up their body or keeping it at a constant temperature, as occurs with terrestrial livestock;
Since they live in water, fish do not require a strong body skeleton to support their weight under the full pressure of gravity, as in terrestrial livestock, nor do they require the costly metabolic processes required to maintain that skeleton;
Nitrogenous waste in fish is eliminated as ammonia directly from the gills which consumes less energy than having to make urea or uric acid and then eliminate it, as is done by mammals and birds.
Figure 1 provides an overview of the balance of nutrients and energy in fish. If we assume that it has ingested all the feed provided, the energy is distributed percentage-wise among different physiological processes, within ranges. If maintained under stressful conditions (poor lighting, low water quality, inadequate stocking densities), where the fish are alive but not comfortable, about 40% of the feed energy will be consumed just to cope with the stress, leaving only 30% for growth. On the other hand, under optimal conditions, fish will use up to 40% for growth. Obviously, the economic viability of an aquaponic system will depend on the optimal use of the energy provided. To do that we have to ensure that they ingest all of the feed, and that we provide optimal housing conditions so that the fish are not overly stressed.
Figure 1: Balance of nutrients and energy for fish kept in recirculating systems
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