General introduction to fish feeding
Feeding and fish nutrition are fundamental aspects of aquaculture, both in terms of fish growth and in economic terms. Proper feeding depends on the development of quality feeds and on choosing appropriate methods to distribute the feed to the fish in the tanks. Apart from affecting growth, feeding can also affect fish health and welfare, which depends in turn on how much we know about the requirements of each species. Each species has its own natural history and well defined stages of growth, which should be understood in order to provide optimal care.
The candidate fish species for aquaponics (see Chapter 3, Table 1) occupy well defined ecological niches in their natural habitat. For that reason we need to provide adequate conditions for proper development, including housing conditions, which means defining the correct temperature, salinity, water quality, and speed of water flow. Normally the most demanding phases are the maintenance of breeders and the fertilization/incubation of ova or eggs, but aquaponic production will normally be dealing with later stages, usually called ‘on-growing’. As the scale of aquaculture and aquaponic farms increases, it becomes more complex to maintain a large number of production phases in the same installation, so companies become specialized in one or two stages, such as breeding or on- growing. In the case of aquaponics, where fish are maintained in recirculating aquaculture systems (RAS), we normally use juveniles which are grown to adults, aiming to simplify the fish production part of the system with only one or two phases, if possible.
In general terms, feeding in aquaculture differs in some fundamental aspects compared to terrestrial mammals. Livestock on land normally self-feed using what are known as ad libitum feeders (each animal can choose when to approach the feeder and how much to eat at any given time of the day). In that case it is relatively easy for the farmer to detect the ration that was really ingested. In the case of aquaculture and aquaponics, fish can also use self-feeders but it is much more difficult to judge how much feed they actually consume. The danger is that any extra feed that falls into the water and is not ingested becomes waste that ‘pollutes’ the system. Efforts need to be made, therefore, to estimate the feed to be distributed and the precise ration that the fish need.
One way to distribute the feed is by hand from outside the tanks, spread over the whole surface area of the water, observing the behaviour of the fish until they seem to be satiated, and then feeding is stopped. Since the fish are feeding underwater, it is not that easy to know when they stop feeding or how much they ate, or even if some fish ate more than others. The more we know about a species, the more we know about their feeding habits. For example, Nile tilapia in the wild are omnivorous when young (juveniles), eating both zooplankton and phytoplankton, while they become more herbivorous as they get older (> 6 cm long) (FAO 2018). Trout, on the other hand, are mostly carnivorous throughout their lives, with a diet almost exclusively based on insects and any smaller fish they can manage to catch. In any case, the perception and knowledge of the people who are in charge of feeding is very important, especially if feeding is done manually. For more information on the feeding habits of different species, see the Aquaculture Feed and Fertilizer
Resources Information System, run by the Food and Agriculture Organization of the United Nations (FAO 2018).
Another way is to use automatic feeders instead of manual feeding. Here we might depend on technological developments such as underwater cameras to detect when the fish are no longer eating. All the feed that goes into the tank becomes a part of the system, whether it is eaten or not. Indeed, fish feed is the main external element of any aquaponic system and should be carefully controlled. Non-ingested feed remains in the tank and causes two problems, one associated with its cost and another associated with its elimination. These two problems underlie the need for adequate designs.
The hydraulics of the system should facilitate the removal of the uneaten feed. Normally this involves tapering the tanks so that the bottom part is narrower than the top, and promoting a swirling motion or current so that faeces settle on the bottom and can be removed efficiently. If the design is deficient, cleaning will be more complex and the fish may be bothered by the frequency of maintenance routines. Any decrease in the sanitary conditions of the tanks will have immediate consequences on the welfare of the fish, and on the profitability of the farm. So, even if we know the nutritional needs of the species, a poorly designed installation will make it difficult to provide adequate requirements for good fish welfare, and feed will be wasted.
Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).