Main interactions between ingestion and environmental factors
As commented above, we should be able to house each species according to its requirements. For that we first need a profound knowledge of the species that we are going to work with before we begin to grow the fish or start the installation. Once we have this information, we should be able to maintain the adequate housing conditions in our system, which in this case is related to aquaponic systems.
Abiotic factors
The main environmental aspects to consider and that have a direct effect on production are the following:
Physico-chemical parameters of the source water, which are independent of the aquaculture activity itself:
Water temperature, which regulates all metabolic processes
Water salinity or conductivity
Turbidity and total suspended solids
Any potentially toxic compounds in the source water. The initial quality of the water is one of the basic success factors in the installation
Physico-chemical parameters of the tank water:
Dissolved gases: fundamentally oxygen, which should be monitored continuously and is required by fish for normal function. In parallel, carbon dioxide is produced by fish respiration, and other gases are present in the circuit, such as nitrogen (that can appear during the over-saturation of pumped water), or hydrogen sulphide or methane from the anaerobic decomposition of sediments
Dissolved micro- or macronutrients, which are related to the feed, including several elements vital for the development of the fish, such as phosphorus, iron, and especially the nitrogenated substances excreted by the fish
Biotic factors
Different species of fish are extraordinarily diverse in terms of their social requirements, such as stocking density. Historically, fish chosen for aquaculture are robust under different conditions, which makes it easier to choose adequate management. That includes carrying out daily tasks on the farm without generating many sanitary complications in the fish. This is also the case for aquaponics, where the most popular fish is tilapia, well known for its hardiness.
However, in the beginning, we first had to domesticate wild species, which were normally difficult to manage, reproduce, and grow, but had a high economic value. That high value covered the costs of production of delicate species. A clear example is rainbow trout, which in the beginning was a very complex species, hard to produce and manage, even though now it seems relatively simple. Any poor management and inadequate movement of the fish produced stress and even loss of scales, which led to infections that brought on or facilitated disease and other common problems of fish that are stressed. Examples of species that are currently being domesticated, and have not reached their full potential in aquaculture, are burbot (Lota lota) and grayling (Thymallus thymallus). Technological development and accumulated knowledge have drastically improved the techniques used in the routine operations on farms, such as sampling of the fish, counting the fish, movement of live fish, etc. The main aspects that will influence the welfare of the fish in the tanks include:
Social structure: depending on the species, some are quite territorial, and we must manage these characteristics in the tanks. For example, we know that trout are quite territorial, and that they require frequent size grading during the initial phases of growth in order to avoid the appearance of dominant fish that will damage the smaller fish. In that case it is better to keep the fish within a narrow size range in separate tanks in order to improve production. We also know that tilapia and Clarias species show two different modes of behaviour: territorial if at low densities, and swarming/schooling if at high densities. Thus, low densities are not always better for all fish species.
Fish density: each species has a minimum and maximum stocking density below or above which problems may arise and fish welfare will be jeopardized. Density is normally measured in kg/m3 and varies depending on the system. Some high output industrial RAS systems grow tilapia above 60 kg/m3 but normally aquaponic systems use lower densities, around 20 kg/m3 (see for example the Aquaponic Gardening Rules of Thumb), although values can range widely depending on fish size and RAS system.
Human disturbance: this depends on the species. Tench (Tinca tinca), for example, are quite flighty, and can hurt themselves by bumping into the tank walls when disturbed or even when they notice human shadows. One solution is to put curtains around the tanks to avoid being seen, or to set tanks on rubber supports to minimize vibrations from human steps or machines.
Prey or feed: the size of the feed should be appropriate for the size of the fish, and distributed throughout the tank so as not to promote dominant fish. Otherwise less proactive fish will not gain weight and tanks will need to be size sorted more often, which is stressful.
Predators. The presence of predators, such as cats, dogs or birds close to the tanks, can stress the fish a lot, and contact needs to be avoided by using artificial boundaries such as fences.
Loud noises, such as music (especially a strong bass sound) can be stressful for fish as well.
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).