How are fish farm marine fishes born and grown?
A fish species can only be reared once the reproduction and the rearing of the juveniles in large quantities have been mastered. For example, the sea bass, the sea bream, the meager, the sole or the turbot, grown fishes are reared all year long in large basins containing several tens of cubic meters of water. Once temperature and light conditions offer an opportunity, the fishes enter sexual maturation and reproduce. The eggs fertilized by the male are, as soon as expelled by the female, spread and collected in an incubator located behind the water exit of the basin. It will takes them 2 days to hatch as tiny larva which need to be fed before the end of the first week because of their very limited energetic reserves. At this stage, larva only accept small zooplankton (rotifer and brine shrimp) which has to be farmed aside in the hatchery and frequently distributed.
The larva slowly obtains the shape and behavior of a small fish. After about one month, they are weaned on an inert food which contains all nutriments required for their health and growth. Juveniles are frequently sorted and only the smallest move through bins with slots and accurately separated. This way they are periodically distributed among the numerous basins of the hatchery. When they weight between 1 and 2 grams, which is between 2 and 3 months after they were born, they are vaccinated against some bacterial diseases they might later be exposed to. The first vaccination consists in immersing the juveniles for a minute in a container filled with sea water in which the vaccine has been diluted.
Between 2 and 3 grams, the juveniles are loaded in oxygenated tanks, transferred by trucks or sometimes by boat to the farms and then released in basins or marine cages. A modest-sized farm needs around 300 000 juveniles each year to produce a hundred of tons of fishes, sold, after 2 years of rearing, as a one-person portion size.
A marine cage is usually composed of a flexible floating tube which circles and supports a weighted net. This net is often replaced and cleaned from seaweeds which colonize it. The size of its net mesh are adapted to the size of the fish. Their volume vary accordingly to the size of the fish and the exploitation, from the size of a room to the size of a big house. In large farms, for example salmon in Norway, marine cages may occupy the volume of a middle-sized building. In France, if we put side by side the cages of all our marine farms, we would cover a hectare, a derisory surface compared to the immensity of our marine coastline.
An unlike rearing method, the particularities of fish farming
For over 10.000 years, in order to emancipate from the constraints and doubts of the hunting and to ensure its subsistence, Man has domesticated animals which all live on the ground and are able to reproduce or be farmed in a closed, or reduced, environment. In France, fish rearing only started to grow a century ago for freshwater fishes, and for only 30 years for marine fishes. This is likely due to the fact that skills and environment were overlooked at that time. In addition, fish is the only husbandry vertebrate who live, eat, breathe and reproduce, but also urinate and deject… in the same environment. This implies that any deterioration in the quality of the water it lives in is incompatible with its own survival. It is its own sentry and the fish farmer will thus pay a lot of attention in preserving this aquatic environment.
Another particularity: Fishes reared in Europe are carnivores. Animals farmed on land are for their part herbivorous or omnivorous. It is due to the fact that the fishes the most demanded by the customers and thus having the highest commercial value are carnivorous: sea bass, sea bream, turbot, sole are usually more demanded than mullet or even sardine. In order to fill their nutritional needs, farmers must give those fishes or at least products of marine origins such as fish-meal and fish-oil. In addition, a fish is at the water temperature and endures its effect. Its appetite and growth greatly rely on the weather, the quality and the temperature of the water. The quantity of food distributed to each batch of fish must be adjusted constantly, sometimes from day to day, such as reducing it, for example, in case of temperature drops.
French research teams for fish food are among the best in the world. Thanks to a better understanding of its needs, the proportion of marine origin products in the food given to trout has been divided by 3 these past 20 years and research are still ongoing to continue decreasing this part of marine ingredients given to all reared species. The marine fish food is currently composed of about a third of marine origin products and two thirds of vegetal products mixed and pressed in the shape of pellets accordingly to the species and size of the fish.