For the 2 main species produced in European aquaculture, Atlantic salmon and rainbow trout, their breeding characteristics are described as being anadromous.
The word anadromous means that spawning and the first phase in life occurs in freshwater. The later stages, growth to the adult and mature phases, take place in the sea.
Other species of fish can be catadromous which means that the fish will normally live in freshwater but will migrate to the sea for spawning and breeding. Eels are an excellent example of a catadromous species.
In professional aquaculture, only fish of the highest quality are used for breeding in order to obtain the best results. For both salmon and trout, the fish are stripped of eggs (from the female) and milt (from the male. This is done in the early autumn for salmon and in the winter for trout.
The milt and the eggs are mixed together, in vitro, in very hygienic conditions, for fertilisation of the eggs. Because the embryo is black in colour, a fertilised egg resembles an eye and, as such, is commonly referred to as an ‘eyed’ egg.
The eyed eggs are laid out on special trays over which clean, cold freshwater runs over.
This is usually done in a hatchery where control over the photoperiod is often done. This is made to simulate day-length and ‘inform’ the young fish that it is time to develop.
Several weeks later, the eggs hatch and the fry (the second stage in the life-cycle) emerges from the egg.
A large yolk-sac, attached to the stomach of the fry, contains the nutrients that the fry needs until it reaches a size where it can feed. This period lasts several weeks.
The fry are fed with a ‘rich’ feed, containing vitamins, proteins, oils and other essential nutrients to assure healthy development. As for all other animals, this is a very sensitive time in the life-cycle and extreme care has to be taken in the husbandry process.
3. The Juvenile stage
As the fish develop, the natural dietary requirements change and the feeds available to the European fish farmer reflect this phenomenon. More than 15 different formulations may be used during the life-cycle and it is the combination of the skills of the farmer and the feed maker that assure healthy development.
Once rainbow trout have reached the size of 60-90 grams, they can be transferred into marine conditions. However, salmon have to wait until they are 12-18 months old at which time they are termed smolts. This represents a physiological change which allows the fish to pass its life in the sea. In the wild, this change is the signal for the salmon to move to the sea.
4. The Adult stage
Growth of the fish to the adult stage is done in floating cages where the installations try to mirror nature as efficiently as possible. Large, deep cages with natural water changes from tidal currents provide an excellent medium for production. After 2 further years of development, the fish become mature. At this stage, in the wild, the fish would return to their river of origin for spawning and the start of a new life-cycle