The present invention relates to a method for commercial edible fish rearing of Euro- asiatic pike-perch, Stitzostedion lucioperca, as specified in the pre-characterising clause of claim 1.

A number of types of fish, such as rainbow trout, for example, are nowadays reared commercially by a large number of edible fish breeders. Common to all the types of fish reared is that that can be reared on dry feed.

Euro-asiatic pike-perch (Stitzostedion lucioperca) is a predatory fish, which as fry normally live on plankton and insect larvae, later going on as young fish to eat fish fry and crustaceans and then, as adult fish, eating only fish. Commercial edible fish rearing of Euro-asiatic pike-perch is not possible at present since the pike-perch is a predatory fish, which in rearing would need to be fed with live feed. Rearing would also be rendered more difficult due to the fact that the reared fish, if not strictly sorted into various sizes, would eat one another. The rearing of live feed in such quantities that they could be used for commercial rearing of edible fish becomes both complicated, resource-intensive and very costly, which is why, as things stand at present, no such rearing is undertaken. At present rearing is confined to fish for release, which are then allowed to grow at liberty in the watercourses into which they are released.

An object of the present invention is to create a method of the type stated in the introductory part, which will render the commercial edible fish rearing of Euro- asiatic pike-perch (Stitzostedion lucioperca) possible. The object has been achieved by a method which has the characteristics specified in claim 1.

Preferred embodiments of the method also have any or some of the characteristics specified in the subordinate clauses.

The method according to the invention allows the pike-perch to assimilate the energy present in non-live low-calorie feed. By means of the method the pike-perch can develop from a certain larva stage to fry and then to fully grown fish using dry feed as the sole source of nutrition. In this way the commercial rearing of edible fish is possible in re-circulating systems or cages.

The invention will be explained in more detail below with the aid of an example of a preferred embodiment of the present invention and with reference to the attached drawing, which shows a system diagram of the rearing method according to the invention.

The rearing method illustrated in the system diagram consists in a method of getting the intestinal system of the pike-perch to accept dry feed through enzymatic opening of the intestine at the larva stage. In the fry stage and the adult stage the pike-perch continues to assimilate dry feed and grows in the fish farm in the same way as fish of the salmon family. Due to this treatment during the larva stage the pike-perch also loses much of its natural hunting instinct, so that it can be reared by aquaculture using the dry feed as sole nutrition without the fish starting to eat one another.

In implementing the method according to the invention a medium consisting of rotifera and Cladocera together with Copepoda is first prepared in a hatching and larva tank. In parallel with this, the fertilised eggs are developed in aqueous mist under approximately 110 dew degrees until they are ready to hatch. Before the mature eggs are delivered to the hatching and larva tank, Cladocera and Copepoda therein are eliminated in a known manner by the addition of organic phosphoric acid (lg/m3 of water). This elimination is performed in order to increase the growth of rotifera. Before hatching, the quantity of rotifera in the water of the hatching and larva tank is then checked. The quantity of rotifera must preferably lie within the range 0.5-2 ml of biomass per 100 litres of water when the mature eggs are released into the hatching and larva tank.

The mature eggs are then released into the hatching and larva tank, where they will hatch, following which the larvae initially feed on their yolk sacs, before eventually beginning to eat rotifera. Around 2 ml of biomass composed of Cladocera and Copepoda is now added to the hatching and larva tank to be developed for the last part of the enzymatic activity. During the first phase, which lasts about 10 days, the temperature of the tank must preferably be approximately 22°C. Towards the end of the ten days the Cladocera and Copepoda will have grown in the hatching and larva tank and are now present in abundant quantity for the pike-perch, which now go over to feeding on these. During this time the enzymatic activity is initiated in the intestinal system of the larvae. The enzymatic activity is then allowed to continue for around a further 20 days at the same tank temperature, that is to say approximately 22°C. The pike-perch now leave the larva stage and enter the fry stage. When the fry have reached a length of approximately 25 to 35 mm they are sorted into a number of

size categories, appropriately 2 or 3 groups, in order to reduce the competition within the groups.

From this stage onwards a periodic bathing of the fry in water with a salt concentration of approximately 0.6% must be performed for around 5 minutes every fifth day, in order to prevent parasitic and fungal attack.

Following the preliminary stages the 25-35 mm long fry, sorted into groups and after salt bathing as outlined above, are transferred to rearing tanks with a maximum salt content of 0.5%. The enzyme and the biological process from rotifera and Cladocera and Copepoda has now prepared the intestine of the fry so that they can assimilate the energy in a dry feed. Accustomisation is begun with a suitable dry feed with a free-flowing character and a feed diameter of around 1 mm Periodic bathing for 5 minutes in water with a salt content of 0.6% every fifth day must be continued until approximately 60 days after hatching. During this time any predators are also sorted out.

Repeated sorting according to size can hereafter be undertaken, as required, and the fish divided up between further rearing tanks for growth.

After about 60 days from the date of hatching the temperature in the rearing tanks is gradually increased to approximately 28°C, following which continuous growth commences, during which the young fish change from fry into fish. When the fish have reached a stock fish stage with a length of approximately 150 mm they can be placed in rearing cages or a recirculating fish farm for continued growth.

Growth then occurs with controlled supply of dry feed and the fish can be transferred to larger tanks or cages, as required. It has emerged from experiments that the rate of growth can be further increased by controlling the temperature and light during the period of growth. Between 12 and 24 months after the stock fish stage the fish, depending on the type of rearing, will have reached a size at which killing can be begun. If the fish are transferred to and reared in a recirculating system with a water temperature in the order of 25-28°C, they will be ready for killing after just 12 months. If the fish are transferred to and reared in cages, the rate of growth will depend on the temperature in the watercourse in which the cages are placed, which usually means a water temperature of essentially less than 20°C, so that the fish will only be ready for killing after approximately 24 months. The fish can finally be killed and distributed in the known way.