Restocking Fish

Until a short while ago, coarse fish where available from a wide variety of sources for restocking angling waters. Most were netted from our own waters and either delivered im-mediately, or kept in holding ponds at the fish farm until sold. Some, however, came from abroad and had to be netted in European waters or bred on fish farms. Increasingly alarmed at the risks involved in im-porting fish from abroad, River Authorities virtually banned the im-portation of such fish in the early 1970s – a necessary move to prevent bringing new diseases into the country, or accidentally importing undesirable ‘alien’ species. Since then, coarse fish have come mainly from our own stocks, but the demand is great and there is a need for farm-bred fish.

Most fish farms on the Continent are able to breed coarse fish by semi-natural methods; the healthy, mature parent fish are placed in specially constructed ponds and allowed to breed in the normal way.

The types of pond in use vary in design, but the most familiar is the Dubisch pond.

This is a shallow pond dug on a suitably sheltered site and is often surrounded by a fence or hedge to reduce the cooling effects of the wind. The bottom and sides of the pond are usually lined with clay to prevent water leaks, and there is a water inlet pipe to fill the pond. The water level is controlled by a sluice situated in one corner of the pond. The main pond is about 1ft deep in the centre, and is surrounded by a deeper channel.

Just before breeding, the central platform is covered with turf and or brushwood to provide a suitable site for egg laying. These also help to introduce a supply of small animals which are a readily available food source for the young fish when they hatch. The pond is then filled and a filtering system on the intake pipe ensures that large insect larvae (which might prey on the fish) are kept out. The parent fish in breeding condition are introduced, and providing the water temperature is sufficiently high and stable breeding should occur in a few days.

After spawning, the parent fish are removed and the eggs left to hatch naturally; this takes four or five days in carp and tench. The young live on the food reserves in their yolk sacs before turning to the small live food items. When the young are required for larger ponds, the water level in the Dubisch pond is lowered and the fry netted from the deeper channel near the bank.

These systems of fish culture have been used successfully in Britain, though changes in the British climate and fluctuations in temperature near spawning time can reduce or rule out the chances of success. (Carp and tench, the two species that are most easy to rear by the Dubisch pond method, in nature fail to spawn during cold summers.)

Recently, solar heating has been investigated as one method of ensuring that the pond is kept at the necessary temperature without investing a large amount of money in expensive heating systems. By covering the ponds in clear plastic or glass canopies, a kind of greenhouse is formed. It now seems likely that commercial breeders and water authorities will use solar heating to help breed fish in this way.

Several other methods of breeding coarse fish are in regular use on the Continent, and the most widely used are the ‘induced breeding’ techniques. These involve the injection of hormones (often called chemical messengers) causing fish to breed at the time of the year that best suits the fish farmer.

The development of the sex cells of fish is under the control of part of the brain called the pituitary gland, and this releases hormones into the blood system. When these reach the sex cells, they stimulate their final development and cause the release of the eggs and milt. The pituitary gland itself is controlled by other parts of the brain, and until the correct environmental conditions occur (temperature and length of daylight, for example), the hormones are not released. It was discovered that if the pituitary glands from mature fish were removed and the hormones extracted, an injection of the extracts into fish in breeding condition would cause them to spawn. This hormone injection technique is the basis of induced spawning.

Induced spawning method

The actual details of the method are quite sophisticated; the female parent fish is given an injection of the hormone (the size of the dose depends on its weight), and about 12 hours later, both male and female fish are given a further injection. Soon after, it is possible to strip the fish of their eggs and milt. These are mixed with water in a bowl – a feather is often used – and the fertilized eggs are washed in salt and urea solutions to prevent them from sticking together.

The fertile eggs are incubated in specially made bottles called Zoug jars – large inverted bottles open at the base, linked to a pressurized water tap. Warm water is passed up into the bottles, round the eggs, and flows out over the top of the bottle. In carp, the species most often raised by the Zoug jar method, the eggs require a constant water temperature of about 20-25°C.

When the eggs hatch after four or five days, the young fish are placed into tanks where they quickly use up their yolk sacs and then live or dried food is given to them.

Though these methods are still new to Britain, several fish farmers are using them with success and more will no doubt follow their lead.