Harvesting and handling of the fish

When harvesting fish, we always must keep in mind that they are—like all other vertebrates—animals that experience pain and fear if they are mistreated. Thus the fish need to be handled with care and respect, especially in the context of organic farming, and harvesting must be done as gently as possible.

A seine net, which is pulled by two persons along the complete pond, is one of the best solutions for harvesting larger numbers of fish and causes little stress and panic among the fish. If the water level of the pond can be lowered, this makes the job for the fish farmer easier. If the pond can be emptied to a harvesting depression near the outlet, the fish can be caught using a scoop net. When the fish realize, however, that their space to move around in is getting continuously smaller, they inevitably panic. This means that quick action is required.

If there are many fish, harvesting the fish out of the filled seine net or out from the harvesting channel with the help of a scoop net should be done by several people simultaneously. From the pond, the fish must be transferred to a clear water tank or to a transport tank in case they shall be transported to a slaughter house.

If it is not intended to transfer the fish alive to another place, the fish must be killed immediately after being taken out of the water with the scoop net. It is unacceptable to let the fish die slowly outside the water by suffocation. The best way to kill small and medium numbers of fish is to strike them by a blow on the head using a beating wood.

If ice is available to make water/ice-slurry, then bigger numbers of fish can be transferred at once into a prepared tank. To prepare the slurry, one part crushed iced is mixed with one or two parts water. The ice should not melt completely. When the fish are transferred from the pond water (~30 °C) to the ice slurry (~0 °C) they will be anaesthetised almost immediately. But as they are not yet dead, they must be killed by a gill cut to let them bleed out.

Tilapia sometimes tends to have an off-flavour or muddy taste that mostly comes from the consumption of specific algae (known as ‘blue-green algae’) that grow in the pond. It is difficult to control these algae, but the muddy taste of the fish can be reduced by transferring the fish for two or three days to a tank or basin with a good supply of freshwater and air before harvesting. During this time, the fish do not get fed.

The necessary water flow depends on the number of fish in the tank and the water temperature. If the stocking density in the tank is 30 kg per cubic meter, the water flow must be sufficient to exchange the water in the tank once every five to ten hours. In addition, a blower should bring additional air to oxygenate the water. For example, if the stock of fish that was harvested is 300 kg (this might be around 800 fish with an average weight of about 350 grams), then the tank the fish are kept in should have a capacity of about 10 cubic meter (10,000 litres). The flow of water therefore should be between 2,000 and 1,000 litres per 5 hours. This corresponds to about 7 litres (1st day) to 3.5 litres (the following days) per minute. If less water is available, only smaller batches of fish can be kept in the tank (meaning lower stocking density).