Novatek gives tips on fish farming: Stocking and stocking density

Novatek gives tips on fish farming:

Stocking and stocking density

by Robert Kanyembo, Aquaculture Research and Development Manager, Novatek Animal Feeds Zambia

The topic of how much fish should be put in a production system has been of a growing concern and somewhat a detriment to aquaculture development. Many are the times fish farmers complain that their fish are not growing as they should or ask why fish die after attaining a certain weight of around 120 g and above. This is because around this stage, the biomass in the growing system would have increased as a result of increase in the average body weight of the fish stocked. Stocking by definition is the introduction of fish species into either a limnological (natural lake) or artificial system such as ponds, tanks, or cages. Stocking density relates to how much fish or weight is added per square area or cubic metre in a production unit. Stocking involves replenishing of limnological systems such as rivers, lakes, oceans, and streams to improve the dwindling fish numbers due to overfishing or natural elimination. In this context, stocking in aquaculture refers to introducing fingerlings into either ponds, cages, or tanks. It is very important therefore to understand how much fish to stock in your system and why. How then do you arrive at how much fish do you stock per production system? The answer lies in your level of management. Management plays an important role in how much fish to stock per production system. In aquaculture or fish farming, there are basically three management levels that include extensive, semi-intensive and intensive, while others also include integrated fish farming.

Extensive fish farming

This is the level of least management in which there is low capital injection with the least yield per square metre. Under this level, we recommend stocking densities of 3 to 5 fish per square metre because fish depends more on natural food than supplementary feeding. Labour and investment costs are low and involves large systems with less fish.

Semi-intensive fish farming

Under this management level, care is taken in the production system to raise fish at a higher stocking density in a smaller area or volume. Moderate investment is made, and as such, supplementary feeding is given as well as natural feeding provided by pond fertilisation. The stocking densities ranges between 5 to 8 fish per square metre with frequent water exchange.

Intensive fish farming

Intensive fish farming involves fish stocked at higher densities of between 8 to 12 fish per square metre and above, and fish fed wholly on manufactured feeds. A significant effort is made to en-

Poorly managed fishpond.

Photo: tradeindia.com.

Paddlewheel aerator. Photo: aquaculturealliance.org.

sure water quality is kept in check and aerators are also used. Aerators provide dissolved oxygen in the production system to support the higher stocking density. In fish cages with constant water flow or better water quality, stocking densities are as high as 200 fish per cubic metre.

Integrated fish farming

Integrated fish farming involves the raising of fish alongside agriculture crops such as rice, mushrooms, sugar cane or vegetables. To explain this in plain language; let us look at a common fishpond size of 25 by 30 m. This gives us an area of 750 square metres. If we compare the different stocking densities, the following variations can be observed.

Extensive 750 m2 3 750 fish Semi-intensive 750 m2 6 000 fish Intensive 750 m2 9 000 fish

The same area of 750 sq/m can have different stocking densities based on the farmer’s financial investment and management. Many times people ask me with how much fish should they stock their ponds. In reaction I always ask, what else is provided by their production systems. This will help to determine the appropriate stocking level for the fish farmer as we have explained in summary. Wrong stocking density has ripple effects that may include overstocking, stunted growth and often fish mortalities. Farmers are often tempted to put in more fish than their ponds or cages can support. Putting in more fish does not imply high yields unless the correct management measures are followed. Management is vital in fish farming success.

750 square metre fishpond (summary illustration): Aquaculture management level Stocking density Support facility in production system

Extensive 3 750 Plankton/less feeding,

Semi-intensive

Intensive 6 000

9 000 Frequent water exchange, feeding Frequent water exchange, aeration, feeding

The three different management methods of fish farming: A: Extensive; B: Semi-intensive; C: Intensive.