Principle of Aquaculture (1+1)

3.1.5.2. Pen culture

Pen culture is defined as raising of fish in a volume of water enclosed on all sides except bottom, permitting the free circulation of water at least from one side. This system can be considered a hybrid between pond culture and cage culture. Mostly shallow regions along shores and banks of the lakes and reservoirs are used in making pen/enclosure using net/wooden materials where fish can be raised. In a fish pen, the bottom of the lake forms the bottom of the pen. Pen has the advantage of containing a benthic fauna which serves as food for the fish and polycultue can be practiced in pens as it is in ponds. The environment in fish pen is characterized by a free exchange of water with the enclosing water body and high dissolved oxygen concentrations.

Advantages:

a. Intensive utilization of available space : Stocking density can be increased compared to that of a pond culture system

b. Safety from predators: Within the enclosure the predators can be excluded. In the larger pens this would be more difficult, but in smaller pens this can be done as efficiently.

c. Suitability for culturing many varied species : Due availability of more space and the natural water system

d. Ease of harvest : In the large pens the harvest may not be as easy as in cage rearing but it more controllable and easier than in the natural waters.

e. The flexibility of size and economy : When compared with the cage, pens can be made much larger and construction costs will be cheaper than that of the cages.

f. Availability of natural food and exchange of materials with the bottom : Since, the bottom of the pen is the natural bottom, the pen cultured organisms are at an advantage that they can procure food/exchange materials from the natural bottom.

Disadvantages:

a. High demand for oxygen and water flow

b. Dependence on artificial feed

c. Food losses : Part of the feed is likely to be lost uneaten, and drifted away in the current, but the loss here would be less than in floating cages.

d. Pollution : Since a large biomass of fish are cultured intensively a large quantity of excrements accumulate in the area and cause a high BOD - also substances such as ammonia and other excreted materials, if not immediately removed/ recycled. They pollute the water and cause damages.

e. Rapid spread of diseases : For the same reason of high stocking density in an enclosed area, any disease beginning will spread very quickly and can cause immense mortality of stock and production decline.

f. Risk of theft : Since the fish are kept in an enclosed area, ‘poaching’ and thefts can take place more frequently than in natural waters, but perhaps less than those from cages.

g. Conflict with multiple use of natural waters : In locations where a pen is constructed, if the water is used for multipurpose like irrigation and recreational activities, such as swimming, boating etc. may lead to conflicts.