Coastal Aquaculture and Mariculture (2+1)

Unit 3 - Methods of coastal aquaculture and mariculture

3.3. Pens

• Pens and enclosures are transitional structures between ponds and cages. They are used for culture of yellow tail in Japan, milkfish in the Philippines and salmon in Norway.
• The success of pens for culture depends on the hydro-biological conditions of the site. The design of the structures is based on adequate knowledge of water quality, floods, waves and currents and prevalence of predatory animals.
• Pens are formed by net barriers to partition off areas of an open water body, such as intertidal areas of the sea, bays or lagoons.
• Different designs of enclosures have been constructed. Generally the enclosure is formed on one side of the shore and on the other three by a wall of nylon netting hung from pole driven into the bottom.
• In many such enclosures, concrete or stone walls are built on each side where it joins the shore or provide adequate support to the nets.
• Around the rest of the perimeter, heavy posts of impregnated timber or concrete piles are driven into the bed extending for about 2m out of water all times.
• Net barriers may be hung from steel cables stung between the poles or the concrete of steel piles. To prevent the lateral movements of these piles some are anchored to fore and aft, to large anchor blocks using strong steel cables.
• The nets are generally made of knotless nylon netting material. In some cases, two walls of netting are used, the outer one to protect the enclosure from floating debris and to prevent escape of fish if the inner wall gets damaged.
• However, if two walls of netting are used it may create hindrance to free flow of water. The net barrier is fixed by a rope along the sea bed for 1m, until it terminates in a lead line. The net is embedded in the sand or silt at the bottom. As a further precaution to prevent fish escaping heavy rubble is piled up at the bottom around the pen.