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Harvesting methods followed in extensive culture systems
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Extensive culture system includes ponds of regular shapes (rectangular, square) in which the fishes are stocked at moderate stocking densities. The harvesting of ponds is facilitated when construction has resulted in regular shapes with properly sloped blank, proper depth, easy access to at least one side by vehicles, large drain pipes that allow rapid and complete emptying and incorporation of a harvest basin near the drain.
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Seining (use of seine nets) of the ponds is followed for harvesting in extensive culture systems. Water is partially drained prior to seining when harvest basins or pits are present in the ponds. Typically water level is reduced by half and a sieve is passed through the pond to capture a portion of fish present. The volume of the pond is then further reduced, depending on the size of the pond, subsequent seine hauls may be made before final harvest is under taken in harvest basin.
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Small ponds can be seined by hand. While tractors or trucks are required to pull seines in large ponds. When the fish have been concentrated in a harvest basin, they may be dip-netted into baskets or pumped, eventually ending up in live-hauling trucks.
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The upper rope on a seine (float or head line) typically consists of a rope to which floats made from cork, plastic, styrofoam, or some other buoyant material are strung at intervals. The bottom rope (lead line) is designed to keep the seine in contact with the sediments. Lead lines may be ropes to which lead weights are attached at intervals or ropes that have a lead core. In ponds with muddy bottoms, traditional lead lines are often not efficient since they tend to burrow into the sediments and dig up mud, which weighs down the seine and causes it to roll up. In soft-bottomed ponds, seines with mud lines tend to be more effective. A mud line is composed of a number of relatively smaller diameter ropes tied loosely together. The ropes are made from a material that readily absorbs water (e.g., cotton). Mud lines tend to maintain contact with pond bottoms without digging in or lifting off the sediments; thus escapement under seines so equipped is reduced. Such lines tend to wear rapidly if used in ponds with firm sediments.
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Harvesting is the most labor-intensive activity associated with an aquaculture operation. Several persons are required on the typical seine crew, even if trucks or tractors are used to do the bulk of the work. The aqua culturist may be required to employ additional help during harvesting, and the added expense should be taken into consideration during business planning.
Harvesting methods followed in intensive culture system
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Intensive culture systems like closed recirculating systems, race ways and cages are stocked with fishes at higher stocking densities. One of the primary advantages of intensive culture is the ease with which the harvesting can be accomplished.
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In large raceways and circular tanks, the water may be partially drained or the animals herded into a relatively small volume with the use of movable screens. As the number of animals is reduced through dip netting, the volume confining those remaining can be further decreased by movement of the screens and additional lowering of the water level is necessary. If continuous harvesting is practiced, grader screens can be used in raceways that will allow sub marketable individuals to escape while crowding marketable individuals into a small space from which they can be dip netted, often without requiring raceway draining.
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In most cases, animals harvested from intensive culture systems can be loaded directly into hauling tank for transport to the processing plant. All such systems, including those located in buildings, should be designed to provide easy accesses to all culture chambers by hauling vehicles; or some suitable technique , such as the employment of fish pumps, should be used to move the fish from the tanks to the hauling truck. Harvesting aquaculture animals from cages can be a relatively simple matter. Cages can be towed to shallow water where the fish can be removed with dip nets or fish pumps.
Specialized Harvesting Techniques
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Harvesting sessile marine animals such as oysters and mussels requires techniques different from those used for fishes. Oysters reared on the bottom can be harvested manually by picking intertidal oysters up at low tide or dredging for sub tidal animals.
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Oysters grown in trays and on long lines suspended from rafts are manually harvested. Mussels are grown on the bottom, on poles, and on longlines. Harvesting technique is based on how the mussels are grown in trays and manually harvested. Scuba divers may also be used for harvesting benthic animals such as abalone. Crawfish and lobsters are harvested by trapping, which are set out at intervals over the pond bottom.
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