Methods of harvesting generally depend on the culture system, the facility, and the form in which the product is to be marketed.
The length of the grow-out stage depends on input size, harvest size and specific growth rate (SGR) in relation to fish size. For that reason, when a pond, tank or cage is to be harvested, it is a great advantage if the fish have been size graded. If not, the slaughterhouse must deal with several sizes of fish which must go to the customers in different weight classes. It is also more difficult to estimate precisely the amount of fish in each weight class and therefore to achieve a good price.
Harvesting from cage farms can be a little more complex, depending on the location and size of the cages. One solution is to size grade during the harvest and send fish that are too small back to another production unit. For this, it is advisable to carry out a representative sampling prior to harvesting, to determine the actual size range of the fish in the cage to be harvested. The production manager will then know if it is advisable to harvest that cage or choose another that has less size variance and that better fits the customers’ specifications.
Extensive rearing of trout can be done in small lakes or ponds. Before use, ponds should be cleared of natural predators. Ponds can be separated into those for fry production and those for on-growing production; the difference is normally the size of the ponds. Commonly, relatively small ponds are used for brood stock, fry, and juvenile fish. In nursery ponds, when fry have reached the required size they can be removed with small-mesh seines, even ones made of mosquito nets, and the smaller pond dimensions enable easy fishing. As the ponds become larger, controlling the water level and managing the stock becomes more difficult.
Full production ponds are also possible, where spawning, fry production and on-growing all occur, although harvesting can be quite difficult. It is important that the levee is sufficiently wide to carry traffic, for instance for feeding, maintenance or harvesting. Drainable ponds offer the advantages of control over the water level and a more effective harvesting process. To carry out the harvesting the pond must normally be emptied and/or drained several times, otherwise the fish density will be too high during harvesting when the water level is lowered. A seine net may also be used for harvesting.
Pond depth is usually between 0.5 and 2.4m, depending on what the pond is used for. For on-growing fish it is normal to choose a depth sufficient to prevent any light reaching the bottom of the pond. In this way growth of vegetation at the bottom is prevented and harvesting is easier. It is important to have a slope towards the outlet on the bottom to make drainage possible and harvesting easier: this can be in the range 1/1000 to 1/100, with the largest slope in the smallest pond. The length–width ratio of ponds is normally about 2:1, but of course is adapted to the site conditions. If ponds are too wide, harvesting will be more difficult. The shape of watershed ponds depends on the terrain. Harvesting with a seine net is easier if the pond is rectangular. Properly designed fish ponds have special provisions for draining and easy harvesting. Special material must be used at the end of the outlet pipe to prevent erosion. Concrete is usually used here. Concrete can also be used to construct the collecting basin for fish so that fry can be harvested or collected.
There are 4 general classifications for harvesting pond facilities.
If there is insufficient water to refill the pond and you only need to harvest part of the population, a seine net can be used to capture the fish without emptying the pond. The seine net will be placed initially on the deeper side and will culminate on the shallow side for harvesting.
If water is a little scarce a siphon can be used to lower the water level in the pond by half. A seine net is used to capture the fish, starting from the deepest area. The smaller fish can be returned to the pond for on-growing.
If there is good water flow, it is better to empty the pond completely. This ensures that all of the fish will be caught. When the pond is almost empty, use hand nets to catch fish that may be in small pools of water.
If a monk is built into the pond, fish can be harvested either in front of the monk inside the pond or outside the pond once they have passed through the monk and the drainpipe.
Figure 1. Concrete monk outlet
For harvesting fish in net pens operation is usefull to have a pump near to the cage to direct the fish to tanks located in the land or transport the fish to the coast and from a dock pump the fish to a harvesting processing area.
Vacuum–pressure pump: A vacuum–pressure pump consists of a tank to which inlet and outlet tubes are connected via valves ; a small pump is also attached. This pump can either pressurize the larger tank or withdraw the air from it, causing a partial vacuum. The function of the pump is first to evacuate the tank; then the valve to the inlet pipe is opened and water and fish are sucked into the tank; after this the inlet valve is closed and the tank is pressurized; lastly the outlet valve is opened and the fish are forced out through the outlet tube. The operation is repeated, and a new batch is pumped through. The pump does not deliver water and fish continuously, because it operates in two phases: vacuum and pressure. However, two pumps can be used alternately to obtain more equal delivery of fish and water. A vacuum head of more than 5m H2O is normally avoided to prevent injuries to the fish; use of less than 40% water relative to fish should also be avoided for the same reason. Here the manufacturer’s recommendations must be followed (Lekang, 2007).
Ejector pump: In an ejector pump, a high velocity, high pressure part flow creates a region of low pressure (suck) in the larger main stream. The fish travel with the water in the main stream. When the water flows past the ejector it will go from low to higher pressure. The pump can therefore deliver fish in a continuous flow of water. The pump has no moveable parts that can injure the fish. A suction head that is too high must be avoided; it is better to take a larger part of the lifting head on the pressure side (Lekang, 2007).
Special care must taken to avoid undue stress to the fish during harvest, since meat quality is known to be affected by stress. This stress response may increase the consumption of glycogen stored in the muscle resulting in an earlier occurrence and shorter duration of rigor mortis after slaughter, reducing flesh quality. Sedatives used before or during harvest (ie, "harvest at rest") can minimize these effects. The use of chemical sedatives is regulated by the Food and Drug Administration of the United States and, sadly, none are approved for harvest at rest. Currently, electro-sedation technology is not subject to the same regulatory restrictions as chemo-sedation, but its effectiveness in the context of harvesting at rest has not been adequately proven. Some scientists have used eugenol treatment at a dose of 10 mg / L, as well as electro-sedation protocols which seem to improve product quality and are perceived as a more humane means of slaughter.
“Cosecha en reposo” puede minimizar el estrés en truchas arcoiris (Salmonexpert)
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