Agriculture and Environment: Shrimp

Better Management Practices: Reduce Nutrient Loads in Effluents

At harvest, if not long before, water must be released from shrimp ponds.

In the past this has been done when the water quality deteriorates, when there is disease, or to remove the shrimp from the bottom during harvest. This effluent will increase the nutrient load in the local receiving water bodies, and should not be released into natural waterways without treatment.


Importace of treating effluents
If the area has a low evacuation or flushing rate or if there are too many farmers in one area, this can lead to eutrophication which can in turn deplete oxygen to the point that aquatic species are stressed or even killed.

All shrimp producers should be required to have settlement ponds, canals, or biological filters (biofilters) that treat the effluent. In Colombia, shrimp farmers are required to treat the effluent so that the water they release back into the environment is of as good or better quality as that which they brought into their ponds. To date most of these systems are passive and treat the water through settlement or holding ponds, canals, or biofilters.

Each of these captures the nutrient-rich sediments so that they do not foul local waters. Suspended solids can take a week to settle out. Many farms have found that an area equal to 10-25% of the surface area of the ponds is required for settlement.

This is a considerable investment as well as an opportunity cost - land used for settlement ponds cannot be used to grow shrimp. However, land used correctly to produce shrimp can be used indefinitely.

Using biofilters
Eliminating nitrogen and other nutrients requires more active management than removing sediments. Biofilters are only beginning to be used for these purposes. These include both natural and artificially created wetlands and mangrove areas.

Decomposing organic matter
Another form of treatment in settlement ponds or canals to ensure decomposition of organic matter is to add populations of microbes that aid decomposition. While this range of practices is more common perhaps than before, there are no global estimates of the prevalence of different practices.

A number of farmers have found that by settling their water, they can then recirculate it back into other ponds for stocking. This allows them to avoid the downtime required to condition the new water until it begins to produce the feed that small shrimp need to survive. Most of the water released during water exchange and the first three-quarters of the harvest can easily be used for recirculation- provided there is no evidence of disease in the pond in question.

Benefits of recirculating water
Recirculating the water has a number of other advantages in addition to reducing the downtime before restocking. It reduces the total amount of settlement ponds required and the amount of time the water stays in them. It reduces the cost of pumping water from more distant areas.

It reduces the chances of bringing disease into the ponds through contaminated intake water. Finally, it reduces the total amount of water used in the operation, as well as the amount that is released back into the environment. Most of these approaches increase resource use efficiency, reduce environmental impacts, and increase net returns.

Polyculture systems
Finally, the use of polyculture systems, in which multiple species are grown in the same space, is another possibility for reducing overall effluent load in the water. This can either be done in the shrimp ponds themselves (which is difficult in commercial operations attempting to maximise shrimp production) or undertaken sequentially with the effluent so that different species are part of the treatment process.

At this time simultaneous shrimp polyculture systems produce shrimp and fish such as tilapia (an omnivore) or milkfish (an herbivore). Sequential polyculture systems tend to focus on bivalves, seaweed, etc. In some areas, sequential polyculture is similar to crop rotation, as different species (e.g., milkfish, tilapia, red claw, or bloodworms) are produced as alternate crops in the same ponds. This also functions as a form of fallowing.

The same water could be recycled into these ponds. Polyculture is practiced in several countries, but it has become most common in countries that have disease problems. In general, polyculture is still not financially viable for most large-scale, commercial shrimp producers. However, as farmers begin to understand better the costs of disease and pollution, or as they are required by government to improve their performance or pay fines, this situation may change.

Credits

Extracts from "World Agriculture & Environment" by Jason Clay - buy the book online from Island Press

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