Water is not only the foundation of life, it is itself a rich environment for life forms. Even perfectly clear, natural water is teeming with microscopic life, which forms the basis of the food chain, from tiny animals and plants too small to be seen, to the fish we eat.
A glass of clean stream water, in which a salad the leaf is placed and left in the sun, will soon be teeming with microscopic life. This is the essence of green water fish farming.
The typical recirculating aquaculture system is, on the other hand, a sterile environment in which the fish depend 100% on artificial feeding. In addition, the high cost of artificial foods and energy are often obstacles to the economic viability of such a system. Green water fish farming offers an alternative.
Not suitable for intensive systems
The very high stocking rate of most intensive fish farming systems requires special attention to water chemistry, as fish are completely dependent on their environment for their survival and growth.
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Enrichment of water with manure or chemical fertilizers is not possible in these systems because filtration will remove the improved nutrients. If nitrates reach high levels, the fortification causes algae blooms that reduce oxygen levels to near critical levels at night.
This excludes the use of foods derived from “green water” in such intensive systems.
Green water fish farming not only promotes the growth of microscopic life (zooplankton and phytoplankton), but also the larger organisms that feed on them: insect larvae, aquatic worms and aquatic crustaceans such as Daphnia and Cyclops.
These organisms, ranging from 2mm to 3mm (daphnia) to 50mm (dragonfly larvae), serve as nutritious food for fish of all sizes.
Enriched water and a suitable substrate are necessary for the growth of these nutrient-rich natural foods.
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A smooth, waterproof plastic liner in a dam is inadequate: species diversity will be low and limited mainly to single-celled algae and microscopic zooplankton. This type of substrate works well enough for fry or fry, but the microscopic organisms are too small to support the rapid growth of larger ones.
An earthen pond with sandy soil or substrate has spaces between soil particles that provide suitable habitat for a more diverse community of organisms.
Many fish feed naturally by probing the substrate for food organisms, even sucking up the soil and extracting insect larvae, before expelling the inedible part. Tilapia and carp usually feed this way.
Fertilizing water in (terrestrial) ponds significantly reduces the cost of feeding fish farms, while producing better results. If 30mm long tilapia fry are stored in green water, which is actually a feed “soup”, they will grow much faster than those who depend on even the best artificial feed in sterile containment. .
This eliminates the need for artificial foods for the first three months. To achieve the fastest growth rate, supplemental feeding can then begin to increase natural foods, resulting in healthy and hardy fish at harvest.
A secondary advantage is that the green water largely hides the fry from birds and other predators.
Nicholas James is an ichthyologist and hatchery owner.