Fish for food!

Fish farms or aquaculture farming is catching on in a big way in India. This is because of rising health awareness and demand for fish meat, and increased logistics bottlenecks along traditional fish sources and lower catches from those regions.

So, new catch is being developed in hinterland fish ponds, the idea being that a long term source of alternate food and earnings can be developed in our farm lands.

The whole idea of modern fish farming is to enhance productivity, thereby the supply, and therefore commercial returns to our farming community.

How does this typically pan out? By stocking more fish per square metre than naturally possible, thereby reaching higher harvest volumes off the same piece of real estate. Very much like employing techniques of precision farming and high density planting to vegetable and fruit crops.

But it is not that simple, nothing ever is! Fish breathe and breed in water, which means they need food and oxygen. Living in the pond water, their natural bodily refuse also needs to be handled or they'd die in their own waste (much like we need to care for the air and earth we pollute through over population).

Fish breathe the oxygen in the water; they do not split water (H2O) but rely on oxygen (O2) gas that naturally diffuses into water.  This oxygen dissolves in the water from two sources; primarily from aquatic plants and from the atmosphere which is in contact with its surface. This means that in artificial ponds with low levels of aquatic plant life, its oxygen retaining capacity is directly related to the surface area available (not the volume but area of water surface).

Ensuring sufficient oxygen levels in a fish pond becomes important aspect of fish keeping. One of the main reasons for pond kills is lack of oxygen in the water. When populating a fish farm, catering to oxygen needs of the fish takes on prime importance (unless low yield levels are acceptable).

The trick is that since the atmospheric oxygen diffusion is off the water surface, we can artificially increase the surface area by cycling the water top with pumps. This naturally happens when winds blow over the water surface or in flowing rivers. In modern fish farms which keep high fish density per acre, this needs to be done by using pumps that gently cycle the surface water multiple of times in a day allowing for more oxygen to dissolve and sustain life in the pond.

Such circulation also allows for oxygenated water to reach fish eggs, when breeding fish. This is why wide shallow ponds are better than deep ones with the same surface area.

Oxygen also serves the purpose to feed aerobic bacteria – very necessary – as these convert the fish waste and other detritus into safe by-products. Fish piss and shit in the water they live in, add to this deaths and plant detritus, the water starts to develop high levels of ammonia. The bacteria trigger the nitrogen cycle, metabolising ammonia to nitrites and finally nitrates. The nitrates are nutrients for aquatic plants which in turn output oxygen (which means it may make sense to have some plants in the fish pond too).

Recently I visited some new fish farms and found that they were probably not being developed so as to sustain life as intended. A surface area that would typically allow for a bout 1,000 fish was intended to stock 30,000 yet there were no signs of catering to such a high population density (we ought to learn a lesson from our cities like Delhi).

Other matters to plan for-

• Do not plan with narrow mud dykes between adjoining fish ponds (specially when under different ownership) as risk of contamination (disease) will be high.
• Do not feed run-off from fish farms into running streams as downstream contamination is likely.
• Plan for biological filtration with aquatic plants for sustainable waste management (of course mechanised filtration systems could be used, but lets try be eco-friendly).
• Investigate integration with other farming activities – bee keeping, live stock, poultry or vegetable farming along with fish farming.
• Segregate ponds as per life cycle of the fish so as to have breeding and stocking ponds, etc.

Just another blog, after an observational visit. More details, comments and inputs from experts are welcome.