If you go to the pond in the morning and see a few fish floating at the surface with their mouths open, the damage was done the night before. The cause is almost always the same: a fall in oxygen under the water, too much algae or toxic gas built up at the bottom. However much money goes into fingerlings and feed, these unseen changes in the water are the root of most losses. Recognising a few signs and keeping a regular eye on the pond can avoid much of this loss.

A pond has two main sources of oxygen: contact with the air and photosynthesis by algae. In daylight, algae produce oxygen, but once night falls the opposite happens and the algae themselves start taking in oxygen, just like fish. If there is a lot of algae, oxygen is lowest in the last part of the night and around dawn, which is why fish deaths happen most at this time. If the sky is cloudy or foggy for two or three days in a row, the same crisis can happen during the day, because without light the algae take in oxygen instead of producing it.

At midday, dip your arm into the pond up to the elbow (about one and a half feet). If you can see your palm clearly, the water is clear and there is little problem. If your palm disappears when you have dipped it only half that far (10-12 inches), there is too much algae or turbidity, and there may be an oxygen shortage at night.

Small sensor devices are now being tested that regularly measure pond temperature, pH and oxygen levels and send a message to a mobile phone when a problem arises. That way farmers can know the state of the pond from home without staying up all night to watch, and act quickly when danger comes.

Rather than searching for the cause after fish die, it is wiser to understand how the water is behaving in advance. A pond is a living environment: the healthier the water, the safer the fish, and the more the fish farmer's hard work pays off.

The writer is an agriculture and technology entrepreneur.

Source: Jagonews24. First published in Bengali on The Agro News.