Fish Pond

A fish pond is a basin dug or dammed to hold the rain that runs off the ground above it, and stocked so the water grows food. Sunlight feeds the tiny plants drifting in it, those feed insects and small creatures, and the fish feed on those, so a crop of protein comes out of water and light. It is not a pond built to store water: the digging happens once, but what it produces is decided again every year.

Two conditions are settled before it is dug. The catchment, the ground that sheds rain into the pond, wants to be roughly fifteen times the water surface at a working depth of 1.8 to 2.4 m (6 to 8 ft). Too small and the level falls away; too large and heavy rain flushes the pond through. The second is alkalinity, the water's ability to resist swings in acidity, which needs to stay above about 20 parts per million; it is corrected with agricultural limestone every two to four years, at a rate set by testing the mud on the bottom rather than the water. Production is then steered: the water is fed with nutrients only while it stays above about 16 C (60 F), judged by how deep a white disc stays visible, 45 to 60 cm (18 to 24 in) being the target.

The failure that empties a pond overnight is loss of oxygen, and it follows warm surface water suddenly mixing with stale water below, an algae bloom dying off, or weeds rotting after treatment. The largest fish go first. Weeds are therefore cleared in spring while the water is cool, a section at a time so the rotting is never all at once. Drought is the same failure slowed down: less water, crowded fish, less oxygen, rising wastes. Feeding is cut back and the stock harvested before depth falls under 0.9 m (3 ft). Digging the basin and building a safe overflow is skilled work. The rest is annual: lime, a feeding season bounded by temperature, spring weeding, and a net check of the fish balance when the fishing falls off.