Fish Refuge

A fish refuge is a deep hole dug into one part of a pond that is otherwise shallow. Fish gather in it when the rest of the water becomes impossible to live in: a cold season chills the shallows, or the pond is drawn down and the shallow ground goes dry. Digging the whole basin deep instead costs far more, because the banks must grow much larger as depth increases. A shallow pond with one deep pocket is cheaper to build and warms faster in spring.

It goes at the lowest point of the basin, so water and fish collect there on their own. At least 50 cm (20 in) deep is the usual minimum, much more if fish are to be held there all year, and in loose soil the hole is made three or more times wider than it is deep - a trench 1 m wide and 1 m deep (3 ft by 3 ft) is one example. Size is decided by how much growing area is worth giving up, not by any standard. But depth brings its own problem. Water deeper than about 3 to 3.5 m (10 to 12 ft) almost always settles into layers in warm weather, and the cold bottom layer, cut off from the surface, loses its oxygen. Cold water a fish cannot breathe is no refuge at all.

So a refuge is dependable against cold and against a drying pond, but serves in hot weather only if something stirs the water or feeds air into it so the layers never settle. The other surprise is that digging one can drain the pond it protects: a basin that has sealed itself over time loses that seal where the ground is cut, and water starts leaking away. Manuring the bottom helps the seal reform, but it is safer to dig the pocket when the pond is first built than to cut into one that already holds water. Being the low point, it is where silt lands first, so it fills in on the same cycle the pond needs clearing, and a refuge that has silted shut looks no different from the bank. Its upkeep is the seal, the desilting, and whatever aeration its summer duty depends on.