Farm ponds, mill ponds and other small waterways are subject to a variety of fish-related problems. Many of these problems are simple to correct, once identified.
A lack of fish in small ponds can be caused by excessive harvesting by humans or predation by wildlife. Pond owners usually harvest a few fish for consumption. In some cases, unknown to the landowner, poachers may also remove fish from private ponds.
In addition to human anglers, a variety of wild predators are skilled at catching fish and in some cases can have considerable impacts on fish populations.
Otters are known to be one of the most efficient predators that visit small ponds. These playful aquatic mammals can eat their weight in fish in a matter of days. Raccoons are another fish-eating mammal, although their efforts are usually limited to shallow areas along the shoreline. Both red and gray foxes are also capable of catching fish in shallow water. Black bears are usually the largest predator to visit farm ponds. These omnivores are capable of causing a considerable disturbance to farm ponds or other small bodies of water.
Birds can have a big impact on fish populations, especially in shallow ponds. Herons and egrets are among the most deadly. Other predatory birds include ospreys, eagles and kingfishers.
Reptiles, including turtles and water snakes take fish from most ponds. Usually these predators do not eat large numbers of fish and the losses are a normal part of fish life cycles.
In addition to predators, environmental factors can affect fish populations in small impoundments.Cold winter temperatures can also harm fish. Most species of freshwater fish have minimum temperature thresholds below which they become stressed or die.
Spring can also be a dangerous time for fish populations, especially in small ponds. In many areas, aquatic vegetation dies off in winter and settles on the bottom. When spring arrives, warm temperatures cause dead plant material to break down, which in turns causes oxygen levels to plummet. The situation, known as hypoxia, can cause large scale fish kills.
During summer, droughts can lower water and oxygen levels, as well as causing water temperatures to skyrocket. Hypoxia can again stress or kill fish as they crowd into small spaces without sufficient oxygen to sustain them.