Finding dead fish floating along the edges of your pond is always a cause for alarm.

In most cases, the majority of fish kills have one or more natural causes. The most common causes are low oxygen levels, spawning activity, temperature fluctuations, and fish parasites or diseases.

Sometimes, toxic algae blooms can result in fish mortality.

Fish need oxygen just like you and I, but unlike humans, fish absorb oxygen from the water.

Ideal conditions for good fish health require a dissolved oxygen level of 5 mg/l (milligrams per liter) of water. Fish generally can withstand short periods of low oxygen with no ill effects. However, if oxygen levels fall below 2 mg/l fish will begin to stress, making them more susceptible to disease. When dissolved oxygen levels fall below 1 mg/l fish will begin to die.

Biological activity in a pond from algae, aquatic plants, bacteria, fish and insects constantly change oxygen levels in the water column. This is normally not a problem during the daylight hours when oxygen is being produced by aquatic plants and algae from photosynthesis. However, when the sun goes down, these plants stop producing oxygen and they become oxygen users. Other factors that affect day-to-day oxygen levels are weather, water temperature and decomposing plant material.

Fish kills can occur from rapid decreases in air temperature and the resulting drop in water temperatures. In addition to the depressed oxygen conditions, late spring and early summer are when most warm water fish, such as bluegill and largemouth bass, begin to spawn.

At this time, large numbers of fish crowd into the shallow waters competing for the best spawning sites. These densely crowded areas are susceptible to disease outbreaks, especially as water temperatures increase. The result is an unavoidable natural fish kill usually consisting of one or two species of fish.

Fish diseases caused by viruses, bacteria, fungi and parasites occur naturally and under certain conditions fish can contract one or more of these diseases.

Of course healthy fish can usually fend off such problems, but if the fish becomes weak from spawning, temperature fluctuations, poor water quality or low dissolved oxygen, it increases the chance of the fish becoming sick and dying.

Sometimes algae blooms can result in fish kills. The most common species of algae that produce toxins are the blue-green algae. Although few cases have been linked to these species, most algae-related fish kills are the result of oxygen depletions rather than toxicity problems.