Fish stay near the surface and, because they cannot get enough oxygen from this new water, die of suffocation. Prevention A good aquatic weed control program will prevent the accumulation of large masses of vegetation, which will use a large amount of oxygen when it decomposes. The use of an aerator will help keep the pond water mixed so that layering is minimized and the surface water is well-oxygenated. Although fish may die of natural causes at any time of the year, they are more stressed in the early spring and during spawning season. This makes them more susceptible to unfavorable environmental conditions and diseases.
Large fish seem to be more susceptible.
Mortality may also be due to old age. Some fish may live up to 10 years, but four to eight years is more common. Prevention Little can be done to prevent natural mortality. Year-round pond management practices will provide the best growth conditions for your fish. Organic Pollution Run-off from barnyards, cropland, feedlots, septic tanks, and intensively managed turf areas such as golf courses can introduce large quantities of organic matter and nutrients into a pond.