Massive fish die-offs have many different causes. These causes range from natural causes to disasters caused by humans. Death due to natural causes is far more common than human negligence or intentional polluting. Oxygen depletion and natural life cycles occur much more often than other causes such as a release of toxins into the water.
The most common cause of fish die-offs is oxygen depletion. This occurs when oxygen saturation levels in the water fall below what is safe for the species of fish found there. Oxygen levels normally fluctuate due to weather, decaying plants and temperature. In some incidences, these natural processes can cause less oxygen absorption leading to depletion. Other causes of oxygen depletion are organic waste, such as sewage or farm waste, entering the water. When these events occur, the fish suffocate and dye of hypoxia.
Toxins entering the water are another cause of fish die-off. These can come from both human and natural causes, but human causes are the least common. Waste from agriculture, chemical spills, and surface runoff cause the water to become toxic leading to a shift in pH levels and eventual death of the fish. Natural causes, such as the change is seasons, can increase in aluminum levels to a point of them becoming a toxin.
In some cases, fish die-offs are a natural part of their life cycle. Spawning fatalities occur shortly after the fish have traveled to their spawning area and released eggs or sperm. These activities, as well as creating nests and courtship weaken the fish. The exhausted fish quickly succumb to the elements and environment. Females are more likely to die from spawning activities.
Seeing a large number of fish dead in one area is alarming, but the cause is more often natural than human-made. Whether the cause is from toxins in the water or a lack of oxygen, fish die-offs will always occur. Humans can help to reduce the number of die-offs, but ultimately they are just a part of natural processes. All of these causes deplete the fish population; however, they rarely cause a species to die-off entirely.
Images via eutrophication&hypoxia