When dead fish are found in large numbers in one place, the incident is referred to as a fish kill and there is significant reason to suspect pollution as the cause. The three main causes of fish kills are poisoning, disease and suffocation.
Fish can be poisoned by a wide range of polluting substances including pesticides, acids, ammonia, phenols, compounds of metals, detergents and cyanide. Many of these substances are used in industrial processes or in agriculture and are released through drains. Some may be accidentally spilled into the waterways as well.
Acid rain resulting from industrial pollutants in the atmosphere causes rivers to become toxic for various kinds of fish. Some types of toxic algal blooms kill fish as well.
In natural environments, disease alone does not usually result in mass mortality but under the artificial conditions of a hatchery or an aquaculture operation, disease can spread rapidly and cause a fish kill. The disease may be caused by viral infections, bacteria, fungi or internal or external parasites.
Suffocation occurs when the oxygen concentration in the water falls below the level at which fish can survive. A common cause is eutrophication, which is the artificial stimulation of plant growth by pollution with fertilisers, sewage or atmospheric fallout.
When the excess plant growth decays, it lowers the oxygen concentration. The discharge of dead organic matter into a watercourse from a sewer or an industrial operation has the same effect. The accidental spilling of a herbicide into a lake or stream may kill large quantities of aquatic vegetation, causing low-oxygen conditions.
Nuisance algal blooms may also cause suffocation. In 1994 at St Helena Bay, South Africa, a large bloom of toxic and nontoxic algae formed in an estuary and extended into the open sea more than 30km from the shore. The bloom sank and decomposed, forming an area with almost no oxygen but with lethal levels of hydrogen sulfide. Approximately 1,500 tonnes of dead fish and 60 tonnes of dead rock lobsters were washed ashore.
Many fish kills can be prevented by reducing the level of pollution entering the waterways.
Application of fertilisers should be matched to the needs of the crop, sewage effluent should receive advanced treatment, and atmospheric emissions from industry and transport should be carefully controlled at source.
Hundreds of dead fish were found floating in Sungai Tonggak near Gebeng in Pahang where bauxite mining was a major cause for concern. But what really surprised me about the incident was hearing the Fisheries Department director in Pahang describing it as "nothing unusual".