Overfishing
This occurs when fishes are catch much faster then they can reproduce. Overfishing increases with the use of highly efficient vessels. Example, a large trawler can have four trawl nets in the water at one time and high-precision sonar screens can detect precisely the schools of fish. Infant fish often cannot escape when they are caught in the very fine mesh of some nets.
This causes the fish stock decrease rapidly. This reduces a major source of food for those dependent on fish. Some species are nearly exhausted. For example, the anchovies off the coast of Peru. The fishermen's income would be affected. Other species of underwater creatures dependent on them might die, due to the upset ecosystem. This will then lead to a lost in the bio-diversity of the ocean.
Water Pollution
This is often caused by the washing of tankers at sea, the dumping of toxic waste, the discharging of untreated waste from factories, ships and sewers. Organic pollutants such as sewage and nitrogenous fertilizers can lead to the growth of phytoplankton which may change the colour of the sea. This phenomenon is called red tides.
Water pollution causes the fishes to die. Contaminated fishes are also unsafe for consumption. Example, the Minamata disaster of Japan in the 1950s, resulting from the consumption of fish poisoned by mercury discharged from a chemical factory, causing deaths, brain damage and the birth of handicapped children. Fish farming are affected. The outbreaks of red tides kills fishes in the coastal fish farms.