Second, overfishing affects the relative abundance of individual species or the mix of different species within an ecosystem. Often, populations of both the target species and the predators that feed on these species decline and are replaced by stocks of lesser commercial value. Overfishing affects other marine species, not just fish stocks. Overhunting has decimated many marine mammal populations. Poor management practices, subsidization of the fishing industry, uncontrolled harvests within international waters, and destructive and wasteful capture methods are to blame for the overexploitation of most marine species.
Contaminants affect marine biodiversity in a number of ways.
Untreated sewage, oil, heavy metals, and other wastes may be directly toxic to some marine organisms. Their effects may be instantaneous or cumulative. Other contaminants such as radioactive waste, pesticides, and other chemicals have cumulative effects, building up within individuals over time, especially within species high on the food chain. Moreover, various contaminants and physical degradation can act together in a cumulative or synergistic fashion. Other contaminants can trigger ecosystem-wide changes, resulting in conditions that are inimical to a range of species.