From the point of view of protection of living resources in the marine environment, the ecological effects of pollution are of the most vital concern. Perhaps the most important effect of pollutants in the marine environment is ecological disruption, such as the imbalance created between organisms and their environment, and between communities of organisms of different species. This is often an insidious, long-term effect which can lead to large changes in populations of commercially important fish. So far, there has been no conclusive evidence that populations of marine fish have been seriously affected by pollution alone. However, this may be related in part to our inability to clearly identify cause and effect in certain fisheries' problems. A whole ecosystem may be modified by input of a particular pollutant, because certain species are reduced in number or eliminated. Other hardier species may fill an ecological niche vacated by a sensitive species eliminated by the pollutant.