There is evidence that fish in polluted waters are more likely to develop various fish diseases. Some of these diseases can cause major economic losses, as they can plague commercial fisheries and result in reduced sales. However, it is still not clear whether pollution itself is a cause of these diseases or if these problems occur simply because the fish in polluted waters are more prone to them. Surveys that try to measure the presence of specific diseases in both polluted and non-polluted sites may be useful in elucidating the links between pollution and disease. However, such surveys have their limitations, as they may not be able to identify individual fish that have been diseased by polluting substances, and they may not be able to distinguish between different types of pollution.