The EPA, and many state regulators, consider aquatic life to be the highest designated beneficial use of water. Closely related to this water quality standard is “fishable and swimmable”. The latter is easier to define and to assess attainment: if fish are present all water quality variables suit their needs; when there are no human parasites or known toxic chemicals water quality is swimmable. The aquatic life water quality standard is not as easy to define and measure.