"Water quality" may include the amounts of dissolved oxygen and the levels of algal growth, pollutants which may be present and the pH level. Water quality effects also which types of organisms can survive in a body of water.
Shellfish require a high level of dissolved oxygen and their abundance is an indication of good water quality. Shellfish are sensitive to changes in the ecosystem and they cannot easily escape changes in the water quality. These are one of the reasons why shellfish are good indicators of water quality.