MASS MORTALITIES OF FISH IN SOUTH AFRICAN ESTUARIES

Abstract

Changes to the physico-chemical environment have been identified as the cause of fish mass mortalities in South African estuaries. Most published accounts have linked these mortalities to a single dominant factor such as low salinity, low or high temperature, low concentration of dissolved oxygen, and high sediment loads. A review of the evidence suggests that, although single factor induced mortalities do occur, the role of supplementary factors in triggering fish kills may be more important than previously acknowledged. The most frequently interacting factors in a southern African context appear to be salinity and water temperature, dissolved oxygen and water temperature, and suspensoids and dissolved oxygen. In some cases all the above factors may be operational in creating an environment which is unsuitable for the survival of certain estuarine-associated fish species.