The development of a fish index to assess the biological integrity of South African rivers

A biological integrity index that uses attributes of fish assemblages was d eveloped and applied to a part of the Crocodile River (Incomati System, Mpu malanga Province). Fundamentally, the fish assemblage integrity index (FAII ) is based on the fish species expected to be present in biological (fish h abitat) segments which are sections of river with relatively homogeneous fi sh habitat. Within this framework fish are categorised according to an into lerance index which lakes into account trophic preferences and specialisati on, habitat preferences and specialisation, requirement for flowing water d uring different life-stages and association with habitat with unmodified wa ter quality. This intolerance index. the expected frequency of occurrence a nd expected health of fish species in a particular fish habitat segment was used to formulate an index for the situation expected under minimally impa ired conditions which was used as the comparative basis for the observed (s ampled) situation. The observed situation was expressed as a fraction of th e expected situation to arrive at a relative FAII index value which was gro uped into one of six descriptive fish assemblage integrity index classes. I t was found that the index reflected several aspects of the modifications t hat have occurred in the Crocodile River, i.e. water quality modifications; flow modifications and introduction of alien fish. However, now releases f rom Kwena Dam hampered fish sampling while the floods that occurred in the summer prior to the survey, extensively modified physical habitat condition s that existed for some years. It was concluded that the index provides a b road, synoptic estimation of the biological integrity of the river. It is h ighly likely that the index in its current form provides an underestimation of the biological integrity due to the species expected to be present, bei ng based on all species listed fora segment, and not on the fish species ex pected to be present in the habitats actually sampled. It is suggested that refinement of the index should include development of improved methodology ;to determine not only the fish species expected per segment, but also spec ies expected per habitat type. In its current form, the index to the potent ial to provide qualitative, descriptive criteria for the desired ecological condition or integrity of rivers for management purposes in terms of the n ew South African Water Law. The development of numerical criteria will, how ever, require development in terms of the statistical properties (i.e. stat istical power) of the index.