A biological integrity index that uses attributes of fish assemblages was developed and applied to a part of the Crocodile River (Incomati System, Mpumalanga Province). Fundamentally, the fish assemblage integrity index (FAII) is based on the fish species expected to be present in biological (fish habitat) segments which are sections of river with relatively homogeneous fish habitat. Within this framework fish are categorised according to an intolerance index which takes into account trophic preferences and specialisation, habitat preferences and specialisation, requirement for flowing water during different life-stages and association with habitats with unmodified water quality. This intolerance index, the expected frequency of occurrence and expected health of fish species in a particular fish habitat segment was used to formulate an index for the situation expected under minimally impaired conditions which was used as the comparative basis for the observed (sampled) situation. The observed situation was expressed as a fraction of the expected situation to arrive at a relative FAII index value which was grouped into one of six descriptive fish assemblage integrity index classes. It was found that the index reflected several aspects of the modifications that have occurred in the Crocodile River, i.e. water quality modifications, flow modifications and introduction of alien fish. However, flow releases from Kwena Dam hampered fish sampling while the floods that occurred in the summer prior to the survey, extensively modified physical habitat conditions that existed for some years. It was concluded that the index provides a broad, synoptic estimation of the biological integrity of the river. It is highly likely that the index in its current form provides an underestimation of the biological integrity due to the species expected to be present, being based on all species listed for a segment, and not on the fish species expected 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 species expected per habitat type. In its current form, the index has the potential to provide qualitative, descriptive criteria for the desired ecological condition or integrity of rivers for management purposes in terms of the new South African Water Law. The development of numerical criteria will, however, require development in terms of the statistical properties (i.e. statistical power) of the index.

Water SA, July 1999, Vol. 25, No. 3, p. 265

http://www.wrc.org.za/wrcpublications/wrcwatersa/wsa_acrobat/jul99_p265.pdf