A primary purpose ofthis overview is to focus on the use of fishes as bioindicators, at the community level, of changes within estuaries. In 2004 a multimetric fish index, the 'estuarine fish community index' (EFCI), was developed and tested in South Africa. The EFCI is based on 14 metrics or measures that represent four broad fish community attributes: species diversity and composition, species abundance, nursery function, and trophic integrity (Table 1). Reference conditions and metric thresholds were derived from fish community data collected during an extensive national study. The individual metrics were evaluated using data that were collected on an estuary that was degraded and in which rehabilitation measures had been implemented. The evaluation demonstrated that the selected metrics adequately measure the conditions of separate but related components of estuarine fish communities and that these reflect environmental condition.
In 2006, the EFCI was applied to data collected for some 190 South African estuaries. Its application spanned three biogeographic regions and included three distinct estuarine typologies. Metric reference conditions and scoring criteria were developed for each estuary type within each zoogeographic region separately. The final EFCI was applied to each estuary by comparing its fish community with the appropriate reference. A comparison of the EFCI with independent measures of estuarine condition revealed that the index was able to effectively differentiate between poor- and good-quality sites. Applying the EFCI to estuaries in which multiple samples were taken also showed that the index is reproducible. The index is both a robust and sensitive method for assessing the ecological condition of estuarine systems; it is also an effective communication tool for converting ecological information into an easily understood format for managers, policy-makers, and the general public.
In conclusion, the basis for using a multimetric biomonitoring technique such as the EFCI to assess environmental condition is that the relative health of a fish community is a sensitive indicator of direct and indirect stresses on the entire aquatic ecosystem (Figure 2). Ideally, parallel investigations should determine stress at the community, population, individual, physiological, and subcellular levels, using techniques such as production ecology, biochemistry, bioaccumulation studies, pathology, genetics, behavior, and physiology. In the real world this complete suite of biomonitoring is almost impossible. Nevertheless, the recommendation that fish studies should be one component in an ecosystem approach, using chemical, hydrographical, and other biological data to interpret the fish results, is fully endorsed.
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