During the past decade there has been an increasing interest in the use of fishes as environmental indicators to assess the 'health' of individual estuaries. The use of fishes as indicators of environmental health or biological integrity is based primarily on the tenet that fish species or fish communities are sensitive indicators of the relative health of an aquatic ecosystem. Some of the reasons that have been put forward to use fishes as bioindicators of environmental changes within estuaries include the following:
• Fish are typically present in all aquatic systems, with the exception of highly polluted waters.
• There is extensive life-history information available for most species.
• When compared to invertebrates, fish are relatively easy to identify and most samples can be processed in the field, with the live specimens being returned to the water.
• Fish communities usually include a range of species that represent a variety of trophic levels and include foods of both aquatic and terrestrial origin. They are therefore able to reflect the adverse effects of stresses on a variety of ecosystem components.
• Fish are comparatively long-lived and therefore provide a long-term record of environmental stress.
• Acute toxicity and stress effects can be evaluated in the laboratory using selected species.
• The general public is more likely to relate to information about the condition of the fish community than data on invertebrates or other ecosystem components.
• Societal costs of environmental degradation are more readily evaluated because of the economic and esthetic values attached to fishes.
However, the use of fishes as indicators of biological integrity does have difficulties and problems, including the following:
• the selective nature of sampling gear for certain sizes and species of fishes;
• sometimes substances physically or chemically harmful to other life forms do not have a detrimental effect on fishes;
• the mobility of fishes on seasonal and diel timescales can lead to sampling bias;
• diverse fish communities may exist in environments significantly altered by humans. It has been suggested that the absence of fishes in a particular environment can be of far greater importance than their presence when assessing biological integrity.
Despite the above problems, fishes have proven to be useful 'tools' when assessing environmental quality, primarily through the use of the 'index of biotic integrity' (IBI). The IBI (see Biological Integrity) incorporates information from individual, population, and community levels into a single, ecologically based index of water-resource quality.
A fundamental tenet of all IBIs is the importance of reference conditions for the application of the index. These reference conditions refer primarily to the natural composition of the community and in some instances this precondition may be difficult to construct, for example, in certain estuaries within Western Europe where preindus-trial revolution fish composition information is unavailable. Another important aspect to be considered when constructing reference fish assemblages for an IBI assessment is biogeography. Indeed, it could be argued that global climate change is causing a change in biogeographic boundaries and that any IBI calculations, especially those in biogeographic transition zones, need to take this into account.
Over the years since its introduction, research and application around the IBI has resulted in its further development to such an extent that it is now applied, in various forms, to streams, rivers, lakes, estuaries, and coastal areas around the world. As a result, the IBI is now widely accepted as a valuable tool in assessing the health of freshwater, estuarine, and coastal marine waters.
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