Estuarine Importance Rating

Biological communities are regarded as an integral component of estuarine ecosystems and the prioritization

Table 1 Metrics used in the 'estuarine fish community index' and their response to environmental stress

Fish community index metric

Response to stress

Species diversity and composition

1. Total number of taxa

2. Rare/threatened species

3. Exotic/introduced species

4. Species composition (relative to reference assemblage) Species abundance

5. Species relative abundance (relative to reference assemblage)

6. Number of species that make up 90% of the abundance

Nursery function

7. Number of estuarine resident taxa

8. Number of estuarine-dependent marine taxa

9. Relative abundance of estuarine resident taxa

10. Relative abundance of estuarine-dependent marine taxa

Trophic integrity

11. Number of benthic invertebrate feeding taxa

12. Number of piscivorous taxa

13. Relative abundance of benthic invertebrate feeding taxa

14. Relative abundance of piscivorous taxa

Reduced Absent Present Reduced

Reduced Reduced

Reduced Reduced

Very low or very high Very low or very high

Reduced Reduced Reduced Reduced

By comparing the fish community within an estuary...

Physical

Chemical

Biological

Physical

Chemical

Biological

Figure 2 Conceptual diagram showing how the fish community in an estuary is dependent upon the physical, chemical, and biological environment and, using a reference fish assemblage, can be used as a basis to determine a health index for the system.

... one can measure the relative health of that estuary

Figure 2 Conceptual diagram showing how the fish community in an estuary is dependent upon the physical, chemical, and biological environment and, using a reference fish assemblage, can be used as a basis to determine a health index for the system.

of potential conservation areas is often necessary for the successful determination of aquatic reserve sites. Estuarine prioritization is also a tool to assist management to make freshwater allocation decisions in arid or semiarid countries where riverine resources are limited.

In 2003, a 'fish importance rating' (FIR), which prioritizes South African estuaries in terms of their importance to estuary-associated fish species, was published. The FIR is based on a scoring system of seven criteria that are considered to reflect the importance of estuaries to estuary-associated ichthyofauna (Figure 3). These criteria may be divided into two primary components as follows:

1. Species importance

• Species richness

• Endemic richness

• Exploitable richness

2. Estuarine importance

• Estuarine type

• Estuarine size

• Estuarine condition

• Estuarine isolation

Scores are allocated to criteria, where factors that influence the estuarine fish community positively are allocated higher scores than those that influence fish communities negatively. Allocated scores within each criterion are subjected to various weighting factors, after which the products are added to generate a fish-based estuarine hierarchy.

This prioritization exercise enabled South African estuarine systems to be ranked according to their importance to estuary-associated fish species, thus facilitating the identification of South African estuaries with a high conservation priority for fishes. The ranking can be presented in both a regional and country context, thus assisting conservation decisions on both a local and national scale.

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