2.2).
Figure 2.1 Map of South Africa indicating the three biogeographic provinces, based on an analysis of estuarine fish assemblages
(modified from Harrison 2002).
Marine estuarine‐opportunist (MEO) species that use estuaries as nursery areas were the dominant taxa in all estuary types within all biogeographic regions (Harrison & Whitfield 2008). Although many MEO taxa utilise estuarine nursery areas, most are not entirely dependent on these environments and are able to use alternative marine nursery areas (Elliott et al. 2007). The paucity of estuaries in the South African cool‐temperate region probably favours those species that are not entirely dependent on estuaries as their nursery areas but are also able to utilise alternative marine nursery areas such as coastal embayments. The mugilid Chelon richardsonii, for example, is a key MEO species in cool‐temperate estuaries and, although their juveniles utilise estuarine nursery areas, they have also been recorded in the inshore waters of sheltered embayments on the Cape coast of South Africa (Clark et al. 1994). A similar situation has been reported in Australia, where the juveniles of MEO species such as the mugilid Mugil cephalus appear to prefer estuarine nursery areas in temperate Western Australia where there are numerous rivers, but further north in subtropical regions, where there are no estuaries, the juveniles of M. cephalus are abundant in nearshore waters (Lenanton & Potter 1987). Marine straggler (MS) species, which are likely to be physiologically stenohaline, generally do not constitute a numerically important component of the ichthyofauna in estuaries, especially along coasts where river flow is substantial and estuarine salinity is low.
Figure 2.2 nMDS ordinations of the ichthyofauna of South African estuaries based on (a) presence/absence, (b) abundance and (c) biomass, and categorized by biogeographic province
(modified from Harrison & Whitfield 2006a).
The key taxa identified during the study by Harrison & Whitfield (2006a) may be divided into a number of groups (Figure 2.3), based on their spatial occurrence and relationships with environmental variables. The first group (Group 1) comprises tropical species (e.g. leiognathid Leiognathus equula) that are largely restricted to the warm, brackish, turbid waters of subtropical estuaries, with the distribution of these fishes strongly linked to water temperature. A second group (Group 2) also comprises tropical species (e.g. haemulid Pomadasys commersonnii) but their distribution extends further south into warm‐temperate estuaries. Although the abundance of most species in this group was also positively correlated with water temperature, they are an important component of the fish community of both subtropical and warm‐temperate estuaries. The third group (Group 3) comprises ‘warm‐water’ endemic species (e.g. sparid Rhabdosargus holubi) that are common in warm‐temperate and subtropical estuaries but are generally not a major component of the fish community of cool‐temperate or tropical estuaries. The fourth group (Group 4) comprises ‘cool‐water’ endemics (e.g. sparid Lithognathus lithognathus) that occur in both warm‐ and cool‐temperate estuaries but are not common in subtropical systems. These species appear to prefer cooler waters and relatively high salinities. The fifth group (Group 5) comprises temperate species (e.g. carangid Lichia amia) that occur in the eastern Atlantic region and extend around the South African coast into KwaZulu‐Natal, but do not constitute a major component of the ichthyofauna of subtropical estuaries. The last group (Group 6) comprises ‘widespread’ species (e.g. M. cephalus) that occur in all estuaries throughout the region (Figure 2.3).
Figure 2.3 South African estuarine fish faunal groupings based on environmental preferences and the zoogeography of the region (after Harrison & Whitfield 2006a).
Several authors have described the biogeographic changes in estuarine ichthyofaunal associations in other parts of the world, but not in the same manner as the above analysis. For example, along the California coast of North America, fishes associated with bays and estuaries were distinctly different north and south of Point Conception, with this location more of a boundary for southern species than for northern ones (Horn & Allen 1978). Vieira & Musick (1993, 1994) established that the majority of the species in tropical and warm‐temperate estuaries of the western Atlantic were young‐of‐the‐year that were maintained by recruitment waves from the adjacent marine environment. Ayvazian et al. (1992) found, however, that in the temperate north‐eastern USA, there was a trend towards a decrease in estuary nursery use by marine species and an increase in diadromous and solely estuarine species with increasing latitude. Dame et al. (2000) described a similar pattern in estuaries of the South Atlantic coast of North America, where systems in temperate North Carolina, South Carolina and Georgia were dominated by estuarine spawning species, while fish assemblages in the more subtropical Florida estuaries contained a higher proportion of marine species.
In the north‐eastern USA, Roman et al. (2000) found that fishes with life history strategies classified as nursery, marine, diadromous or transient species represent a greater percentage of fishes using estuarine habitats in more southern latitudes, while resident fishes and seasonal residents dominate the fauna of estuaries in the northeast. Emmett et al. (2000) raise the important point that, unlike the situation on the east coast of the USA, where most fish species reside in estuaries during most of their life history, many North American west coast species (especially anadromous taxa) use estuaries only during a short period of their life cycle. However, these estuaries play an important role in the life histories of these species, e.g. salmonids. From studies in temperate New Zealand estuaries, McDowall (1985) suggested that, with increasing latitude, there was a change from species of marine origin to freshwater/diadromous taxa, and thus similar to the findings above for fishes in estuaries along the eastern North American coast.
The composition and diversity of estuarine fish assemblages in Europe are also often related to latitude and thus biogeographic zones. Records of fish species from European estuarine waters obtained from information published in Elliott & Hemingway (2002) included data of fish taxa reported in 23 European estuaries covering nine countries (Figure 2.4). The presence/absence of each species from each estuary was used to generate a Bray‐Curtis resemblance matrix and subjected to non‐metric multidimensional scaling (Figure 2.5). The results indicate that, based on their fish assemblages, the compositions of European estuaries are broadly arranged according to a latitudinal gradient, with those in systems below 45 °N situated towards the bottom half of the ordination and those at latitudes above 45 °N located in the top half of the ordination (Figure 2.5a).
