Estuarine Typology and the Structuring of Fish Communities in South Africa

Abstract

Some 190 South African estuaries, covering all biogeographic provinces within the region, were classified into three types based on a combination of mouth condition and estuary size (surface area). The fish communities of the estuary types within each zoogeographic region were described and compared. Multivariate analyses revealed that each estuary type contained somewhat distinct fish communities. In addition, the study identified common patterns in species richness and ichthyofaunal composition. Open estuaries have relatively high species richness; this is a reflection of a permanent or near-permanent connection with the sea which allows access into these estuaries by all marine migrant species within the region. Intermittent connection with the sea limits the recruitment and utilisation of closed estuaries by marine migrant species; this results in reduced species richness in moderate to large closed estuaries. Small closed estuaries exhibit the lowest species richness and this is probably a result of their limited habitat and increased isolation from the sea. The key fishes that utilise estuaries could also be categorised into a number of groups based on their relative importance within each estuary type. Some species are largely restricted to predominantly open systems. Other taxa, while important in predominantly open estuaries, also occur in moderate to large closed systems. Some estuarine-associated species are well represented in all estuary types but exhibit a greater importance in closed estuaries. This study has shown that South African fish communities not only reflect estuarine typology but also respond to these differences in a consistent manner that spans all zoogeographic regions. The prevalence of similar patterns in other parts of the world suggests that estuarine typology is a major driver in the structuring of global estuarine fish communities.

The Importance of Regional, System-Wide and Local Spatial Scales in Structuring Temperate Estuarine Fish Communities

“…Nevertheless, as demonstrated in this study and supported by others (e.g. Edgar et al 1999;Harrison and Whitfield 2006a), estuarine type and, in particular bar-state, is a major driver in structuring spatial differences in estuarine fish communities in temperate, microtidal waters. estuaries in this study have characteristics that probably reflect evolutionary adaptations to being disconnected from the sea and its moderating influences (e.g.…”

“…Whitfield 1999;Harrison and Whitfield 2006a). Indeed, while not the main focus of their study, Harrison and Whitfield (2008) demonstrated the latter to be true for fish guild compositions in three estuary types and bioregions across South Africa.…”

“…Moreover, when factors across multiple scales are examined, they are typically tested at the level of individual environmental variables rather than as a collective group representing a scale as a whole. For example, in an extensive study of the fish assemblages in 190 estuaries throughout South Africa, Harrison and Whitfield (2006a) demonstrated that species composition differed significantly between most estuarine types (i.e. permanently-open, small normally-closed and moderate to large normally-closed estuaries) within each of the three bioregions along the coast.…”

Fish under influence: A macroecological analysis of relations between fish species richness and environmental gradients among European tidal estuaries

“…At this large scale, estuarine fish communities are related to environmental conditions (river flow and temperature) but also to biogeographic species distribution. Main features of estuaries thus strongly influence fish habitats and communities at different scales (Roy et al, 2001;Saintilan, 2004;Harrison and Whitfield, 2006). Consequently, taking into account the large-scale driving factors (Keddy, 1992), macroscopic studies of the relationships between environment and fish assemblage structure may help, first, to understand the functioning of estuarine ecosystems, and then, to assess their ecological status so that appropriate management decisions can be taken (Brown, 1995;Peterson, 2003;Engle et al, 2007).…”

“…Power et al, 2000;Daufresne et al, 2003) or from spatial comparisons of a number of different sites (e.g. Ley, 2005;Harrison and Whitfield, 2006). Previous very large scale American, South African and Australian studies have found estuary mouth configuration (Monaco et al, 1992;Edgar et al, 2000;Ley, 2005;Harrison and Whitfield, 2006), estuarine water area (Monaco et al, 1992;Pease, 1999;Harrison and Whitfield, 2006), latitude (Edgar et al, 1999;Pease, 1999;Harrison and Whitfield, 2006) and catchment area hydrology (Ley, 2005) to be the main abiotic determinants of species richness.…”