T.D. Harrison

Estuarine Typology and the Structuring of Fish Communities in South Africa

SynopsisSome 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.

Application of a multimetric fish index to assess the environmental condition of South African estuaries

A multimetric fish index, the Estuarine Fish Community Index (EFCI) of Harrison and Whitfield (2004), was applied to data collected for 190 South African estuaries. Estuaries spanned three biogeographic regions and included three distinct estuarine typologies. 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. Metric reference conditions and scoring criteria were developed separately for each estuary type within each zoogeographic region. The final EFCI was applied to each estuary by comparing its fish community with the appropriate reference. Index values ranged between 18 (very poor) and 66 (very good). 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 EFCI 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.

Gilchristella aestuaria (Pisces: Clupeidae) biomass and consumption of zooplankton in the Sundays Estuary

Gilchristella aestuaria is an important resident zooplanktivorous fish species in most southern African estuarine systems. The dry mass of this species in the Sundays Estuary during 1986/87 ranged from 11 g·m−3 in the lower reaches to 178 g·m−3 in the middle reaches. The average total biomass of G. aestuaria in the estuary was 219 kg, with a maximum value of 723 kg dry mass recorded in November 1986. Daily food consumption by the estuarine round herring ranged from 0,6 mg·m−3·day−1 in the lower reaches to 20,4 mg·m−3·day−1 in the middle reaches. On average, G. aestuaria consumed almost 19 kg dry mass of food per day in the Sundays Estuary during 1986/87, or 6,9 tons dry mass per year. Most food consumption occurred during summer (37 kg·day−-1), compared to only 1 kg·day−1 in winter. The higher summer consumption coincides with increased zooplankton abundance in the Sundays system during the warmer months. Calculations indicate that production by the calanoid copepod Pseudodiaptomus hessei in the Sundays E...

A preliminary survey of the estuaries on the southeast coast of South Africa, Cape Padrone – Great Fish River, with particular reference to the fish fauna

ABSTRACT A basic ichthyofaunal and physico-chemical survey of 10 estuaries on the southeast coast of South Africa from Cape Padrone to the Great Fish River was conducted during October and November 1991. The greatest diversity of fishes was found in the freshwater-deprived but permanently open Kariega, Kowie and Bushmans estuaries and this can be attributed in part to the influence of clear, marine waters and the greater number of marine stragglers recorded. The number of species recorded in the freshwater-dominated Great Fish Estuary was lower but published information suggests that a higher fish biomass per unit area occurs in this system when compared to equivalent freshwater-deprived estuaries. Six systems (Boknes, Kasouga, Rufane, Riet, West Kleinemonde and East Kleinemonde) were temporarily open/closed estuaries and although supporting lower species diversity also provided important habitat for a number of estuarine associated species, particularly resident species, which were recorded in high numbers.

A preliminary survey of the estuaries on the southeast coast of South Africa, Cape St Francis – Cape Padrone, with particular reference to the fish fauna

ABSTRACT A basic ichthyofaunal and physico-chemical survey of the coastal outlets on the southeast coast of South Africa (Cape St Francis – Cape Padrone) was conducted during September and October 1995. Eleven systems have been identified along this stretch of coastline. Four systems (Kromme, Gamtoos, Swartkops and Sundays) are permanently open and provide important habitats for a number of estuarine-resident and marine fish species. Three systems (Seekoei, Kabeljous and Van Stadens) are moderately sized estuaries characterised by intermittently open mouths. The Seekoei Estuary has undergone extreme human disturbance, while the Kabeljous and Van Stadens are in a less disturbed state. All three systems provide nursery habitats for both estuarine-resident and estuarine-associated marine species. The Maitland is a small, shallow sandy system that is typically closed to the sea for extended periods; despite this it was still found to support estuarine-associated species particularly mugilids. The remaining three systems (Bakens, Papkuils and Coega) are all highly altered systems that provide little littoral habitat for either resident or migrant marine species.

Fishes as Indicators of Estuarine Health and Estuarine Importance

The use of fishes as indicators of environmental health is based primarily on the tenet that fish assemblages effectively integrate and reflect the condition or health of aquatic ecosystems. Bioindicators of ecosystem condition have traditionally relied on single measures such as species diversity, species dominance, and presence/absence of indicator species. Individually, however, these indicators provide limited information on ecosystem health. In contrast, the estuarine fish community index follows a more integrated approach by combining several attributes into a single multi-metric index. Similarly the fish importance rating, which is also based on several criteria, serves to prioritize estuaries on a regional basis in terms of their importance to estuary-associated fish species.

A preliminary survey of the estuaries on the south coast of South Africa, Robberg Peninsula - Cape St Francis, with particular reference to the fish fauna : ecological overview article

A basic ichthyofaunal and physico-chemical survey of the coastal outlets on the south coast of South Africa (Robberg Peninsula to Cape St Francis) was undertaken during November 1994 and September 1995. Some 27 systems were identified along this stretch of coast and of these, 48% (Brak, Helpmekaars, Klip, Witels, Geelhoutbos, Kleinbos, Bruglaagte, Langbos, Sanddrif, Eerste, Boskloof, Kaapsedrif and Klipdrif [Wes]) comprised the inlets of coastal streams that offer little habitat for fishes, 26% (Sout, Bloukrans, Lottering, Elandsbos, Storms, Elands and Groot [Oos]) are located within deeply incised valleys within the Tsitsikamma region and provide limited littoral habitat for estuary-associated fishes; 26% of the systems (Piesang, Keurbooms, Matjies, Groot [Wes], Tsitsikamma, Klipdrif [Oos], Slang) appear to serve some estuarine function for both marine migrant and resident species although some of these may also be limited due to their small size and infrequent connection with the sea. This study serves to emphasize the ecological importance of these latter estuaries with in this coastal sector.

A preliminary survey of the estuaries on the southeast coast of South Africa, Old Woman's – Tyolomnqa, with particular reference to the fish fauna

A basic ichthyofaunal and physico-chemical survey of estuaries on the southeast coast of South Africa from Old Womans to Tyolomnqa (former Ciskei Region of the Eastern Cape) was undertaken during September 1996. Sixteen coastal outlets have been identified along this stretch of coastline. The greatest diversity of fishes was found in the permanently open Keiskamma and Tyolomnqa estuaries and this can be attributed in part to the permanent connection with the sea that allows year-round access for marine species. Ten systems were predominately closed estuaries and, although supporting lower species diversity also provided important habitat for a number of estuarine-dependent marine species as well as resident species, which were often recorded in high numbers. One system was a small coastal stream, which is of limited value for estuarine-associated fishes. Three of the systems were dry stream beds and provided no habitat for fishes. Twelve of the estuaries in this region therefore provide important habitat for estuarine-associated species and also support a number of endemic species which further emphasises the importance of these estuaries in maintaining ichthyofaunal diversity in the region.

Grey mullet (Mugilidae) as possible indicators of global warming in South African estuaries and coastal waters.

The grey mullet usually occur in large numbers and biomass in the estuaries of all three South African biogeographic regions, thus making it an ideal family to use in terms of possibly acting as an environmental indicator of global warming. In this analysis the relative estuarine abundance of the dominant three groups of mugilids, namely tropical, warm-water and cool-water endemics, were related to sea surface coastal temperatures. The study suggests a strong link between temperature and the distribution and abundance of the three mullet groups within estuaries and indicates the potential of this family to act as an indicator for future climate change within these systems and adjacent coastal waters.

Fishes as Indicators of Estuarine Health

The use of fishes as indicators of environmental health is based primarily on the tenet that fish assemblages effectively integrate and reflect the condition or health of aquatic ecosystems. Bioindicators of ecosystem condition used to rely on single measures such as species diversity, species dominance, and presence/absence of indicator species. Individually, however, these indicators provide limited information on ecosystem health and have been overtaken by more holistic approaches, especially those based on fish community indices. A variety of estuarine fish community indices have been developed in the past decade, all of which follow a more integrated approach and combine several attributes into a single multi-metric index. This review highlights developments within this field. Emphasis is also placed on recent work that combines biomarker information at the cellular and individual fish level with index information at the fish community level, thus giving a more useful and meaningful bioindicator of estuarine health.