L Vivier

Fish assemblages in the Mfolozi-Msunduzi estuarine system, KwaZulu-Natal, South Africa, when not linked to the St Lucia mouth.

The fish community of the Mfolozi–Msunduzi estuarine system was investigated with particular emphasis on its role as a nursery area for marine fish when the adjacent St Lucia mouth is closed. The mouth was open during March 2007 and 2008 with high-turbidity river water flowing into the sea, and was closed during August 2007 and 2008. Fish were sampled biannually with seine-nets during March and August in 2007 and 2008 at five sites throughout the system. A total of 5 886 fish from 59 species were recorded, with a higher number of species and CPUE in the Mfolozi Estuary than in the Msunduzi Estuary. Seine-net catches were dominated by Ambassis gymcocephalus, Ambassis natalensis, Leiognathus equula and Valamugil cunnesius. Juveniles of marine spawning species were present throughout the study period, even when the mouth was closed. The sampled fish assemblage structure was influenced by significant temporal differences between the four biannual sampling seasons, with relatively small spatial differences between the sampling areas. Temperature, salinity and sediment characteristics were most responsible for the structure of the fish assemblage as sampled by seine-nets. The results emphasise the importance of the Mfolozi–Msunduzi estuarine system as an alternate nursery area.

Fish breeding in, and juvenile recruitment to, the St Lucia Estuarine System under conditions of extended mouth closure and low lake levels

Ongoing drought conditions along the northern coast of KwaZulu-Natal resulted in the St Lucia Estuary mouth closing, and to evaporation leading to extremely low lake levels and the development of hypersaline conditions in most parts of the system. The mouth closed in June 2002 and has remained closed for more than three years. The St Lucia Estuarine System is an important nursery ground for juveniles of many marine fish species. Ezemvelo KZN Wildlife initiated a once-off survey to investigate the effects of the drought on the fish fauna and to establish if any fish were breeding in the system under the existing drought conditions. Fish were sampled at six sites throughout the system, using seine and gill-nets. In December 2004, the system had become partitioned into four isolated compartments, and at this time a total of 4 475 individuals comprising 30 species were recorded. An analysis of size class data indicated the presence of juveniles of at least 12 species, including nine estuarineassociated marine species, which had spawned after mouth closure. Although some estuarine species had spawned within the system, most appeared to have recruited into the Narrows and lake sections of the St Lucia system during January 2004 when the adjacent Mfolozi River, which is linked to the open Mfolozi Estuary, flooded and overtopped into St Lucia Estuary. Should the St Lucia mouth remain closed for four or more years, it can be expected that the lack of recruitment of first-time spawners from the St Lucia nursery grounds into the off-shore marine breeding stocks will have major impacts on these estuaryassociated marine species.

Benthic faunal distribution and abundance in the Mfolozi–Msunduzi estuarine system, KwaZulu-Natal, South Africa

The Mfolozi–Msunduzi estuarine system historically shared a common mouth with the St Lucia estuarine system. In 1952, a separate mouth was created, 1.5 km south of St Lucia mouth, to prevent silt carried by the Mfolozi River from entering Lake St Lucia. Despite its proximity to the comparatively well-studied St Lucia Estuary, there is very little information on the Mfolozi– Msunduzi estuarine system in general and no information on its benthos. In the present study, 17 taxa were recorded from biannual quantitative sampling in 2007 and 2008. Results indicated that the system was dominated by the polychaetes Ceratonereis sp., Dendronereis arborifera and Capitella capitata, the crab Paratylodiplax blephariskios and the tanaid Apseudes digitalis. The main factors influencing the distribution of the benthos were oxygen concentration, temperature, the open or closed state of the mouth, and salinity, with particle size and organic content of the substratum also being important in determining community structure. Although the dominant taxa were previously recorded as abundant in St Lucia, numbers present in the Mfolozi–Msunduzi system were generally lower than those recorded in St Lucia, suggesting that periodic flooding and the unstable nature of the sediments in the Mfolozi–Msunduzi prevented its benthos from attaining the densities recorded in the adjacent St Lucia Estuary. While the Mfolozi–Msunduzi system is classified as a river mouth, its benthos was more similar in composition to that of the Mhlathuze and Mlalazi estuaries, which are classified as an estuarine bay and a permanently open estuary, respectively. It is suggested that reclassification of the Mfolozi–Msunduzi system as a permanently open estuary would be more appropriate in terms of its benthos.

Juvenile fish recruitment through wave-overtopping into a closed subtropical estuary

The fish fauna of the subtropical Nhlabane estuary in northern KwaZulu-Natal was studied during a regional drought over the period 1991–1995. The estuary mouth closed in late 1991, due to the drought and to water abstraction from the system, and reopened only at the end of the drought in late 1995. The fish community underwent considerable changes in composition and abundance, from marine-dominated during 1992 to freshwater/estuarine-dominated in 1995. The adverse effect of extended mouth closure on the fish fauna and the lack of contact with the marine environment was clearly illustrated. Due to mouth closure, recruitment of estuarine-dependent marine species almost stopped, although some recruitment by means of wave-overtopping was found to continue, even though the estuary mouth remained closed. Recruitment of twelve estuarine-dependent marine fish species was found to occur in this way, suggesting that recruitment through wave-overtopping is an important aspect of estuarine functioning during drought conditions.

The potential for zoobenthic fauna dispersal into the St Lucia Estuary from the Mfolozi–Msunduzi estuarine system, KwaZulu-Natal, South Africa

The onset of a prolonged drought in the St Lucia catchment in 2002 and subsequent mouth closure raised concern for the biota of the estuary. Ezemvelo KZN Wildlife proposed a scheme whereby the Mfolozi–Msunduzi mouth would be closed, causing fresh water to enter the St Lucia Estuary to prevent excessively hypersaline conditions (>65) from developing in the lower and middle reaches of the St Lucia system. Although this proposal caused concern about the impact of mouth closure on the ecology of the Mfolozi–Msunduzi estuarine system, it raised its potential as a source of benthic fauna dispersal into, and the restocking of, the St Lucia system. Quantitative benthic sampling in the Mfolozi–Msunduzi system in 2007 and 2008 revealed a benthic community generally dominated by polychaetes which, although low in terms of total species richness compared with St Lucia, included elements of a core benthic group present in the St Lucia system throughout the current drought. The presence of the gravimetrically important crab Paratylodiplax blephariskios in the Mfolozi–Msunduzi system was considered important as a source from which the once-abundant population in the St Lucia Narrows could be restocked via dispersal through the channel linking the Mfolozi–Msunduzi Estuary to St Lucia. It was concluded that, while the input of fresh water from the Mfolozi River is necessary to ameliorate hypersaline conditions and conserve the important core benthic group in the Narrows and South Lake, consideration must be given to allowing sufficient tidal exchange in the Mfolozi–Msunduzi system to maintain its stocks of benthos with an obligate marine phase.

Ecological status and role of the Mfolozi–Msunduzi estuarine system within the iSimangaliso Wetland Park, a World Heritage Site on the south-east coast of South Africa

The original structure and functioning of the historical Mfolozi–St Lucia estuarine system are described and anthropogenic impacts, which resulted in the Mfolozi and Msunduzi rivers being separated from the St Lucia Estuary and having their own combined mouth to the sea, are detailed. An overview is provided of the current ecological status of three major faunal groups, zooplankton, benthos and fish, present in the Mfolozi–Msunduzi estuarine system, a system upon which, until recently, virtually no scientific research had been conducted. Currently, decreased rainfall and extended closure of the St Lucia mouth have resulted in this system being under major ecosystem stress. Management options, both short- and long-term, related to reconnecting the Mfolozi–Msunduzi to a common mouth with St Lucia are considered. These include the re-establishment of sediment-filtering swamps in the lower reaches of the Mfolozi and Msunduzi rivers, as well as the construction of levees that would protect the bulk of the adjacent land on which sugar cane is farmed. Concern is expressed regarding potential problems that may arise as a result of the reconnection of the two systems.

Initial characterisation of the mesozooplankton community of the Mfolozi– Msunduzi estuarine system, KwaZulu-Natal, South Africa, during a low-flow period

The mesozooplankton of the Mfolozi and Msunduzi estuaries, which share a combined mouth, was sampled twice per year during a two-year period of relatively low river flow. Samples were collected during March, after the rainy season when the estuary mouth was open, and during August, after the low-rainfall winter months when the mouth was closed. The estuarine calanoid copepods Pseudodiaptomus stuhlmanni and Acartia natalensis were numerically dominant, making up 75% of the total number of mesozooplankton recorded. Relatively high abundances were recorded for these copepods, with P. stuhlmanni reaching peak densities of 79 000 individuals m−3. In the low-salinity regions of the estuaries, especially the upper regions of the Msunduzi, the cladoceran Moina sp. also attained high densities, reaching 50 000 individuals m−3. During sampling sessions when the mouth of the estuaries was closed the systems became relatively fresh, in contrast to open-mouth conditions when seawater dominated the lower reaches. Peak densities of estuarine copepods occurred during times of mouth closure. When the mouth was open these copepods generally declined in number, and an influx of coastal marine species such as paracalanids, Corycaeidae and chaetognaths was evident. Although higher zooplankton densities were mostly recorded in the Msunduzi Estuary, the species compositions in the two systems were relatively similar, with no significant differences between them. The Mfolozi Estuary is normally classified as a river mouth type, but during the present study this estuary effectively functioned as a temporarily open/ closed type estuary. The low-flow conditions did not adversely affect the estuarine mesozooplankton, largely because the mouth never remained closed for any extended period.

Diet of the demersal feeding Leiognathus equula in the Mfolozi–Msunduzi estuarine system, South Africa, in response to an impoverished macrobenthic invertebrate community

Leiognathus equula, a species generally regarded as a zoobenthic feeder, was the most abundant fish species sampled in the Mfolozi–Msunduzi estuarine system in 2008–2010, despite the zoobenthos in this system being impoverished. Four dietary assessment methods all confirmed that its diet, both spatially and temporally, was overwhelmingly dominated by the copepod Pseudodiaptomus stuhlmanni. Plankton data indicated the presence of high P. stuhlmanni densities supporting this fish species. It appears likely that L. equula are present in relatively large numbers due to their ability to target P. stuhlmanni, probably preying on them during daylight hours when these copepods are on or near the estuary bottom.

Prawn community structure in the subtropical Mfolozi–Msunduzi estuarine system, KwaZulu-Natal, South Africa

The life cycles of many marine and freshwater prawn species are closely associated with estuarine habitat. Here, the prawn community of the Mfolozi–Msunduzi estuarine system is described and the systems potential as an alternative nursery for prawns during prolonged closure of the adjacent St Lucia estuarine system is examined. Sampling was conducted at five localities in summer and winter in 2007–2012. The Mfolozi–Msunduzi Estuary is a river-dominated system with mouth condition, sediment grain size and turbidity being the most important factors affecting the structure of its prawn community. Twelve prawn species were recorded, including five marine penaeid and five freshwater palaemonid species. The prawn community was numerically dominated by freshwater Macrobrachium equidens and two penaeids, Fenneropenaeus indicus and Metapenaeus monoceros. Significant differences were recorded between summer and winter samples, with freshwater species dominating the catch in summer, while penaeid species were dominant in winter. The marked seasonal change in the prawn community was related to strong river flows during summer creating low-salinity conditions, unsuitable for penaeid postlarval development, throughout most of the system. Compared to St Lucia, the Mfolozi–Msunduzi Estuary can be regarded as a poor alternative nursery area for penaeids during the summer peak postlarval recruitment period.

Penaeid and carid community changes in the St Lucia estuarine lake system, South Africa, under low water level, extended closed periods and marine reconnection conditions

Penaeid and carid communities were assessed in St Lucia over a wide range of saline conditions, water level conditions and mouth states, including a six-month marine connection and various connections to the Mfolozi Estuary. Samples were collected biannually in spring and autumn from November 2004 to May 2012 using seine nets. Seven penaeid and seven carid species were recorded, of which Palaemon pacificus, P. peringueyi, Penaeus indicus and Metapenaeus monoceros were the most abundant. The decline from 2004 to 2007 in both species recorded and densities of freshwater, estuarine and marine species during the initial closed period was linked to increased salinity, lack of recruitment and reduced estuarine surface area. Increased wave action and high seas following Cyclone Gamede opened the mouth in March 2007 after a 57-month closed period, resulting in recruitment of marine species, predominantly P. indicus, followed by a decline in densities after reclosure in August 2007. Increases in marine and freshwater species were evident after Mfolozi flood connections during 2008, 2009 and 2010. Highest densities were recorded after breaching in 2007 and after flood connections, highlighting the importance of a marine link to maintain the recruitment of penaeid species.