H L Jerling

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.

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.

Effect of feeding frequency and feeding rate on growth of Oreochromis mossambicus (Teleostei: Cichlidae) fry

The effect of feeding frequency and feeding rate on growth of Oreochromis mossambicus fry was investigated over 30 d under temperature- and light-cycle-controlled conditions on the University of Zululand campus, South Africa, in October and November 2009. Weight gain, specific growth rate and gross food conversion ratio were significantly affected by feeding frequency, but survival rate was not significantly affected; the best results were obtained at a feeding frequency of four times per day. Feeding rate significantly affected weight gain and gross food conversion ratio, but not the specific growth rate or survival rate. Taking into consideration weight gain, specific growth rate and gross food conversion ratio, feeding four times per day at a rate of 15% body weight per day is optimal for fry growth.

Mesozooplankton of the Kosi Bay lakes, South Africa

The Kosi coastal lake system, a chain of four interconnected basins, is located in the subtropical north-eastern corner of South Africa. Little information is available on zooplankton of the system and the main aim of this study is to report on zooplankton samples collected during 2002 and 2003. The set of samples consists of seasonal, subsurface mesozooplankton samples that were collected during nighttime in each of the lakes. A well-developed salinity gradient was evident along the interconnected lakes in the subsurface water during all seasons, ranging from freshwater in the upper lake Amanzamnyama to a maximum of 22 recorded in Lake Makhawulani. The zooplankton community structures of the lakes reflected the salinity gradient of the system, with some coastal marine taxa recorded in the lakes closer to the mouth and only freshwater taxa recorded in Lake Amanzamnyama. Mesozooplankton diversity and abundance were relatively low compared to other estuarine systems along the eastern coast of South Africa. The dominant taxa were calanoid copepods Acartiella natalensis and Pseudodiaptomus stuhlmanni and the mysid Mesopodopsis africana in the lower lakes, whereas cyclopoids Mesocyclops sp. and Thermocyclops sp. dominated the freshwater lake Amanzamnyama.

Mesozooplankton community structure changes in the Mfolozi–Msunduzi estuarine system, South Africa, during contrasting river flow conditions

The Mfolozi–Msunduzi estuarine system is subject to periodic dry and wet cycles, with subsequent changes in the abiotic and biotic characteristics of the system. The aim of the current study was to compare its mesozooplankton composition during relatively dry and wet periods. Mesozooplankton samples were collected between 2007 and 2010 in both the Mfolozi and the Msunduzi, covering a dry period between 2007 and 2008 and a period of relatively high freshwater inputs during 2009 and 2010. High flows during the wet period reduced the densities of most of the dominant estuarine mesozooplankton taxa in the Mfolozi Estuary, such as estuarine calanoids Pseudodiaptomus stuhlmanni (Poppe & Mrázek, 1895) and Acartiella natalensis (Connell & Grindley, 1974). The Msunduzi Estuary functioned as a reservoir from which recolonisation by estuarine taxa would quickly take place after the Mfolozi was scoured by floodwaters. Densities of dominant meroplankton taxa, such as zoeae of the crab Paratylodiplax blephariskios and Macrobrachium spp., were not noticeably different in the Mfolozi–Msunduzi system between the low- and high-flow periods.