Nicola C. James

A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system

In 2005/2006 a multidisciplinary research programme that included studies on the hydrodynamics, sediment dynamics, macronutrients, microalgae, macrophytes, zoobenthos, hyperbenthos, zooplankton, ichthyoplankton, fish and birds of the temporarily open/closed East Kleinemonde Estuary was conducted. Particular attention was given to the responses of the different ecosystem components to the opening and closing of the estuary mouth and how this is driven by both riverine and marine events. Using a complementary dataset of daily estuary mouth conditions spanning a 14-year period, five distinct phases of the estuary were identified, including closed (average = 90% of the days), outflow (<1%), tidal (9%) and semi-closed (<1%). The open-mouth phase is critical for the movements of a number of estuary-associated fish (e.g. Rhabdosargus holubi) and invertebrates (e.g. Scylla serrata) between the estuary and sea. The timing of this open phase has a direct influence on the ability of certain estuaryassociated fish (e.g. Lithognathus lithognathus) and invertebrates (e.g. Palaemon peringueyi) to successfully recruit into the system, with a spring opening (October/November) being regarded as optimal for most species. The type of mouth-breaching event and outflow phase is also important in terms of the subsequent salinity regime once the berm barrier forms. A deep mouth breaching following a large river flood tends to result in major tidal inputs of marine water prior to mouth closure and therefore higher salinities (15–25). Conversely, a shallow mouth breaching with reduced tidal exchange during the open phase often leads to a much lower salinity regime at the time of mouth closure (5–15). The biota, especially the submerged macrophytes, respond very differently to the above two scenarios, with Ruppia cirrhosa benefiting from the former and Potamogeton pectinatus from the latter. River flooding and the associated outflow of large volumes of water through the estuary can result in major declines in zooplankton, zoobenthos, hyperbenthos and fish populations during this phase. However, this resetting of the estuary is necessary because certain marine invertebrate and fish species are dependent on the opening of the estuary mouth in order to facilitate recruitment of larvae and post-larvae into the system from the sea. Slight increases in the numbers of certain piscivorous and resident wading bird species were recorded when the estuary mouth opened, possibly linked to increased feeding opportunities during that phase.

Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review

The majority of estuaries along the coastline of southern Africa are termed temporarily open/closed estuaries (TOCEs) and are closed off from the sea for varying periods by a sandbar which forms at the mouth. It is therefore important to understand the processes occurring within TOCEs and their importance to fishes in order to make sound management recommendations. Estuaries along the coast of South Africa and their associated fish assemblages are biogeographically distinct and occur in either a subtropical, warm-temperate or cool-temperate zone. There are 125 TOCEs found within the cool-temperate and warm-temperate zones. Most fish species found in TOCEs are the juveniles of marine taxa that breed at sea. Permanently open estuaries generally have a higher diversity of species than TOCEs, but TOCEs still provide important nursery areas for many marine species and numerically often have a higher proportion of estuarine resident species. Important taxa in terms of abundance and biomass in warm-temperate TOCEs include the sparids Rhabdosargus holubi and Lithognathus lithognathus, several mugilid species, estuarine residents (particularly Gilchristella aestuaria and Atherina breviceps) and the freshwater cichlid Oreochromis mossambicus. The diversity of fishes in cool-temperate TOCEs is low when compared with warm-temperate systems and Liza richardsonii tends to dominate catches by number and mass in most systems. Several species recorded in TOCEs show clear longitudinal distribution trends. For example Atherina breviceps is generally more abundant in the lower reaches of estuaries. Mouth state, particularly the frequency, timing and duration of mouth opening plays a key role in determining species richness, composition, diversity and abundance in TOCEs. Mouth state is directly linked to freshwater input. Reduced river inflow leads to prolonged mouth closure and shorter open phases, which inhibits immigration and emigration of marine fish species between estuaries and the sea. Understanding of the effects of various processes occurring within these systems, particularly variation in freshwater input, on the biota of these important systems facilitates the development of informed management recommendations.

Abundance, recruitment and residency of two sparids in an intermittently open estuary in South Africa

The recruitment of distinct year-class cohorts in two sparid species, Rhabdosargus holubi and Lithognathus lithognathus, were linked to records of daily mouth state in the intermittently open East Kleinemonde Estuary, South Africa, between 1995 and 2006. L. lithognathus only recruited into the estuary in years when the mouth opened between late August and January. This was attributed to a limited spawning season and inability to recruit during wave overwash events. In contrast, R. holubi recruitment was uninterrupted and not influenced by seasonality of estuary access opportunities (mouthopening and overwash events). Estuarine-dependent residency periods ranged from 27 months to 48 months for L. lithognathus and 12 months to 23 months for R. holubi. The interannual abundances of these estuarydependent sparids were determined by reproductive seasonality, recruitment strategy and seasonal timing of estuarine access opportunities.

Ecomorphological analysis as a complementary tool to detect changes in fish communities following major perturbations in two South African estuarine systems

Ecomorphological changes as a result of natural perturbations in estuarine fish communities were investigated in two South African estuaries (Swartvlei and East Kleinemonde), both before and after the loss of aquatic macrophyte beds in these systems. The fish communities were analysed using an ecomorphological diversity index (EMI) and the results compared to a traditional index, the Shannon-Wiener diversity index. The EMI revealed that the major changes in fish community composition recorded in both estuaries were associated with quantitative variations at the species level. Both estuaries essentially lost their macrophyte beds and ended up with the same type of bottom habitat (bare sediment). In both cases the fish morphological variability decreased immediately after aquatic macrophyte loss and then increased to end above the initial value. The ecomorphological analysis appeared to be sensitive to major ecological disturbances that occurred during the study period and this was confirmed by the morphospace configuration. The results indicate that the ecomorphology of the fish community responds to habitat changes and that this change corresponds to alterations in the representation of the different feeding types. These findings therefore contribute to the measurement of morphological changes in estuarine fish assemblages as a result of habitat changes within the ecosystem and we propose that ecomorphological analyses add another dimension to the information provided by existing diversity indices in studying changing fish communities.

Temporal variability of a temperate fish assemblage in Africa's oldest marine protected area

A standardised research-based, shore-angling survey was conducted in the 47-year-old Tsitsikamma National Park Marine Protected Area (MPA) between 1998 and 2005. The aim of this study was to examine variability in community structure, relative abundance (catch per unit effort, CPUE) and size frequency of the shore-angling fish assemblage in this no-take MPA. A total effort of 8 141 angler hours yielded 8 236 fish of at least 54 species from 19 families. The overall CPUE showed no significant trend over time (r 2 = 0.015, p = 0.1). The interannual variation in CPUE indicated relatively stable populations for some species (e.g. blacktail Diplodus capensis) and patterns of recruitment failure for others (e.g. galjoen Dichistius capensis). However, the lack of a significant trend in taxonomic distinctness (r 2 =0.008, p = 0.8), the stable rank order of species abundance (χ2, r = 361, p < 0.05) and the high coefficient of concordance (W = 0.84) indicated a relatively stable fish assemblage. This was attributed to the extended length of protection afforded by this MPA. One noticeable trend was an increase in the abundance of species with an affinity for warm water and whereas this correlated to an annual increase in water temperature of 0.024 °C between 1998 and 2005, a longer time-series is necessary to identify rise in sea temperature (climate change) as a driver of change in this warm-temperate fish assemblage.

Review of the projected impacts of climate change on coastal fishes in southern Africa

The coastal zone represents one of the most economically and ecologically important ecosystems on the planet, none more so than in southern Africa. This manuscript examines the potential impacts of climate change on the coastal fishes in southern Africa and provides some of the first information for the Southern Hemisphere, outside of Australasia. It begins by describing the coastal zone in terms of its physical characteristics, climate, fish biodiversity and fisheries. The region is divided into seven biogeographical zones based on previous descriptions and interpretations by the authors. A global review of the impacts of climate change on coastal zones is then applied to make qualitative predictions on the likely impacts of climate change on migratory, resident, estuarine-dependent and catadromous fishes in each of these biogeographical zones. In many respects the southern African region represents a microcosm of climate change variability and of coastal habitats. Based on the broad range of climate change impacts and life history styles of coastal fishes, the predicted impacts on fishes will be diverse. If anything, this review reveals our lack of fundamental knowledge in this field, in particular in southern Africa. Several research priorities, including the need for process-based fundamental research programs are highlighted.

Marine Connectivity and Fish Length Frequencies of Selected Species in Two Adjacent Temporarily Open/Closed Estuaries in the Eastern Cape Province, South Africa

The effects of timing, frequency and duration of mouth opening and marine overwash events on fish assemblages in two, adjacent temporarily open/closed estuaries were investigated. Sampling was conducted biannually over two years during summer and winter, using selective sampling gear to target estuarine-associated marine species. During this period the East Kleinemonde Estuary opened more frequently than the West Kleinemonde Estuary and it also experienced a greater number of bar overwash events when larval recruitment can occur. The timing of mouth opening and the frequency of marine overwash events did not influence species richness of marine-spawning species but had a pronounced effect on abundances (catch-per-unit-effort) of species. The species composition of both estuaries was similar, but the catch-per-unit-effort in the East Kleinemonde Estuary was more than three times greater than that of the West Kleinemonde Estuary. The East Kleinemonde also supported a greater proportion of immature individuals of Rhabdosargus holubi and Monodactylus falciformis, and the length frequency distributions of both these species differed significantly between the two estuaries. The larger fish sizes in the West Kleinemonde Estuary were attributed to the prolonged mouth closure that resulted in the trapping of larger juveniles and mature fish within the estuary.

Population dynamics and biology of an invasive population of mosquitofish Gambusia affinis in a temperate estuarine lake system

This study investigates the distribution, relative abundance, population growth and biology of the alien invasive fish Gambusia affinis in the Wilderness Lakes System, a coastal lake system in the warm-temperate region of South Africa. Fish were collected throughout the system during four seasonal sampling trips in spring (October 2010), summer (February 2011), autumn (May 2011) and winter (July 2011). Gambusia affinis were widespread and abundant in all habitats from the freshwater reaches of the inflowing Touw River to the more saline lakes and estuary. Sex ratio was generally female dominated and mean length at maturity was 14.7 mm standard length (SL) for males and 20.3 mm SL for females. Reproduction was strongly seasonal with reproductively active fish sampled in spring and summer but not in autumn and winter. Relative abundance also varied seasonally, with populations following typical ‘boom and bust’ population dynamics. By winter, the population in all habitats was lower with mortality rates as high as 85%. The stronghold of the population appears to be in the channels between the lakes, where relative abundance was more consistent and winter mortality rates were lower. The survivors, which are primarily females, then reconstitute the population in the spring and summer months.

Resilience of fish assemblages in the Breede Estuary, South Africa, to environmental perturbations

Biogeographical transitional areas are often characterised by fluctuating environmental conditions and high variability in species composition and abundance. A 10-year summer sampling programme was undertaken in the permanently open Breede Estuary, situated between the winter and bimodal rainfall regions and in the cool/warm-temperate biogeographical transition zone. This allowed for the collection of data highlighting inter-annual changes in the fish community of a transition zone estuary and relating these changes to selected environmental variables (salinity, temperature and flow). Although the abundance of individual species varied on an inter-annual basis, the ranking of fish taxa in the Breede Estuary varied little over the 10-year period, indicating a fairly stable species composition. Multivariate analysis showed that the fish communities sampled in summer within each reach (upper, middle, lower) remained similar, regardless of year, and this was largely driven by salinity patterns which showed little inter-annual variation during the low-flow summer period. The Breede Estuary is, however, subject to regular winter and spring flooding, where up to 50% of the annual rainfall may occur over a few days. Although the magnitude of flooding prior to sampling had little effect on species abundance, changes in the frequency or return-time of events did; such that when two floods occurred in succession species abundance and richness declined.

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.