Fish and tadpole composition as an indication of differences between micro-estuaries and micro-outlets in the Eastern Cape Province, South Africa
Introduction
Estuaries are highly productive ecosystems that are characterised by frequent fluctuations in physico-chemical parameters such as salinity, temperature, nutrients and turbidity (Baltz and Yanez-Arancibia, 2011; Cowan et al., 2013). Many of the world's permanently open estuaries and temporarily open/closed estuarine systems play an important role as spawning and foraging areas, as well as nursery grounds for marine migrant and estuary resident fish species (Potter et al., 1990). They also serve as a corridor for the movement of species which need to access upstream freshwater areas as part of their life-history strategies (Bruton et al., 1987). As a direct result of the highly dynamic nature of estuaries, fish which inhabit these systems must be resilient and also possess the ability to adjust to the frequent fluctuations in physico-chemical conditions (Cowan et al., 2013).
Major salinity fluctuations exert acute physiological demands on the osmoregulatory processes of fishes and, as such, salinity plays a major role in influencing the presence and/or absence of both marine and freshwater fish species in southern African estuaries (Whitfield et al., 1981; Childs et al., 2008; Whitfield, 2015). Some of the other factors which have been found to have a direct relationship with the longitudinal distribution and diversity of fish assemblages in estuaries include temperature, freshwater inflow, turbidity and mouth state (Whitfield et al., 1994; Ter Morshuizen et al., 1996; Vorwerk et al., 2003). The opening and closing of an estuary mouth has implications for the system's fish community, as the open mouth state affords an opportunity for marine species to either enter or exit the estuary (Harrison and Whitfield, 2006).
A survey of the current South African literature reveals that there has been limited research conducted on fish diversity and abundance in estuarine systems located within the eastern warm-temperate section of the country's coastline, particularly between East London and the Kei River mouth. The review by James and Harrison (2016) is one of the very few studies that solely focused on an assessment of fish assemblages in multiple estuaries located in the above mentioned area. However, the aforementioned study did not consider the majority of smaller coastal systems, especially the micro-estuaries and micro-outlets (Fig. 1), which are spread throughout the warm-temperate region as well along other parts of South Africa's coastline (van Niekerk and Turpie, 2012; van Niekerk et al., 2020). The systems, collectively referred to as coastal microsystems, are characterised by being smaller in size compared to temporarily open/closed systems while also being isolated from the nearshore environment for extended periods of the year (Magoro et al., 2019). Furthermore these systems have lower abundance and diversity of estuary-associated biota, when compared to larger systems such as permanently open and temporarily open/closed estuaries (Magoro et al., 2020).
Based on the large number of microsystems around the coast, it is important that the species composition and abundance of fish assemblages in these systems, along with the underlying physico-chemical conditions, be investigated in order to determine their value as fish habitats. Whitfield and Baliwe (2013) recommended that studies be conducted on micro-estuaries and micro-outlets, the vast majority of which are unnamed (van Niekerk et al., 2020), in order to ascertain whether or not they function as estuaries and therefore contribute to the national estuarine diversity and productivity. The presence of estuary-associated fish in micro-estuaries and micro-outlets would therefore confirm that these systems share some characteristics with other larger estuarine systems at an ichthyofaunal level.
The Eastern Cape microsystem research programme has delivered a number of published studies that have attempted to use various biotic components in an attempt to differentiate between micro-estuaries and micro-outlets (Dalu et al., 2018, 2020; Magoro et al., 2019, 2020). The aforementioned studies have elucidated that micro-estuaries and micro-outlets differ in terms of phytoplankton, microphytobenthos, zooplankton and macrozoobenthos assemblages, with micro-estuaries having relatively higher diversity and abundance of the respective biotic components.
Furthermore, in a comparison between the two system types, micro-outlets have a much lower estuarine function since they have a higher proportional representation of freshwater biota for much of the year, while micro-estuaries contain a more balanced mixture of freshwater, estuarine and marine biota. The present assessment is the first from the above programme to formally record fish and tadpole assemblages inhabiting these warm temperate microsystems and to highlight how these assemblages can be successfully utilised to differentiate between micro-estuaries and micro-outlets.
Detailed studies of the structuring of fish assemblages in temporarily open/closed and permanently open Eastern Cape estuaries is contained in a series of publications (e.g. Whitfield et al., 1994; Ter Morshuizen et al., 1996; Cowley and Whitfield, 2001; Vorwerk et al., 2003) and important reviews of these systems are also covered (e.g. James et al., 2007, 2016; Perissinotto et al., 2010). The main aim of this study was to elucidate the diversity and abundance of fish assemblages in six microsystems in the warm-temperate region of the Eastern Cape Province, primarily to use these assemblages as a possible tool to differentiate between micro-estuaries and micro-outlets. Secondly, the study sought to investigate the environmental parameters that are responsible for structuring the fish assemblages within these systems. Due to a combination of system mouth size, location and surface area, the fish assemblages in the micro-estuaries were expected to consist of both a freshwater and estuarine/marine component, while freshwater taxa were expected to be overwhelmingly dominant in the micro-outlets. The presence of anuran tadpoles in the microsystems was unexpected and is documented here in an attempt to help distinguish micro-estuaries from micro-outlets.
Section snippets
Study area
The present study was conducted in three micro-outlets (Black Rock, Palm Tree and Sandy Bottom) and three micro-estuaries (Kwesani, Mtwendwe and Mtendwe) microsystems located along an eastern section of the warm-temperate region of the Eastern Cape coast (Fig. 2). The study systems were selected in order to reflect the wide spectrum between the conceptual micro-outlets (smaller-sized systems) and micro-estuaries (larger-sized systems). The smaller-sized systems (<0.5 m deep with <0.5 ha of open
Results
Kwesani Micro-estuary had the highest average channel depth at 1.1 m, while Black Rock and Palm Tree micro-outlets had the shallowest average channel depth of 0.2 m (Table 1). The highest average salinity was recorded at Kwesani (16), with average salinity levels at all three micro-outlets being below 3, and at all three micro-estuaries being above 6.
The nektonic community in most of the study systems was dominated by fish. Tadpoles belonging to the African clawed frog Xenopus laevis were
Discussion
During winter, all systems excluding the Kwesani Micro-estuary had an unimpeded connection to the sea due to strong catchment flows in the preceding days. The mouths of all the systems, with the exception of the Sandy Bottom Micro-outlet (which had a trickle of water into the boulder beach beyond the mouth), were in a closed state during both summer and autumn. The Mtwendwe Micro-estuary was the only larger-sized system which experienced mouth breaching over two consecutive seasons.
Seasonal
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was funded by the SARChI Chair in Shallow Water Ecosystems (DST/NRF), South African Institute for Aquatic Biodiversity (SAIAB) and the Nelson Mandela University (NMU). Tris Wooldridge, Lyndle Naidoo, Janine Adams, Shaun Deyzel, Nelson Miranda and Lucienne Human are thanked for their assistance with field and laboratory work. Any opinions, findings, conclusions or recommendations expressed in this paper are those of the authors, and the DST/NRF, SAIAB and NMU do not accept any
References (47)
- et al.
Do environmental factors influence the movement of estuarine fish? A case study using acoustic telemetry
Est. Coast. Shelf Sci.
(2008) - et al.
Microphytobenthos diversity and community structure across different micro-estuaries and micro-outlets: effects of environmental variables on community structure
Environ. Pollut.
(2020) - et al.
Morphometric assessment of intermittently open/closed coastal lagoons in New South Wales, Australia
Est. Coast. Shelf Sci.
(2006) - et al.
Natural nutrient enrichment and algal responses in near pristine micro-estuaries and micro-outlets
Sci. Total Environ.
(2018) - et al.
Choice chamber experiments to test the attraction of postflexion Rhabdosargus holubi larvae to water of estuarine and riverine origin. Estuarine
Coastal and Shelf Science
(2008) - et al.
Structure and function of south-east Australian estuaries
Est. Coast. Shelf Sci.
(2001) The effects of prolonged aquatic macrophyte senescence on the biology of the dominant fish species in a southern African coastal lake
Est. Coast Shelf Sci.
(1984)- et al.
The ichthyofauna of an intermittently open estuary: implications of bar breaching and low salinities on faunal composition
Est. Coast. Shelf Sci.
(1997) - et al.
What do tadpoles really eat? Assessing the trophic status of an understudied and imperiled group of consumers in freshwater habitats
Freshw. Biol.
(2007) - et al.
Ecosystem-based management of coastal fisheries in the Gulf of Mexico: environmental and anthropogenic impacts and essential habitat protection
Ubiquity and persistence of Escherichia coli in a midwestern coastal stream
Appl. Environ. Microbiol.
The diets of fish in three south-western Cape estuarine systems
S. Afr. J. Zool.
The Demography, Breeding Biology and Management of Two Mullet Species (Pisces: Mugilidae) in the Eastern Cape, South Africa. PhD Thesis
Allotopic distribution of native frogs and introduced fishes in high Sierra Nevada lakes of California: implications of the negative effect of fish introductions
Copeia
Life history styles of diadromous fishes in inland waters of southern Africa
Am. Fish. Soc. Symp.
Estuarine nekton
Ichthyofaunal characteristics of a typical temporarily open/closed estuary on the southeast coast of South Africa
Ichthyol. Bull. J.L.B. Smith Inst. Ichthyol.
Assessing phytoplankton composition and structure within micro-estuaries and micro-outlets: a community analysis approach
Hydrobiologia
Food and feeding habits of Oreochromis mossambicus (peters) in hypertrophic hartbeespoort dam, South Africa
S. Afr. J. Zool.
A practical strategy for analysing multispecies distribution patterns
Mar. Ecol. Prog. Ser.
Estuarine typology and the structuring of fish communities in South Africa
Environ. Biol. Fish.
Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa – a review
Rev. Fish Biol. Fish.
A preliminary fish survey of the estuaries on the southeast coast of South Africa, Kayser's Beach – Kei Mouth: a comparative study
WaterSA
Cited by (5)
Boosting spring runoff into the sea by reservoir regulation and its potential for estuarine fishery recovery
2024, Science China Earth SciencesSouth African coastal outlets and estuaries: what defines an estuary versus an outlet?
2023, African Journal of Aquatic ScienceHabitat Use and Connectivity
2022, Fish and Fisheries in Estuaries: A Global PerspectivePredation on small juvenile fishes in shallow estuarine nursery areas: Reply to Baker & Sheaves (2021)
2021, Marine Ecology Progress Series