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The St Lucia estuarine system on the east coast of South Africa is a declared World Heritage Site and Ramsar Site of International Importance. A major ecological feature of St Lucia during the last century was the annual spawning migration of the flathead mullet Mugil cephalus down the system in the first half of each year. Top predators, such as t...
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... flathead mullet Mugil cephalus has a life-history cycle that encompasses mainly marine and estuarine environments but can also extend into fresh waters (Whitfield et al. 2012) (Figure 1). In most parts of the world M. cephalus spawns in the nearshore marine environment, the egg and embryo stages drift passively in ocean currents, and there is an onshore migration at the postflexion larval stage and first entry into estuaries at about 10 mm standard length (Wallace 1975a(Wallace , 1975b). ...
Citations
... However, the freshwater input was still not sufficient to facilitate a natural mouth breaching event that could reconnect the estuary with the adjacent Indian Ocean, restoring connectivity and flushing the system of the accumulated sediment (Jones et al. 2020). In response to a number of concerns including the system's prolonged mouth closure and freshwater state, back-flooding in the adjacent sugarcane fields, sediment build-up and proliferation of reeds, the decision was taken to artificially breach the St Lucia Estuary on 6 January 2021 (Daly et al. 2021;Fox and Mfeka 2021;Whitfield 2021) by skimming the berm down to the estuary level, which was 1.25 m above mean sea level (MSL) at the time. ...
... This is lower than those recorded during a previous wet phase where densities of up to 600 000 ind m -3 were recorded, but similar to densities during the 2007 natural mouth breaching event (Carrasco et al. 2010;Carrasco and Perissinotto 2015). The generally lower densities here could be due to lower freshwater input, the open mouth state that facilitated export of some zooplankton production to the Indian Ocean, or possibly due to predation as the marine connectivity may have allowed for the introduction or recruitment of fish that could have depressed zooplankton stocks (Daly et al. 2021;Whitfield 2021). ...
... Mugil cephalus used to be a dominant marine fish species in the gillnet catches of illegal gillnet fishers in False Bay and North Lake (Mann 1995(Mann , 2003 but this fishery ceased when the lake nearly dried up during the 2000s (Supplementary Figure S2). The prolonged closed phase during the first two decades of the 21st century led to the loss of M. cephalus from the St Lucia system, details of which are provided by Whitfield (2021a). Since a wide variety of top predators such as African fish eagles Haliaeetus vocifer, white pelicans Pelecanus onocrotalus and Nile crocodiles Crocodylus niloticus were dependent on M. cephalus as a primary food source (Whitfield and Blaber 1978a, 1979a, 1979b, the trophic impact of this loss on the lake ecosystem must be significant. ...
... (the mouth was still open at time of writing), have failed to provide evidence of the re-establishment of the large biomass of M. cephalus that was historically present in the St Lucia system (Whitfield 2021a). A re-establishment of the annual autumn spawning migration by this species from the lake, down the Narrows and into the estuary, will signal the return of Lake St Lucia into a fully functional estuarine system. ...
... Reduced river flow can modify connectivity between the catchment and the ocean by its effect on estuarine salinities and gradients, water temperatures, turbidity, nutrient status, and primary and secondary productivity (Gillanders et al. 2022), all of which will influence the stresses experienced by fishes in estuaries (Whitfield 2021a). When occurring over decades, such changes can result in the complete loss of all marine and diadromous fish species from affected estuaries (Fig. 4, Krispyn et al. 2021;Whitfield 2021b). ...
The dependence on connectivity and use of estuaries by two major groups of fishes, namely estuary-associated marine and diadromous species, are reviewed. The former group comprises marine estuarine–opportunists and marine estuarine–dependents, and the latter anadromous, catadromous, and amphidromous species. Examples of ingress to estuaries by larvae and juveniles of species from each group illustrate the importance of freshwater-estuarine-marine connectivity to the life cycles of those species. Factors that threaten estuarine connectivity, including the potential/possible consequences of global climate change on the contribution of these key taxa to coastal fish assemblages at pristine or recent historical levels, are highlighted. The implications of reduced connectivity on the current and future status of these major groups in estuaries are also discussed. Finally, it is noted that the abundance of fishes in the above guilds has already declined substantially and that there are no clear prospects for a reversal of this trend. Possible future research on fishes and coastal connectivity include applications of environmental DNA and otolith microchemistry, and the assessment of fish responses to the removal of dams to restore connectivity in catchment rivers.
... During drought, the mouth status in TCEs tends towards a prolonged closed state, but depending on catchment size and management, whereas PrOEs are likely to close more often. When drought induced closure of the estuarine inlet occurs, not only are the extent and variability of the physico-chemical environment, and community composition affected, but also recruitment migrations into the estuary and back out to sea of larvae, juveniles and adults, which may even lead to temporal species extinctions (Carrasco et al., 2013;Whitfield, 2021). At current estimates, 20% of South African estuaries are severely threatened by flow modifications as a result of the abstraction of more than a third of the freshwater that previously reached the coast (van Niekerk et al., 2019). ...
... This comparison emphasises that, while estuaries play a very important role as a nursery area for M. cephalus in many regions, they are not essential for the survival of this species as a whole. An ancilliary example is the probable existence of contingent populations of M. cephalus at Lake St Lucia, with a strong possibility that the estuarine contingent of this species may have disappeared from this system as a result of the loss of marine-estuarine connectivity for more than a decade (Whitfield, 2021). ...
... Prolonged estuary mouth closure lasting more than a decade resulted in a cessation of access to estuarine nursery areas for both marine estuarine-opportunists and marine estuarine-dependent fish species. The consequences for these species was devastating, with major declines in the sparid Rhabdosargus sarba and mugilid Mugil cephalus being documented (Mann and Pradervand, 2007;Whitfield, 2021). This lack of habitat access to a 35 000 ha estuarine lake, that constitutes approximately 50% of the estuarine area in South Africa, is likely to detrimentally affect a number of other estuary-associated marine species that were once abundant in the system (Schutte et al., 2020). ...
The concept of estuarine dependence by various fish species has been widely discussed and written about by
ichthyologists for decades. In this review we selected single, well studied species that have the potential of being
classified as dependent on estuaries for survival of the taxon on each of the following continents – Africa,
Australia, Europe, North America and South America. The three main guilds used in our assessment are marine
estuarine-dependents, estuarine resident-dependents and catadromous estuarine-dependents. Of the three categories
reviewed by this study, diadromous taxa are perhaps the most dependent on estuaries as a physical
environment, even though these species spend less time in estuaries when compared to marine or estuarine
categories. This is particularly the case with estuarine resident species that complete their entire life cycle within estuaries and whose primary population is found in estuaries and not in adjacent freshwater or marine environments. We also provide an overview of which guilds can be regarded as estuary-dependent and which are estuary-opportunist. Issues that are related to estuary dependency by fishes in estuaries, for example, the existence
of contingents within a species population and the impact of global/climate change on estuary-dependent species, are also discussed.
... The cause(s) of this extreme hypersalinity may be linked to the removal of natural vegetation from the river catchment that led to salinization of the river water when present, as well as a prolonged drought in the region that was possibly linked to climate change. A second example of compromised estuarine function is due to the loss of marine connectivity for more than a decade, and is epitomized by the estuarine Lake St Lucia tending to be transformed into a freshwater dominated coastal lake, with the consequent decimation of many estuary-associated fish and invertebrate species in the process (Whitfield, 2021). Table 2 The estuarine definition adopted for nature conservation purposes under the European Union Habitats and Species Directive ...
... The ecological reasons why some scientists supported the assisted breach of the St Lucia Estuary mouth in January 2021 are outlined in a review by Whitfield (2021). A natural opening of the St Lucia Estuary mouth occurred in April 2022, but visual evidence suggests that the compacted sediments in the estuary basin and Narrows were not scoured from the system by outflowing iMfolozi River floodwaters. ...
Major threats to South African estuaries include freshwater flow alterations, urbanisation, habitat transformation, deteriorating water quality, increased biological invasions and resource over-exploitation. Although South African estuary restoration is in its infancy, important lessons have been learnt through past efforts undertaken in some of the most impacted systems. This review presents case studies focused on hydrological and physical restoration actions, while recognizing the need to implement urgent remedial measures to restore the populations of targeted estuary-associated marine fish species. Common interventions designed to arrest declining estuary health include artificial management of estuary mouths and the use of artificial structures to restore tidal action and remediate erosion damage. Limited natural habitat restoration actions have been conducted on the subcontinent. Fortunately, estuaries are resilient ecosystems and can recover fairly rapidly if appropriate restoration interventions are successfully applied. Restoration needs to be co-ordinated through a national strategy which is currently not in place. It is particularly important to address the root cause of deterioration through active restoration and/or rehabilitation measures rather than only managing symptoms. The implementation of existing and future conservation legislation regarding the exploitation of natural resources within estuaries needs to be prioritised, so that over-exploited angling and subsistence fish species populations can recover.
... The management authority, based on scientific advice from a specially commissioned GEF (Global Environment Facility) Project, supported a natural mouth breaching policy. In retrospect, however, the absence of an adaptive management policy when more than a decade had elapsed with no mouth opening caused some estuary-associated marine species to disappear from the system (Whitfield 2021). In addition, flooding of floodplain agricultural land during high Mfolozi River flow periods led to protests by farmers whose livelihood was threatened owing to the closed estuary mouth phase. ...
Estuarine and coastal waters are acknowledged centres for anthropogenic impacts. Superimposed on the complex natural interactions between land, rivers and sea are the myriad consequences of human activity – a spectrum ranging from locally polluting effluents to some of the severest consequences of global climate change. For practitioners, academics and students in the field of coastal science and policy, this book examines and exemplifies current and future challenges: from upper estuaries to open coasts and adjacent seas; from tropical to temperate latitudes; from Europe to Australia. This authoritative volume marks the 50th anniversary of the Estuarine and Coastal Sciences Association, and contains a prologue by founding member Professor Richard Barnes and a short history of the Association. Individual chapters then address coastal erosion and deposition; open shores to estuaries and deltas; marine plastics; coastal squeeze and habitat loss; tidal freshwaters – saline incursion and estuarine squeeze; restoration management using remote data collection; carbon storage; species distribution and non-natives; shorebirds; modelling environmental change; physical processes such as sediments and modelling; sea level rise and estuarine tidal dynamics; estuaries as fish nurseries; policy versus reality in coastal conservation; developments in estuarine, coastal and marine management.
... However, since 2002, increasing water extraction for irrigation upstream, compounded by a period of prolonged drought in the region, meant the mouth did not connect with the sea, except for a short period in 2007 following a big-sea event (Cyrus and Vivier 2006;Taylor 2006;Cyrus et al. 2010Cyrus et al. , 2020Schutte et al. 2020). This, and the historic separation of the Mfolozi River from the St Lucia Estuary in 1953 and various management interventions over decades, led to low lake levels and extreme hypersalinity (up to 200 in parts of the lake), causing most of the marine-dependent ichthyofaunal biodiversity to die out (Taylor 2006;Mann and Pradervand 2007;Whitfield and Taylor 2009;Carrasco and Perissinotto 2012;Whitfield 2021). Subsequently, on 6 January 2021, the beach berm separating the St Lucia Estuary from the sea was artificially breached, allowing the system to reconnect with the sea-in anticipation that it would restore some of the estuarine ecological functioning of the system and in response to ongoing concerns from the public relating to the accumulation of sediment, the continued closed mouth, the freshwater state of the entire St Lucia system, the growth of reeds choking part of the estuary, and the back-flooding of sugarcane fields in the Mfolozi floodplain, among other concerns (AK Whitfield, South African Institute for Aquatic Biodiversity, pers. ...
... Although bull sharks may not represent an important component of the diet of the Nile crocodile at St Lucia year-round, a single neonate shark weighing approximately 2.5 kg may constitute a large portion of a crocodile's daily ration and may be an important opportunistic source of food (Whitfield and Blaber 1979). Crocodiles have been documented to respond opportunistically to prey availability at St Lucia, as observed, for example, during the historic mullet run (Whitfield and Blaber 1979;Whitfield 2021). In this study, crocodiles were observed foraging at the mouth, where they were regularly seen preying on sharptooth catfish that had succumbed to the increased salinity and were being washed out to sea. ...
... Although it remains unclear how the St Lucia ecosystem will respond over time to the artificial breaching of the mouth, this study provides evidence to suggest that recruitment of a top marine predator back into the system may be rapid. Despite decades of management effort to restore the ecological functioning of St Lucia, its resilience remains in the balance (Taylor 2006;Whitfield and Taylor 2009;Everett et al. 2015;Cyrus et al. 2020;Forbes et al. 2020;Whitfield 2021). St Lucia warrants the World Heritage Site status conferred upon it for its natural beauty and biodiversity, and we hope that unique observations such as reported here will further motivate the need to conserve and manage this system in a way that promotes its ecological functioning and protects its unique biodiversity. ...
Estuaries provide critical nursery habitat for juvenile bull sharks Carcharhinus leucas, as they have the ability to withstand a wide range of salinities. St Lucia is the largest estuarine lake in Africa and was once a key nursery for bull sharks until a prolonged and near-continuous period of mouth closure and drought between 2002 and 2021. The estuary mouth was opened for the first time in 13 years on 6 January 2021, and, within 10 days, bull shark pups recruited into the estuary. On 16 January, an adult Nile crocodile Crocodylus niloticus was observed preying on a live neonate bull shark which it swallowed whole. This observation provided the first photographic evidence in Africa and highlighted a unique interaction between these species, which are top predators in the freshwater and coastal environments, respectively. Estuaries remain important nursery habitats for bull sharks in the region and we assembled the known records of bull shark occurrence in all South African estuaries. In summary, the rapid recruitment of bull shark pups into St Lucia Estuary is notable for the management and conservation implications for this important estuarine system, as well as for regional bull shark populations.
This article addresses the history of a resident population of bull sharks (Carcharhinus leucas) in an isolated stagnant body of water in subtropical Australia. From 1996 to 2013, six bull sharks were landlocked in a golf course lake near Brisbane. The adjacent Logan and Albert rivers trapped sharks due to major floodings. When floodwaters receded, these sharks remained in the lake, which is normally isolated from the riverʼs main channel. While this event was extensively reported in the media and recieved much public attention, it has not been investigated in depth, yet it provides an opportunity for insights into the tolerance of bull sharks to low salinity habitats and euryhalinity in this species. Currently, information on the extent of the bull sharkʼs capability to endure low salinity conditions and its longevity in these environments is scarce. The case reported here provides information on the occurrence of bull sharks for 17 years, which represents the longest uninterrupted duration in a low salinity environment that ever has been recorded in this species. Bull sharks arrived first in the lake as juveniles but through time, they have reached maturity. This occurrence presents not just another ordinary bull shark record from a low salinity environment but instead a record of physiological and scientific importance. Therefore, details of the residency of C. leucas in an Australian golf course lake are reported here.
