The Wilderness Lakes is comprised of three coastal lakes: the first two, Eilandvlei and Langvlei, are barrier lagoons and the third, Rondevlei, is a deflation basin that has been artificially connected by a channel to Langvlei (Hall et al. 1987). The lakes provide a flood plain for the Touw River and Estuary (Figure 1). The depth of the Touw Estuary varies from 1.5 m to 3 m (Hall et al. 1987; Russell 2003). Rondevlei is the smallest lake with a surface area of 1.43 km² and a maximum depth of 6 m. Langvlei is the shallowest lake with a maximum depth of 4 m, but has the largest surface area of 2.16 km². Eilandvlei has a maximum depth of 6.5 m and is the deepest in the system, with a surface area of 1.5 km² (Hall et al. 1987). Under drought conditions, the water levels in the system drop significantly, resulting in the separation of Rondevlei, Langvlei and the connecting channel. This scenario was observed between March and May 2010 during the study period.

The Touw Estuary is a temporarily open and closed estuary that is closed off from the sea for varying periods by a sandbar at the mouth. Under pristine conditions, the estuary opened approximately 40% of the time. Opening of the estuary mouth has been managed by SANParks since the 1970s to prevent flooding of private property adjacent to the estuary (Russell 2013). When water levels in the estuary reach between 2.1 m and 2.4 m above mean sea level, the mouth is opened manually by SANParks. Declining freshwater inputs and artificial breaching have resulted in reduced opening of the estuary, with the estuary now only open approximately 28% of the time (Russell 2013). During this study, the estuary mouth was only open in July 2010 (4 days) and between December 2010 and January 2011 (28 days) (Figure 2b).

The Wilderness Lakes often exhibits a reverse salinity gradient, with the salinity increasing higher up the system towards Rondevlei (Allanson & Whitfield 1983). Tidal penetration into the lakes is minor during open phases, with higher salinities in Rondevlei and Langvlei caused by low or no freshwater input into these lakes (Russell 2013). A reverse salinity gradient was present throughout the study period, with the highest salinities (mean ± s.d.: 10.1 ppt ± 0.2 ppt) recorded in Langvlei and the lowest in the Touw Estuary (mean ± s.d.: 4 ppt ± 2.3 ppt). Surface water temperature varied little across the system, but varied seasonally with relatively warm summer (mean ± s.d.: 26.0 °C ± 0.9 °C) and mild winter (mean ± s.d.: 13.6 °C ± 0.6 °C) temperatures.

The Wilderness Lakes are bordered by a margin of emergent aquatic plants predominantly made up of Phragmites australis (common reed) and, to a lesser extent, Juncus kraussii (dune slack rush), Schoenoplectus scirpoides (Biesiegoed) and Typha capensis (bulrush) (Russell 2003). Submerged aquatic macrophytes occur in each of the lakes and are mostly made up of stands of Potamogeton pectinatus (fennel pondweed), Charophyta and filamentous algae (Cladophora spp.). Zostera capensis (eelgrass) occurs sporadically in the Touw Estuary (Allanson & Whitfield 1983; Howard-Williams & Liptrot 1980; Weisser & Howard-Williams 1982). During drought conditions, the density of aquatic vegetation decreases, which is assumed to be the result of increased salinity levels.

During this study, a total of 25 species were caught (Tables 1, 2 and 3). A total of 23 species were recorded in the Touw Estuary and Eilandvlei, 11 in Langvlei and 15 in Rondevlei. Of the species caught, euryhaline marine species were dominant (15 species), followed by estuarine resident species (7 species) and freshwater species (3 species). Estuarine resident species, particularly estuarine roundherring (Gilchristella aestuaria) and Cape silverside (Atherina breviceps), were the most abundant species recorded (with more than 10 000 individuals caught). Seine net catches in all water bodies were dominated numerically by estuarine roundherring (G. aestuaria) and Cape silverside (A. breviceps), which together comprised between 66% (Rondevlei) and 99% (Touw Estuary) of the catch (Table 1). The freshwater species Mozambique tilapia (Oreochromis mossambicus) was also numerically abundant in the system, with more than 5000 individuals sampled. Mozambique tilapia comprised between 63% of the catch (Touw Estuary) and 99.9% (Langvlei and Rondevlei) of the fyke net catch (Table 2). Gill nets, which sampled larger individuals of marine and freshwater species, were dominated numerically by euryhaline marine species, particularly southern mullet (Liza richardsonii), which comprised between 10% (Langvlei) and 40% (Eilandvlei) of the catch, and oval moony (Monodactylus falciformis), which comprised between 3% (Eilandvlei) and 48% (Langvlei) of the catch.

Oval moony was present throughout the system and was common in Langvlei and Rondevlei, where it comprised, respectively, 48% and 27% of the gill net catch (Table 3). Although this species commonly uses the middle and upper reaches of estuaries as nursery areas, adults (> 120 mm standard length [SL]) are generally found in the near-shore marine environment (Whitfield 1998). Individuals captured during this survey ranged in size from 13 mm FL to 228 mm FL and showed a bimodal length frequency distribution. The majority of small individuals in the 90 mm FL – 100 mm FL size class were caught in the Touw Estuary. Larger specimens represented by the 190 mm FL – 200 mm FL size class were caught in Langvlei and Rondevlei (Figure 3). The presence of individuals > 120 mm FL in the upper lakes suggests that they were trapped in the system.

Southern mullet was present in all water bodies and common (comprised more than 20% of the catch) in gill net catches in Eilandvlei and Rondevlei (Table 3). Common as juveniles and adults in estuaries in the Eastern and Western Cape, southern mullet is normally more abundant in the adjacent near-shore marine environment (Bennett 1989). Individuals captured during this survey were all large, mature individuals ranging in size between 200 mm FL and 483 mm FL, with most individuals in the 340 mm FL – 350 mm FL size class (Figure 3). The smallest individuals were recorded in Eilandvlei. Maturing at 225 mm FL, with spawning taking place in the near-shore marine environment (De Villiers 1987; Van der Horst & Erasmus 1981), they are usually more abundant in the lower and middle reaches of southern Cape estuaries (Whitfield & Kok 1992). The abundance of adults, particularly in Langvlei and Rondevlei, during this study indicates possible entrapment in the upper lakes.

Although it is found as juveniles and adults in both estuaries and rivers, freshwater mullet (Myxus capensis) is often referred to as a catadromous species, with spawning occurring in the near-shore marine environment (Bok 1979). Freshwater mullet was present throughout the system, and increased in abundance in Rondevlei where it comprised 31% of the gill net catch (Table 3). Individuals showed a bimodal length frequency distribution; the first size class comprised individuals with a mode of 300 mm FL – 310 mm FL and the second size class comprised individuals with a mode of 380 mm FL – 390 mm FL. The larger individuals were found in Rondevlei (Figure 3).

Cape stumpnose (Rhabdosargus holubi), considered to be entirely dependent on estuarine habitats for the first year of life (Whitfield 1994), was present throughout the system and more common in Rondevlei where it comprised 5% of the gill net catch (Table 3). Cape stumpnose is often the most abundant marine species recorded in both temporarily open and closed and permanently open estuaries in the warm-temperate region of South Africa (Harrison 2005; Vorwerk et al. 2001). Individuals captured during this survey ranged in size from 120 mm FL to 420 mm FL with a polymodal length frequency distribution; the first size class comprised individuals with a mode of 140 mm FL – 160 mm FL, caught mainly in the Touw Estuary. The second size class comprised individuals with a mode of 230 mm FL – 240 mm FL, mainly from Eilandvlei, and the third size class comprised individuals with a mode of 340 mm FL – 350 mm FL from Rondevlei (Figure 3). Cape stumpnose normally migrate back to sea after reaching 120 mm SL. Some individuals, however, remain trapped in closed estuaries, where they reach sizes greater than 200 mm SL (James, Cowley & Whitfield 2007a).

Large, predatory species such as leervis (Lichia amia) and springer (Elops machnata) were present in the estuary, Eilandvlei and Rondevlei, with one large leervis caught in Rondevlei (> 900 mm FL). Spotted grunter (Pomadasys commersonnii) and white steenbras (Lithognathus lithognathus), two important fishery species, showed a similar trend, with a greater proportion of smaller individuals sampled in the Touw Estuary and Eilandvlei and only a few very large individuals caught in Rondevlei (Table 3 and Figure 3). These species are all dependent on estuaries as nursery areas, but usually return to sea as sub-adults (Whitfield 1998).

Seven estuarine resident species were caught, of which estuarine roundherring and Cape silverside were common or abundant in seine net catches in all water bodies. The other estuarine resident species, including prison goby (Caffrogobius gilchristi) (Touw Estuary and Eilandvlei) and speckled sandgoby (Psammogobius knysnaensis) (Touw Estuary, Eilandvlei and Rondevlei), were not caught in all water bodies. Cape silverside captured during this survey were between 16 mm FL and 100 mm FL. The majority (> 85%) caught in Eilandvlei and Rondevlei were mature individuals (< 40 mm, Ratte 1989). In contrast, only 53% of the individuals caught in the Touw Estuary and 59% of the individuals caught in Langvlei were mature (Figure 4). Estuarine roundherring captured during this survey were between 20 mm FL and 89 mm FL, with the largest individuals caught in Rondevlei (Figure 4). Estuarine roundherring is estimated to mature at a size of 28 mm SL (Talbot 1982).

Three alien freshwater species, Mozambique tilapia, western mosquitofish (Gambusia affinis) and common carp (Cyprinus carpio), were sampled. Mozambique tilapia was abundant throughout the system (dominated fyke net catches in all sampling areas). Although endemic to southern Africa, Mozambique tilapia does not occur naturally south of the Bushmans Estuary in the Eastern Cape, but the species has been introduced extensively into farm dams as an angling fish in systems further south along the coastline, from which it has escaped. It is most abundant in temporarily open and closed estuaries during the closed phase (Whitfield & Blaber 1979), possibly because of a combination of increased suitable nesting habitats during the closed phase and reduced recruitment success during open phases (Ellender et al. 2008). Although large individuals (> 200 mm FL) were caught throughout the system, the majority were small (< 180 mm FL) individuals (Figure 5).

The first formal record of the invasive common carp within the Wilderness Lakes was from Langvlei in 2009 (Olds et al. 2011), with fish measuring between 150 mm and 300 mm. Larger specimens (500 mm FL – 800 mm FL) were sampled in 2010 in each of the lakes, whilst in 2011 three juveniles were sampled in the Touw Estuary (Figure 5). Together the data show a progression in size classes from 2009 to 2011 and confirms a successful spawning event having occurred in the Wilderness Lakes. Western mosquitofish were sampled in the 30 m seine net within the Touw Estuary during this study and previously in the Touw and Duiwe rivers by Russell (1999). This small species is most likely underrepresented in seine net samples because it is generally associated with marginal vegetation in the shallow littoral zone (Sloterdijk et al. 2015). Sloterdijk et al. (2015) sampled the Wilderness Lakes using scoup nets concurrently with this study and showed this species to be widespread and abundant throughout the system.

Improving or maintaining the ecological functioning of the Wilderness Lakes requires management actions to halt observed directional changes in water quality trends and improve connectivity between lakes. In particular, as in other systems, the structure of the marine fish community is determined by the timing of the mouth opening. In addition, the community is also influenced by the degree of flow and the ease of movement between lakes. The directional change in water chemistry from an estuarine to a more lacustrine system noted by Russell (2013) needs to be halted. Recognition of the national and international importance of the system dictates that the artificial mouth opening needs to take into consideration biodiversity and ecosystem processes and should only occur at the required height of 2.1 m above mean sea level. Premature breaching due to human pressure (flooding risk) will only continue to exacerbate the trends in water quality and overall ecosystem functioning. To ensure adequate water flow and fish recruitment throughout the system, restrictions (sediment deposition) within the connecting channels need to be assessed and, where possible, removed. In addition, the dense macrophyte stands occurring within these channels not only restrict water movement but also could limit fish movement. Both submerged and emergent aquatic plants within the channels should be regularly cut. Importantly, the cut plants should be removed from the channel to prevent possible anoxic conditions during decomposition. Monitoring of fish recruitment within and throughout the system should continue to assess efficacy of management actions, whilst research should focus on the impacts of the invasive fish species on endemic species and broader ecosystem functioning.