Lubabalo Mofu

In many animal species, males frequently have alternative reproductive tactics (ART) to maximise their reproductive success. These reproductive strategies or tactics are often characterised by differences in traits such as behaviour, physiology and morphology. However, there is very limited evidence for the occurrence of such tactics in Gobiidae. In this study, we investigated the population of Glossogobius callidus from artificial ponds located in the Lower Sundays River Valley, in the Eastern Cape, South Africa across three seasons (Winter, Spring and Summer). We found that the urogenital papilla can be used to sex G . callidus , without sacrificing the fish. Urogenital papilla of G . callidus was longer and wide in females and shorter in males. In addition, three morphologically distinct male morphs, which represented alternative reproductive tactics were identified (parental, intermediate and sneaker). There was a significant relationship between ART and the different male morphs of G . callidus . These three male morphs were distinguished by body coloration, with parental males displaying dark colouration, while sneaker males exhibited light colouration, and intermediate males showed characteristics between the two extremes. Males exhibiting dark coloured morphs were significantly heavier than those with light coloured morphs. There was a high abundance of dark morphs in Spring (355 individuals) and Summer (202 individuals), which corresponds to the reproductive season. In contrast, the high abundance of light morphs observed during winter (n = 142) suggests that alternative reproductive tactics or traits may be established prior to the breeding season. This study contributed to knowledge on the ART of G . callidus .

Rivers are recognized as significant pathways and transportation for microplastics (MPs), an emerging contaminant, to aquatic environments. However, there is limited evidence on how riverine reservoirs influence MPs transport. To fill this gap and provide baseline empirical data and insights to South African context, the current study assessed the seasonal variation in MP densities from sediments collected upstream, within the reservoir, and downstream of the Vondo Reservoir along the Mutshindudi River. We hypothesised that MP densities would be highest within the reservoir, due to the lack of constant flow that would otherwise transport accumulated particles downriver. Additionally, we expected the cool–dry season to be associated with the highest MP densities. As expected, high MP densities were observed within the reservoir (117.38–277.46 particles kg⁻¹ dwt) when compared to the downstream (72.63–141.50 particles kg⁻¹ dwt) and upstream (28.81–91.63 particles kg⁻¹ dwt) sites of the reservoir. The cool–dry season (91.63–277.46 particles kg⁻¹ dwt) exhibited the highest MP densities compared to the hot–wet season (28.81–141.50 particles kg⁻¹ dwt). However, MP densities downstream the reservoir were higher during the hot–wet season (141.50 ± 24.34 particles kg⁻¹ dwt) compared to the cool–dry season (72.63 ± 48.85 particles kg⁻¹ dwt). The most dominant MP particles identified were white, transparent, and black fibres/filaments composed primarily of polypropylene (PP) and polyethylene (PE). This suggests diverse sources of MP particles. No significant correlations were found between water parameters and MP densities across sampling sites and seasons, indicating a widespread and context-independent presence of MPs. These findings contribute to MP studies in freshwater environments and further reinforce the role of sediments as sink for MPs and suggest that riverine reservoirs similar to dams can trap MPs, which may then be remobilized downstream during high-flow periods. Importantly, the results of this study can support local municipalities in implementing targeted plastic pollution mitigation strategies and public awareness campaigns, particularly because the Vondo Reservoir serves as a critical water resource for surrounding communities.

Invasive freshwater fish can often have severe negative effects on native fishes in river systems. The interactions between hydrology and habitat variability can mediate the speed and success of individual invasions and the consequent impact on biodiversity. The rivers within Addo Elephant National Park (AENP) in the Eastern Cape, South Africa experience cyclical droughts and wet periods and as a result are naturally episodic. These rivers were recently invaded by three non-native species, the invasive largemouth bass (Micropterus salmoides) as well as the extralimital sharptooth catfish (Clarias gariepinus) and Mozambique tilapia (Oreochromis mossambicus). Monitoring of key sampling sites along two rivers over a 12-year period that included two major droughts revealed unexpected patterns in the spread of these species and their interactions with native fishes. On the Coerney River, C. gariepinus repeatedly invaded and was extirpated from a seasonal reach of the river, wherein O. mossambicus was only occasionally captured. On the Wit River, two apparently independent introductions of M. salmoides in the lower and upper reaches of the river resulted in patchy habitat occupancy over the course of 12 years. While C. gariepinus regularly co-occurred with native species, M. salmoides appeared to locally extirpate the endangered Eastern Cape redfin (Pseudobarbus afer). During drought, both species persisted in close but disconnected pools, suggesting that the episodic hydrology and geomorphology of these rivers may offer temporary predation refugia for native species during drought. Conservation implications: Drought in episodic rivers can mitigate against the impact and spread of freshwater invasions within protected areas. Effects of drying on invasion corridors and spatial interactions with native species should be taken into consideration when managing such invasions. Severe droughts also offer an opportunity to actively control invasive species when they are confined to accessible drought refugia within the protected area.

Reservoirs are human-made ecosystems with diverse purposes that benefit humans both directly and indirectly. They however cause changes in geomorphological processes such as sediment cycling and influence the composition and structure of aquatic biota. This study aimed to identify water and sediment quality parameters as drivers of macroinvertebrates and fish communities during the cool-dry and hot-wet seasons in the littoral zones of three subtropical reservoirs (Albasini, Thathe and Nandoni). Macroinvertebrates and fish were collected from three sites (n = 3 from each site) in each reservoir. A total of 501 and 359 macroinvertebrates and fish individuals were collected throughout the sampling period, respectively. The present study employed a two-way ANOVA in conjunction with redundancy analysis (RDA) to assess the relationships that exist between water and sediment variables, macroinvertebrates diversity and species abundances across seasons. Based on the two-way ANOVA model, significant differences were observed across reservoirs for evenness, Simpson’s diversity, and total abundance, while seasonal differences were observed for most metrics, with exception for evenness. The RDA results identified four water variables (i.e., water temperature, oxidation–reduction potential, pH and conductivity) and one sediment metal (Mg) as the most important parameters in driving the fish community structure. Field observations and metal results attest that the Nandoni reservoir shows high concentrations of metals in sediments as compared to other reservoirs, suggesting that anthropogenic activities such as car washing, brick making, recreation, fishing, wastewater treatment work and landfill site may be the major contributor of metals to the Nandoni reservoir, which accumulate in the littoral zones. Findings of this study highlight the need to analyze reservoir ecological conditions at several scales. The study of macroinvertebrates and fish, water, and sediment chemistry in the littoral zone laid the groundwork for proposing measures for conserving aquatic ecosystems.

The analysis of food web structures has increased the understanding of the dynamics of organisms belonging to different trophic levels. In this study, the diet of two native species, Glossogobius callidus and Gilchristella aestuaria, was assessed in the presence of two non‐native species, Oreochromis mossambicus and Gambusia affinis, in irrigation ponds, Eastern Cape Province, South Africa. The proportion of dietary items consumed and assimilated by the four fish species were inferred from gut contents and carbon and nitrogen stable isotope analysis. Stable isotope analysis revealed that both G. affinis and O. mossambicus had a larger isotopic niche size than G. callidus and G. aestuaria. Although G. callidus fed on benthic resources and G. aestuaria fed on phytoplankton, gut content analysis showed that G. callidus, O. mossambicus and G. affinis fed predominantly on benthic resources, whereas G. aestuaria fed mainly on plankton resources. Considerable niche overlap corroborates the view that resource competition is a major factor shaping the composition of the four fish species. This study highlighted the low diversity of the food web within the Sundays River Valley irrigation ponds, where food items are shared by all the small‐bodied fishes.

Freshwater ecosystems show substantial loss of biodiversity as a result of anthropogenic induced stressors. In this study, we evaluated the distribution of freshwater fishes in the Kowie River catchment using historic fish distribution records supplemented by field survey data. Fishes were collected using a multi-method approach: seine nets, fyke nets and gill nets. Historic fish distribution data showed that 22 freshwater fishes from 11 families historically occurred, while in this study, we recorded a total of 16 freshwater fish species from 9 families. Overall, a decrease in the number of native species was recorded with a total of five species absent and two new non-native species recorded during the current survey. Coptodon rendalli constituted a new record in the Kowie River catchment while Clarias gariepinus was recorded for the first time in the mainstem of the Kowie River. The presence of these two non-native species in the Kowie River catchment may have implications for the conservation and management of the freshwater diversity in the catchment.

In fisheries science, life-history trait information is widely used to estimate fish population recruitment, growth, and mortality. The aging and growth of Oreochromis mossambicus from the Sundays ...