African Journal of Aquatic Science

Spatio-temporal dynamics of anthropogenic land-use pressures on temporarily closed estuaries in South Africa: A four-decade remote sensing analysis

B McKelvey, CF MacKay — Fri, 30 May 2025 00:00:00 +0000

Land-cover change is the leading cause of habitat loss globally. The extent of estuarine habitat loss and land-use pressures are understudied in South Africa, particularly in the numerically dominant temporarily closed estuaries. Remote sensing techniques are effective tools for monitoring estuarine habitat loss, and the type and intensity of anthropogenic influences that derive from land-use, such as water quality deterioration. This study quantified land-use/land-cover within and adjacent to 57 temporarily closed estuaries in KwaZulu-Natal, on the subtropical east coast of South Africa, to investigate spatio-temporal land-use trends over the past four decades. Urban and agricultural land-use pressures were high in the estuarine functional zone and adjacent ecotones (32.1% and 52.1% transformed, respectively). The rate of land-cover change has stabilised, following a decline in transformation over time due to the withdrawal of cultivation from some wetland areas. There was, however, evidence of land-use intensification between the 1980s and 2018 as former agricultural land was progressively developed for urban or peri-urban use, and sand mining was identified as an important local land-use. High intensity sand mining, continued urban expansion and the introduction of urban land-uses in rural areas will intensify local land-use pressures, leading to further declines in regional estuary health.

Assessment of the inland wetland ecosystem types in South Africa: Threats and protection

H van Deventer, Jeanne L Nel — Fri, 30 May 2025 00:00:00 +0000

Ecosystem threat status (ETS) and ecosystem protection levels (EPLs) are headline indicators that can assess freshwater ecosystems at a country-wide scale. A spatial layer of freshwater, inland wetland ecosystem types of South Africa was combined with a range of spatial data sets to model their ecological condition. The ETS and EPL of each ecosystem type were determined using the area of that type in good ecological condition relative to a biodiversity target, which represented 20% of the total area of that ecosystem type. Thresholds were applied to distinguish four ETS categories ranging from Least Concern to Critically Endangered, and four EPL categories ranging from Not Protected to Well Protected. A total of 79% of the 135 of South African inland wetland ecosystem types were found to be threatened, of which 83 (62% of the number of types) are Critically Endangered, 12 (9%) are Endangered, 12 (9%) are Vulnerable and 28 (21%) of Least Concern. Of the 135 inland wetland types, 61% were Not Protected, with 6% being Well Protected, 3% Moderately Protected, and 30% Poorly Protected. Protected and Ramsar sites hosted only 7% of the total area of inland wetlands, which means that the Aichi Biodiversity Target 11 for 2020 (17%) was not met.

Influence of spatio-temporal variations in precipitation and temperature on the productivity of Eastern Lake Kyoga, Uganda

Fri, 30 May 2025 00:00:00 +0000

Spatio-temporal variations in precipitation and temperature affect productivity in aquatic systems, mainly through nutrient dynamics. Shallow lakes are more vulnerable to spatio-temporal variations, yet they are an important resource for biodiversity conservation, water supply and fisheries. The eastern arm of Lake Kyoga experiences two dry and two wet seasons annually, yet there is no information on how they affect its productivity. This study, therefore, focused on determining how spatio-temporal variations in temperature and precipitation influence production using chlorophyll-a (Chl-a) as a proxy indicator parameter. Seventeen sampling sites were established for data collection during both dry and wet seasons. Fifteen physico-chemical and biological parameters like total suspended solids (TSS), total phosphorus (TP), and chlorophyll-a were used for assessment. Thirteen of the 15 parameters had increased concentrations during the dry season, with only TSS and water temperature increasing during the wet season. Chlorophyll-a showed significant (p = 0.007) spatio-temporal variations between seasons. It significantly correlated with nitrate (r = 0.964), SRSi (r = 0.901), conductivity (r = 0.970) and pH (r = 0.936) during the dry season. Spatio-temporal variations influence nutrient dynamics, and therefore productivity in Lake Kyoga. These results are critical and should guide the water managers, decision, and policy-makers to develop appropriate mitigation approaches against impacts of spartio-temporal variations on the productivity of Lake Kyoga and its ecosystem.

Influence of physico-chemical parameters on benthic macroinvertebrate community dynamics in Isiukhu River, Kenya

Emmanuel Mzungu, William A Shivoga, Dorothy K Lukhabi, Anthony W Sifuna — Fri, 30 May 2025 00:00:00 +0000

The current study investigated the influence of physico-chemical parameters on macroinvertebrates distribution, abundance and diversity throughout the Isiukhu River. The sampling sites were selected along the Isiukhu River based on distinct land use types at the upstream, midstream and downstream sections. Sampling was done twice monthly from March 2018 to March 2019. The DO and %DO saturation decreased downstream; and an increase in temperature, conductivity, turbidity, salinity, pH and ORP downstream. A total of (N = 993) macroinvertebrate individuals made up of 21 families were found. Veliidae, Gerridae, Notonectidae and Heptageniidae families dominated. The diversity and taxon richness declined from upstream to downstream. Senyende’s upstream site had the highest taxon richness (11.4) while Mutono’s downstream site had the lowest (1.4). The Kimangeti site upstream had a higher Shannon diversity index (2.2), whilst the Ekero site downstream had the lowest (1.2). A regression analysis showed that physico-chemical water quality parameters accounted for 54.8% of the spatial variability of macroinvertebrates. The study identified DO and temperature as the main physico-chemical parameters affecting distribution patterns and abundance of macroinvertebrate communities. The results provide information for the management and conservation of streams and rivers in Africa.

First report of Dactylogyrus fimbriphallus (El Gharbi, Birgi & Lambert, 1994) infecting Luciobarbus biscarensis (Boulenger, 1911) in Oued Tadmit River of Djelfa Governorate (Algeria)

Djouhina Chabira, Badreddine Attir, Nurten Aydoğdu, Adel Mammeri — Fri, 30 May 2025 00:00:00 +0000

This study aimed to determine the occurrence of dactylogyrid communities in Sahara barbel, Luciobarbus biscarensis (Boulenger, 1911), an endemic species to Algeria, according to season and host factors. A total of 78 samples of L. biscarensis were collected seasonally, over four seasons from 2023 to 2024, from Oued Tadmit River in Djelfa Governorate with the help of local fishermen. Thirty-three L. biscarensis were infected with one or more individuals of dactylogyrid parasites (42.31%), and a total of 306 individuals were recorded. All monogenean species collected from the gills of the host fish were identified based on the presence of the following morphometric characteristics: two pairs of anchors, dorsal bar, ventral bar, vaginal armament and copulatory tube. Only one Dactylogyrus species, Dactylogyrus fimbriphallus (El Gharbi, Birgi & Lambert, 1994), was found on the gills. The mean intensity and abundance were 9.27 ± 12.50 parasites per fish and 3.92 ± 9.28, respectively. The infection levels of D. fimbriphallus were also investigated in relation to length and sex of the host fish as well as seasons, and the differences in its prevalence, mean intensity and mean abundance were statistically evaluated. There were significant differences in prevalence, mean intensity and abundance values between seasons. The infestation rate was higher in female hosts than in males. The host fish represents a new host record for D. fimbriphallus and new insights into the geographical distribution and host range have been gained. This study highlights the need for further research on helminth parasites, particularly monogeneans, affecting Algerian freshwater fishes, given the limited existing data.

Evaluation of antibacterial and antioxidant potentials of epidermal mucus of Clarias batrachus L. from freshwater bodies of India and its biochemical characterisation

MR Patel, JK Barot — Fri, 30 May 2025 00:00:00 +0000

The epidermal mucus of fish serves as a vital line of defence against pathogens and oxidative stressors in aquatic environments. In this study, we evaluated the antibacterial and antioxidant properties of the epidermal mucus of Clarias batrachus L., commonly known as the walking catfish, sourced from freshwater bodies in India. The mucus samples were collected and assessed for their efficacy against a panel of pathogenic bacterial strains, including Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa and Escherichia coli. This was done using the agar cup/well diffusion method, along with determination of Minimum Inhibitory Concentration (MIC). The results of the MIC assay showed that E. coli and S. aureus exhibited greater susceptibility to the 50 µg ml⁻¹ mucus sample compared to B. subtilis and P. aeruginosa. The antioxidant capacity of the mucus was assessed through DPPH scavenging activity, demonstrating notable antioxidant potential. Furthermore, biochemical characterisation of the mucus was conducted using gas chromatography-mass spectrometry (GC-MS), which identified several bioactive compounds, including cyclotrisiloxane (hexamethyl), cyclotetrasiloxane (octamethyl), cyclopentasiloxane (decamethyl), naphthalene, phenol, 3,5-bis(1,1- dimethylethyl), and hexadecanoic acid methyl ester. Based on the available reviews, these bioactive compounds are recognised for their antimicrobial and antioxidant properties. These findings also highlight the diverse protective functions of C. batrachus epidermal mucus, acting both as a barrier against bacterial pathogens and a natural source of antioxidants. Overall, this study provides valuable insights into the potential biomedical applications of C. batrachus epidermal mucus and lays the groundwork for further research in this field. This research is particularly applicable in Africa given the prevalence of Clarius spp. in Africa, which may exhibit the same physiological features as C. batrachus.

Effect of altitudinal gradient on reproductive and feeding behaviours of four congeneric endemic freshwater fish in the Zat Catchment (Morocco)

A Ahnich, F Aitboumalassa, H Khalki, M Naimi — Fri, 30 May 2025 00:00:00 +0000

The adaptive capacity of freshwater fish enables them to undergo morphological and structural changes in response to environmental fluctuations. Documenting these responses is crucial to assess species vulnerability to ecological perturbations. This study investigated the reproductive, feeding and physiological adaptations of four endemic fish species (Luciobarbus ksibi, Carasobarbus fritschii, Atlantor reinii and Luciobarbus magniatlantis) across an altitudinal gradient (690–1570 m) in Moroccan river catchment areas in 2023. Field sampling targeted a range of key biological and statistical indicators utilising 129 specimens. Gonadosomatic Index (GSI) and Viscerosomatic Index (VSI) analyses revealed significant variations driven by altitude, species and sex, with L. ksibi consistently exhibiting the highest values across gradients, indicating its superior reproductive and energy allocation strategies. Scaled mass index (SMI) further demonstrated distinct interspecies and altitudinal patterns, with marked differences in smaller size classes at higher altitudes. Total weight, digestive tube weight and gonad weight were key drivers of physiological differentiation, with species and altitude emerging as the dominant factors influencing variability. These insights are essential for informing conservation efforts and understanding the impacts of climate change on freshwater ecosystems.

The bio-invasion and population dynamics of Cherax quadricarinatus in Zambian waters

Fri, 30 May 2025 00:00:00 +0000

Since the 1990s, the exotic Australian redclaw crayfish, Cherax quadricarinatus, has been spreading in the Zambezi River basin in Zambia. The population structure of crayfish was determined at locations along the north shore of Lake Kariba 'near Siavonga' and the Kafue River. Crayfish were sampled from the sites using the catch of fishers at the sites on a monthly basis. Population structure was analysed by length-frequency plots using total length or carapace length measurements. Estimates of population growth rates were estimated by cohort analysis. Data reveal that population structures of crayfish in both areas are robust with multiple year classes present. Estimates of the Von Bertalanffy growth parameters of crayfish were L_∞ = 196.15 mm, ranging from 168 to 215.25 mm total length (TL), k = 0.238, ranging from 0.14 to 0.43 y⁻¹, and mean t_o = −0.465 y, ranging from −0.71 to 2.107. Gravid females were trapped primarily during the austral spring and early summer. There is evidence of growth overfishing at heavily fished sites, but no evidence of recruitment overfishing, suggesting difficulty of extirpation once populations become established. The presence or absence of crayfish at sites throughout Zambia was determined by use of an online survey administered to staff from the Fisheries Department at the Ministry of Fisheries and Livestock, Zambia, assigned to various districts around the country, and to local officials at towns along the Zambezi, Kafue and Kwando Rivers in areas where crayfish may have extended their range. Data were verified by focus group interviews of fishers. Crayfish in Zambia have been spreading unabated throughout western and southern regions of the country. The crayfish have appeared in six previously unreported locations in the Zambezi River basin including the Kabompo River in the Northwestern Province and the Chingola Reservoir in the Copperbelt Provinces, and in the Kwando sub-watershed of the Zambezi Basin in Western Province that borders with Angola and has a seasonal aquatic connection to the critical Okavango Delta Wildlife Refuge in Botswana.

Littoral substrates and nest properties of Coptodon rendalli vary with distance from the shoreline in a tropical impoundment, southeast Zimbabwe

RJ Chateya, HGT Ndagurwa — Fri, 30 May 2025 00:00:00 +0000

The change in littoral substrates and Coptodon rendalli nest properties along the inshore–offshore gradient of Mushandike Dam in southeast Zimbabwe, impacted by long-term water-level fluctuations, were studied in March 2016. Substrate and nest properties were measured in four contiguous 25 m2 plots along the inshore–offshore gradient at five sampling stations. We used one-way ANOVA to test for differences between treatments, and multiple linear regression to determine the substrate variables significantly influencing nest characteristics. Percentage sand, pebble/gravel/boulder, coarse organic matter and vegetation significantly declined but clay/silt increased with increasing shoreline displacement. Nest density and diameter significantly decreased with increasing distance from the shore. While vegetation had a positive relationship with nest density and diameter, clay/silt negatively influenced nest diameter indicating that vegetation and substrate properties play an important role in nest establishment by C. rendalli. In conclusion, water-level fluctuations causing large-scale shoreline displacement through changes in littoral substrate properties can, potentially, significantly impact the reproduction of C. rendalli and other such littoral biota, impairing the structure, productivity and composition of littoral communities. Further, these assessment methods are transferrable, efficient and cost effective, enabling the assessment of potential impacts of changing water levels on littoral biota to better inform water management decisions.

Age and growth of the invasive bluegill Lepomis macrochirus (Rafinesque, 1810) from the Howison’s Poort Dam, Eastern Cape, South Africa

PM Ndaleni, L Mofu — Fri, 30 May 2025 00:00:00 +0000

The present study assessed the age and growth of bluegill, Lepomis macrochirus (Rafinesque, 1810) from Howison’s Poort Dam, Eastern Cape. Precision between readers and structures (burnt and unburnt) of otoliths were estimated, and the aging error was determined by the coefficient of variation (CV, %) and average percentage error (APE, %). The lowest APE (18.73%) and CV (17.28%) were determined for burnt otoliths. Growth for male bluegill was best described by the von Bertalanffy Growth Model of the form Lt = 201.2 (1-e-0.312(t+ 0.61)) and Lt = 161.8 (1-e-0.409(t+ 0876)) for females. Growth zone counts on sectioned sagitta showed that L. macrochirus attained a maximum age of 6 years and can grow up to 197 mm TL. A comparison of growth parameters from fish in the present study with those from its native range suggests that the individuals can live for a maximum of 12 years and attain length of 260 mm TL in their native range. Though the species did not attain the same age as with their native range, early reproduction (four years) could have contributed to its establishment success.