African Journal of Aquatic Science

Restoration of South African estuaries: successes, failures and the way forward

L. Claassens — 2023-04-24

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.

Assessment of water quality for suitability and human health risk: a study of the Owan River, Edo State, Nigeria

Nkonyeasua Kingsley Egun — 2023-04-24

Suitability assessment of water bodies for human consumption and its implication on human health is critical to water resources utilisation and sustainable development. This study investigated the water quality of the Owan River and the potential health risk to users. Examined physico-chemical parameters were within the Nigerian National Environmental Standards and Regulation Enforcement Agency permissible limits, except for the lead concentration. Water quality index values (Water Quality Index: 84.50–316.20) indicated that surface water from the sampling locations were of poor quality and unsuitable for drinking (Water Quality Index: > 50). The comprehensive pollution index values (Comprehensive Pollution Index: 0.31–0.57) indicate the water is sub–clean to slightly polluted. Evaluated non–carcinogenic risk to human health showed that children in the receptor population were predisposed to risk from heavy metals through oral ingestion (Oral Hazard Index: > 1), while the water was safe for dermal contact (Dermal Hazard Index: < 1) for adults and children. Integrated carcinogenic risk values classified the surface water as Grade IV (Medium risk), with cadmium having the highest carcinogenic risk. Cadmium accounted for approximately 94% of the total carcinogenic risk and hence a concern for public health. Natural runoff and anthropogenic activities were identified as sources of pollutants within the watershed. The study has provided data for water resource quality monitoring and public health management.

Water quality of the Blue Nile at Khartoum, Sudan, before complete filling of the Grand Ethiopian Renaissance Dam

A.M.H. Bashir — 2023-04-24

Physico-chemical variables were monitored biweekly for the Blue Nile at Khartoum between July 2017‒June 2019. The aim was to document the present status of water quality at Khartoum before complete filling of the Grand Ethiopian Renaissance Dam (GERD). Physico-chemical parameters within acceptable limits recommended by WHO for drinking water included pH (7.2–10.8), temperature (13.7‒30.4 °C), conductivity (137‒281 μS cm^‒1), total dissolved solids (75‒141 mg l^‒1), total residue (5‒3194 mg l^‒1), alkalinity (93‒176 mg CaCO3 l ^‒1), dissolved oxygen (3.5‒10.9 mg l^‒1), nitrite–nitrogen (<detection limit (DL)‒43 µg l^‒1), ammonium‒nitrogen (<DL‒940 µg l^‒1), nitrate– nitrogen (<DL‒1468 µg l^‒1), phosphate‒phosphorus (<DL‒63 µg l^‒1), silica (4.0‒23.6 mg l^‒1), iron (<DL‒250 µg l^‒1) sodium (3.2‒15.7 mg l^‒1), potassium (0.7–5.6 mg l^‒1), calcium (6.2–30.3 mg l^‒1), and magnesium (1.5–11.5 mg l^‒1). Total residue and pH were amongst the measured variables that were outside of the WHO recommendations. This study revealed that the Blue Nile remains generally unpolluted, and has not been significantly negatively affected by runoff from urbanisation or sewage contamination. This study presents further evidence that industrial wastes discharged upstream of Khartoum have not accumulated downstream as they are carried into the Main Nile by the annual floodwater pulses. The water is therefore currently suitable for drinking and all other uses. However, accumulation of pollutants in the Blue Nile may be problematic once filling of the GERD is complete as the discharge during flooding events necessary to flush the river will be reduced.

Zooplankton distribution and community structure as a function of environmental variables in the Niger River and its tributaries in Niger

H. Souley Adamou — 2023-04-24

The Niger River and its main tributaries (Sirba, Tapoa and Mékrou) in Niger were sampled during high and low water flow with the objectives of (1) comparing the taxonomic structure, diversity and abundance of the zooplankton community (rotifers, copepods, crustaceans) along the river and (2) determining which environmental or biological factors influence the distribution of the zooplankton communities most strongly. The impact of the tributaries at high-flow was also evaluated. Rotifers (32 taxa) were the most abundant and diverse followed by Cladocerans (13 taxa) and Copepods (three taxa). The mean number of taxa (16.5 and 17.6, respectively), the Shannon-Weaver diversity index (2.7 and 2.8, respectively) and the evenness (0.7 and 0.7, respectively) did not vary significantly between the high- or low-flow periods, or along the length of the Niger River and its tributaries. The mean zooplankton abundance was highest during low-flow periods (40 × 103 ± 45 × 10³ ind. m^–3) but showed no specific spatial pattern. During high-flow sampling, the mean zooplankton abundance (3 × 10³ ± 2 × 10³ ind. m^–3) increased progressively downstream. RDA analysis showed that rotifers are associated with high conductivity and NO₂ − concentrations, copepods with chlorophyll a, while cladocerans were abundant when conductivity, chlorophyll a and NO₂ − concentrations were low.

A trait database for southern African freshwater invertebrates

Oghenekaro N. Odume — 2023-04-24

Invertebrates are often used as indicators of ecosystem health. In community ecology, the use of organism traits (e.g. behavioural, biological or ecological) can be of value in relation to impact diagnosis and assemblage prediction. It has also been recognised that certain traits are related to ecosystem function, and their use in biological monitoring provide insights into ecosystem functional responses to stressors. However, the paucity of trait information and non-availability of a local trait database for southern African invertebrates impede progress in using this approach for biological monitoring. To address this critical gap, we compiled an updateable trait database for southern African freshwater invertebrates. The database contains information on 40 trait categories and 204 trait attribute/modalities. Of the 40 trait categories included in the database, 12 are biological, 12 are behavioural, and 16 are ecological preferences. The database is designed to accommodate taxonomic flexibility. In this regard, trait information is entered at multiple taxonomic levels: family, genus, and species, allowing researchers working at different taxonomic resolutions to retrieve relevant information. While there are still gaps in the database, this effort represents the first attempt to synthesise available trait information on southern African freshwater invertebrates.

Molluscicidal and cercaricidal effects of Persicaria senegalensis on Radix natalensis snails and their echinostome-shed cercariae in South Africa

M.C. Mathole — 2023-04-24

Persicaria senegalensis is an aquatic plant of the Family Polygonaceae with apparent molluscicidal properties. The research evaluated the molluscicidal effect of P. senegalensis on the freshwater snail Radix natalensis, and its echinostome-shed cercariae. Various compounds from Persicaria senegalensis were extracted and qualitatively analysed. Radix natalensis and the cercariae were exposed to different extract concentrations of P. senegalensis and tested for a period of 24 and 3.5 hours, respectively. Glycosides, saponins, tannins and phenols were found to be present in the leaf, flower and stem extracts. The roots only contained saponins, with traces of tannins and phenols. Terpenoids were absent in the plant. The LC₅₀ values of P. senegalensis extracts against R. natalensis were determined at 18.20 ± 14.78, 30.90 ± 31.25, 42.66 ± 55.67 and 77.62 ± 132.31 for leaves, flowers, stems, and roots respectively. For the cercaricidal activities of leaf, flower, stem and root extracts, the LC₅₀ values against R. natalensis-shed cercariae were 20.89 ± 8.91, 30.90 ± 22.10, 42.66 ± 40.94 and 77.62 ± 93.62, respectively. The mortality rate of tested organisms increased with increased concentrations. It is concluded that extracts of P. senegalensis have molluscicidal and cercaricidal properties; and, therefore, has potential as an alternative biological control agent for freshwater snails, their cercariae and, subsequently, other snail-transmitted parasitic diseases

Southern mullets (Chelon richardsonii) as a potential biological indicator for mercury pollution in the Walvis Bay lagoon, Namibia

Veronica Kaleinasho Kapula — 2023-04-24

Southern mullet Chelon richardsonii is one of the important fish species found in the Walvis Bay lagoon (WBL) that is commercially exploited for local consumption. The southern mullet is, however, exposed to mercury (Hg) found in sediments, which forms part of its diet. This study evaluated the Hg concentration in muscles, stomach content, liver, and gills of C. richardsonii over two years. The Hg concentration was higher in the liver (0.005 mg kg⁻¹) and relatively lower in the gills (0.001 mg kg⁻¹). There were temporal variations in Hg concentration among the tissues; a positive correlation between mean Hg concentration and fish sizes (total length and total weight) was documented in the gills; and a negative correlation for size versus Hg concentration was recorded in the livers, stomach contents, and the muscles of southern mullets. The Hg concentrations of all tissues were below the World Health Organisation (WHO) limit for fish (0.5 mg kg⁻¹), which indicates that it does not pose a human health risk at the current levels. This study’s findings form a baseline for using southern mullets as biological indicators of Hg pollution in WBL. The study also highlights the need for regular monitoring programmes to protect and maintain the ecological characteristics of WBL as per Namibia’s mandates under the Ramsar Convention.

Colonisation theory and invasive biota: the Great Fish River case history, 35 years later

Pule P. Mpopetsi — 2023-04-24

Using the Great Fish River, South Africa, Laurenson and Hocutt (1986) proposed a colonisation theory outlining the characteristics of successful invaders and the properties of aquatic habitats that facilitate successful fish invasions. The Great Fish River has modified flow due to an inter-basin water transfer (IBWT). The present study evaluated the current state of invasions and probable mechanisms facilitating establishment of non-native fishes within this river. We used historical literature on the non-native fishes and associated this information with the recent distribution records of these fishes within this system. We hypothesised that non-native fishes would reflect life-history traits and environmental associations that were consistent with adaptability to flow-modified disturbance. Of the 11 non-native fish species recorded within this system, seven have established, three have failed to establish and one is uncertain. The Orange–Fish IBWT and angling were the main vectors of invasions, accounting for 36% and 46% of the non-native species, respectively. Life-history traits, such as large size, greater longevity and wide habitat tolerance, appear to have facilitated establishment of most species. Our findings indicate that both intrinsic and extrinsic factors may explain the invasions of non-native fishes within the Great Fish River and possibly other similarly disturbed systems.

A comparison of the water quality and plankton diversity of the Okamini Stream to the freshwater systems within the New Calabar River catchment, Port Harcourt, Nigeria

B.B. Otene — 2023-04-24

The water quality and phytoplankton diversity of the Okamini Stream, Port Harcourt, Nigeria was studied between April and June 2018 at three stations. The data was used to assess the health of the system for supporting local communities. Compared to national and international standards, the stream was found to be acidic, but pH levels (5.87 ± 0.40) were similar to other systems in the New Calabar River catchment. Analysis of variance showed that there was no significant variation (p < 0.05) in water quality between the stations, with the exception of PO4 (0.82 ± 0.40 mg l^-1), which was significantly higher than other local systems. A total of 26 species of plankton were recorded during the study. These species belonged to the Bacillariophyceae (n = 10), Chlorophyceae (n = 6), Cyanophyceae (n = 5), Euglenoidea (n = 3), and Chrysophyceae (n = 2), but phytoplankton density was low. The most frequently occurring phytoplankton species were indicators of organic pollution, such as Cyclotella comta, while Melosira varians was recorded at all sampling occasions. Relatively high diversity indices indicated a stable environment. We conclude that the Okamini Stream is not yet degraded, but that the water quality should be monitored, and anthropogenic activities managed to avoid potential future deterioration.

Validation of growth zone formation in Oreochromis mossambicus otoliths collected from an irrigation pond in the Sundays River Valley, Eastern Cape, South Africa

L. Mofu — 2023-04-24

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 River Valley irrigation pond in the Eastern Cape province of South Africa were determined using otoliths. Mark recapture of chemically-tagged wild fish was used to confirm the periodicity of growth zone creation. A total of 150 specimens of O. mossambicus were obtained using a combination of seine and fyke nets. Female total lengths (Lt ) ranged from 20 to 340 mm, while male Lt ranged from 82 to 374 mm. Growth zone deposition rates of wild O. mossambicus otoliths fluorochrome-marked with oxytetracycline hydrochloride (OTC) and recaptured after one year was consistent with the deposition of one growth increment annually. According to the three-parameter von Bertalanffy model, growth was described using different coefficients for females and males, respectively. The maximum age estimate for both females and males was 12 years. Oreochromis mossambicus in this warm temperate pond had relatively long life spans compared to subtropical populations, which suggest likely long- term population persistence within cooler, novel environments.

New host and distribution records for five species of Enterogyrus Paperna, 1963 (Dactylogyridae: Ancyrocephalinae) from cichlid hosts in South Africa

Iva Přikrylová — 2023-04-24

Between March 2015 and September 2016, three species of Cichlidae (Coptodon rendalli, Oreochromis mossambicus and Tilapia sparrmanii) were screened for the presence of stomach monogeneans. Four localities across three provinces of South Africa (Limpopo, Northern Cape and North West) were sampled and a total of five Enterogyrus species were identified, viz. E. cichlidarum, E. coronatus, E. malmbergi, E. mashegoi and E. multispiralis. Finding E. malmbergi from O. mossambicus and T. sparrmanii represents new host and geographic records. Enterogyrus coronatus was reported from C. rendalli and T. sparrmanii from three localities. Both hosts are new host records for the species and show a wide distribution of E. coronatus within South Africa. Tilapia sparrmanii is a new host record for E. cichlidarum, E. mashegoi and E. multispiralis. Enterogyrus mashegoi and E. cichlidarum are reported from new localities.