African Journal of Aquatic Science

Potential live feeds for larval fish culture in Ethiopia

2022-11-23T16:06:09+00:00

The artificial propagation of commercially important fish species in Ethiopia is constrained by high mortality rate at the early stage of larval rearing and a subsequent poor growth performance during later stages. This is mainly due to absence of live feeds suited to the requirements of the larvae of the species. Although not organized, several studies have indicated that microalgae, copepods, cladocerans and rotifers are the dominant inhabitants in several water bodies that are the precursors in the larviculture of commercially important finfishes in the world. However, apart from studying the abundance and diversity of these important planktonic organisms, efforts made to culture any of them for use in the larviculture of commercially important fish species in Ethiopia is minimal. This may be due to the lack of comprehensive and organized information on the distribution and abundance of these important species in the context of their potential in aquaculture of live feeds in Ethiopia. The objective of this review is therefore to compile the available information on the abundance and distribution of the major potential live feed organisms in the Ethiopian water bodies with a special emphasis on freshwater live feed organisms currently used in larviculture of commercially important freshwater finfishes. In line with this, four potential live feeds (i.e. microalgae, rotifers, copepods and cladocerans) were included in the review. It is hoped that the review will provide baseline information for future research in the culture of economically important larval live feeds.

Investigating the potential for saltpan restoration for the provision of multiple ecosystem services

2022-11-23T16:20:30+00:00

Saltpans are increasingly being abandoned around the world, leading to the loss of ecosystem services provided by these unique semi-natural wetlands. The desertion of a saltpan at the Swartkops Estuary, South Africa has left behind a large area of desiccated hypersaline sediment and a sharp decrease in waterbird abundance and diversity. Here, we explore the potential for restoring this saltpan’s wetland function using stormwater inflow to support multiple ecosystem services and improve estuary health. Stormwater will be able to flow into the saltpan as a passive restoration approach that can maintain the site as a wetland habitat. This will contribute to improving the health of a nationally important, yet highly degraded, estuary by retaining 758 ML of stormwater to achieve full capacity, containing an estimated 2908 kg of dissolved inorganic nitrogen and 68 kg of dissolved inorganic nitrogen. Additional ecosystem services such as biodiversity maintenance, carbon storage and societal values can be expected. However, this would create a novel hypersaline stormwater wetland and a strategic adaptive management approach will be required. Management must be guided by monitoring, which should comprise collection of basic environmental data to establish a baseline condition, and against which, long term changes and responses resulting from stochastic events can be assessed and mitigated through the use of achievable ecological restoration targets.

Flow alterations and nutrient loading reduce primary producer variability in a temporarily closed microtidal estuary in KwaZulu-Natal, South Africa

2022-11-23T16:30:17+00:00

Primary producer responses to cumulative regulating factors were investigated in a temporarily closed microtidal estuary. We hypothesised that the phytoplankton chlorophyll-a concentration (Chl a) and invasive alien aquatic plant (IAAP) abundance would be highest during the low-flow season and that shifts in community structure would be driven by salinity and nutrient gradients. The augmented freshwater inflow limited phytoplankton Chl a (<5 µg Chl a l⁻¹) due to low freshwater residency, while isolated instances of increased Chl a (≥10 but ≤20 µg Chl a l⁻¹) were recorded following a brief period of mouth closure. Cyanophyceae proliferated for 67% of the study period with maximum abundance (>10 000 cells ml⁻¹) recorded in spring owing to increased water retention. A community consisting of co-dominant Cryptophyceae, Euglenophyceae and IAAPs was recorded during winter, indicating a shared preference for slow-flowing, low-salinity (<5) eutrophic habitats. Dinophyceae was the only phytoplankton class that showed niche differentiation by favouring the mesohaline lower reaches. The homogenisation of dynamic estuarine gradients and the prevalence of nutrient-tolerant primary producers is indicative of the degraded state of this estuary as a result of consistent nutrient-enriched freshwater baseflows. The application of a multidisciplinary restoration approach is required to improve the ecosystem health.

Validation of lipid extraction and correction methods for stable isotope analysis of freshwater food webs in southern Africa

2022-11-23T16:40:35+00:00

Stable isotope analysis is ubiquitous as a method to investigate food-web dynamics at various scales in aquatic ecology. Most studies make use of dorsal muscle tissue, which involves lethal sampling of the fish. The sampling of muscle tissue is often followed by chemical lipid extraction pre-treatment before stable isotope analysis. In this study we tested whether stable light isotope results obtained from fin tissue were comparable to those from muscle, and we investigated whether lipid correction could be used as a substitute for lipid extraction. Various lipid correction equations were evaluated. Based on our results, we propose ethical and efficient methods of sample collection and preparation for stable isotope analysis of freshwater fish. We found that dorsal muscle and fin tissue samples could yield similar interpretations of freshwater food-web dynamics in South Africa, demonstrating that fin clippings might be more widely applied as a nonlethal sampling method for stable isotope studies. Existing lipid correction equations either over- or underestimated true lipid extracted δ13C values, therefore an amended lipid correction equation is proposed as it was successfully tested against a population of wild fish. The errors arising from existing lipid correction equations suggest that site-specific calibration should be employed .

An assessment of water and sediment quality of aquatic ecosystems within South Africa’s largest floodplain

2022-11-23T16:53:53+00:00

Ndumo Game Reserve (NGR), a Ramsar site, situated in South Africa’s largest floodplain (Phongolo River floodplain) in northern KwaZulu-Natal, with important aquatic ecosystems: the Usuthu and Phongolo rivers and their associated floodplain lakes. While the Phongolo River is regulated, the Usuthu River is unregulated and subjected to fewer anthropogenic activities that influence the downstream environmental quality of this Ramsar system. The present study evaluates the environmental quality of floodplain systems in the NGR. Water and sediment were sampled and analysed from both rivers and associated floodplain lakes during different hydrological periods (flow). Using multivariate statistical techniques, ionic compositions, as well as water and sediment quality indices of the ecosystems were evaluated. Key findings suggest spatial and flow-related differences, which highlight the human-driven impacts from the upper catchments and the rivers influence on their respective floodplain lakes. The environmental quality (water and sediments) could be classified as ‘good’ and predominantly unimpacted. Notably, ionic composition of river water showed the importance of the unregulated Usuthu River and its contribution to aquatic ecosystems within the NGR and their continued functioning. The quality indices further indicated that metals do not currently pose any ecological risks to these systems.

Comparison of physico-chemical parameters with macroinvertebrate and vertebrate fauna of Lake Ogelube and Lake Ojii, Opi-Agu, south-eastern Nigeria

2022-11-23T17:13:14+00:00

The physico-chemical parameters of water provide crucial information on the condition of a waterbody at a point in time. Physico-chemical parameters determine the primary and secondary productivity of an aquatic ecosystem. Data on water temperature, pH, turbidity, water depth, total dissolved solids, total hardness, biochemical oxygen demand, dissolved oxygen, chloride, nitrate and phosphate, together with information on the aquatic macroinvertebrate and vertebrate species composition and abundance were collected for two Nigerian lakes, Lake Ogelube and Lake Ojii at Opi-Agu. The physico-chemical parameters were analysed using standard methods. Macroinvertebrate and vertebrate (Actinopterigii and Amphibia) species composition and abundance in the lakes were correlated with the physico-chemical parameters. Temperature, depth, biochemical oxygen demand and phosphate were significantly higher in Lake Ojii than in Lake Ogelube, while pH, turbidity and nitrate were significantly higher in Lake Ogelube (p < 0.05). In total, 1 442 animals were collected from the lakes of which 1 101 were macroinvertebrates and 341 vertebrates. The family Libellulidae (Order: Odonata) and the species Coptodon zillii (synonym: Tilapia zillii) (Gervais, 1848) (Perciformes: Cichlidae) were the most abundant macroinvertebrate and vertebrate taxa, respectively. Libellulidae were negatively associated with biochemical oxygen demand, temperature, turbidity, phosphate and chloride and Coptodon zillii was positively associated with dissolved oxygen.

Comparison of oxidative stress biomarkers in Oreochromis mossambicus in minimally and highly disturbed aquatic environments in the Matabeleland region, Zimbabwe

2022-11-23T17:42:50+00:00

Owing to their ability to provide a functional measure of organismal response to chemical stressors, oxidative biomarkers are useful in ecotoxicological studies to assess disturbance in aquatic environments. This study assessed the use of oxidative stress biomarkers in Oreochromis mossambicus (Peters, 1852) (Perciformes: Cichlidae) to distinguish between minimally and highly disturbed aquatic environments. A water quality index (WQI) and overall index of pollution (OIP) were used to characterize the target sites, namely the Mananda Dam (control, reference site) and Lower Mguza Dam (disturbed site). Forty male O. mossambicus samples were collected from each dam between April and August 2013. Values for the WQI and OIP indices were significantly higher for the Lower Mguza Dam than for the Mananda Dam (p < 0.05). Oxidative stress biomarker evaluation results showed that the activities of glutathione peroxidase (GPx), superoxide dismutase (SOD) and glutathione S-transferase
(GST) as well as the malondialdehyde (MDA) concentration in the liver of O. mossambicus were significantly higher in fish collected from Lower Mguza Dam than those collected from Mananda Dam (p < 0.05). The activities of DT-diaphorase (DTD) and catalase (CAT) were significantly inhibited in fish from the Lower Mguza Dam, when compared to those collected from the Mananda Dam (p < 0.05). From these findings, it is evident that oxidative stress biomarkers, such as antioxidant enzyme activity and MDA accumulation, can be used to differentiate minimally and highly disturbed aquatic environments.

The extent of hybridisation between largemouth bass and Florida bass across two river systems in South Africa

2022-11-23T17:40:55+00:00

Native to North America, largemouth bass Micropterus salmoides (Lacepède, 1802) were introduced in South Africa in 1928. Florida bass Micropterus floridanus (Lesueur, 1822) were introduced to enhance existing largemouth bass fisheries in 1980. While largemouth bass and Florida bass readily hybridise and produce offspring that are difficult to identify morphologically, confirmation of hybridisation requires genetic analysis. This study sought to understand: (1) the extent to which Florida bass spread within a catchment once introduced; and (2) whether a subset of samples taken from within a catchment accurately characterises hybridisation throughout the catchment. Samples were collected from the Breede River and Kowie River catchments and screened using 38 species-diagnostic single nucleotide polymorphisms to assess hybridisation. Collections from the mainstem of both rivers represented hybrid swarms, and neither pure largemouth bass nor Florida bass were observed. The absence of genetic differentiation among sampling sites suggests that hybridisation will occur throughout systems where both species are present. Hybridisation levels in dams located off the mainstem rivers were significantly variable and represented potential sources for Florida bass alleles observed within rivers. This finding, in conjunction with our limited knowledge of the distributions of the two species, suggests that applying independent management strategies to control and monitor the spread of both species may prove difficult.