Ketlhatlogile Mosepele

Fish, Floods, and Ecosystem Engineers: Aquatic Conservation in the Okavango Delta, Botswana

The Okavango Delta, Botswana, is a major wetland surrounded by the Kalahari Desert. The delta supports a diverse fish fauna that depends on highly seasonal flooding from inflowing rivers, and on the actions of ecosystem engineers (hippopotamuses, elephants, and termites), for creation and maintenance of their habitats. Conflicts in resource use, especially water, are likely to affect fish populations and the Okavango ecosystem in the near future. We present conceptual models of this remarkable aquatic ecosystem in relation to fish and fisheries as the basis for future research and conservation efforts. Developing understanding of the environmental flow requirements of the delta is key to the management of the Okavango Delta as an ecosystem supporting diverse and abundant fish and wildlife. Once developed, this understanding can be used to allocate water within the Okavango watershed.

FOOD WEBS AND ENERGY FLUXES ON A SEASONAL FLOODPLAIN: THE INFLUENCE OF FLOOD SIZE

The world’s largest inland delta, located on the Okavango River in Botswana, faces major changes in the annual flooding size and duration due to climatic shifts and increased water use. We examined several parameters of a seasonal floodplain in the Okavango Delta during two years of contrasting flooding size. The small flood (2003) was characterized by high concentrations of total nutrients (2.5 mg N and 1 mg P L−1), high primary production (0.8 mg C m−3 d−1), and zooplankton biomass (30 mg DW L−1). Methane production and consumption was considerable and stable isotope analysis suggested that methane oxidation provided a significant input of C to the aquatic food web. The large flood (2004) was characterized by lower volume-specific productivity, lower concentrations of nutrients (1 mg N and 10 μg P L−1), lower primary production (45 mg C m−3 d−1), reduced zooplankton biomass (10 μg DW L−1), and low methane production. The density of fish (CPUE) was significantly higher for the large flood compared to the small one. The findings point to the overall importance of flooding size on primary and secondary production, as well as basic food web properties in the delta. Low floods mean higher volume-specific production at the base of the food web. Seasons of large and long lasting floods cause improved circulation and enhanced reproductive success for fish.

Spatial and temporal variations in densities of small fishes across different temporary floodplain types of the lower Okavango Delta, Botswana

Small-sized and juvenile fishes as well as physicochemical water parameters in various primary, secondary and rarely flooded temporary floodplains of the Okavango Delta were assessed during the different hydrological phases of the 2009–2010 flooding season. Small fishes were sampled in the marginal zone of the floodplains using a throw-trap net and a suite of physicochemical properties were measured. Both physicochemical water quality parameters and densities of small fishes showed spatiotemporal variations across the temporary floodplain types. Turbidity, dissolved oxygen, conductivity, woody debris and chlorophyll a were all highest in rarely flooded floodplains compared to both primary and secondary floodplains (ANOVA, p < 0.05). Fish from 38 fish taxa belonging to 11 families were identified and classified during the study. The fish families Cichlidae (dominated by tilapias), Poeciliidae (Aplocheilichthys spp.) and Cyprinidae (Barbus spp.), were the most abundant across the study floodplains. The poeciliids were most abundant in frequently flooded primary and secondary floodplains, whereas juvenile cichlids dominated in rarely flooded floodplains. During high floods the rarely flooded portions of the delta function as important nursery habitats for juvenile cichlids, implying that a significant reduction in flooding may have negative effects on the deltas tilapia stocks due to reduced recruitment success.

Age and growth of sharptooth catfish, Clarias gariepinus (Burchell, 1822) (Clariidae), in the Lower Okavango Delta, Botswana

Managing any inland fishery requires reliable age data and accurate estimates of growth rate. Clarias gariepinus, the largest catfish species in most tropical African floodplain river systems, is an important constituent of subsistence fishery catches. We used otolith-derived age estimates to describe patterns of age and growth for C. gariepinus from the Lower Okavango Delta in northern Botswana. Edge analysis of sectioned sagittal otoliths showed that growth zone deposition occurred during the annual flood peak between July and September. The maximum recorded age was 8 years. Growth was best described by the von Bertalanffy Growth Model of the form E[L|age] = 528.70(1–e–0.72(age+1.35)). A comparison of growth parameters from this study with those from previous studies suggests two distinct populations of C. gariepinus between the Upper and Lower Okavango Delta, emphasising the importance of regional stock assessment for key fishery species. The fairly short life span and high variability in growth of C. gariepinus are important indicators that it is imperative to maintain natural habitat and flow regime for sustainable management of fishery resources in the Lower Okavango Delta.

Dynamics of the feeding ecology of selected fish species from the Okavango River delta, Botswana

Background. This study contributes to the understanding of the potential impact of changes in flooding patterns and (potential) fish production as a consequence of upstream developments in floodplain systems. Therefore, stomachs of eight fish species from the Okavango Delta were analysed to evaluate the feeding ecology of floodplain fish (and the effect of seasonal flooding), using the delta as a case study. Materials and methods. In total, 2101 fish stomachs of eight species, representing six families, were collected (in all seasons) from the delta using experimental fishing nets, from 2004 to 2009. Frequency of occurrence, Levin’s diet breadth index, Pianka’s overlap index, trophic levels, and Bray–Curtis similarities were used to evaluate feeding preferences. Detrended correspondence analysis was used to study temporal variations in diet. Multiple linear regressions were used to determine the influence of flooding on diet. ANOVA and MANOVA were used to determine the level of significance among variables, while LSD post hoc analysis revealed the source of significance. Results. Cluster analysis and Pianka’s index highlighted interand intra-specific competition for food among different species and age classes, ANOVA highlighted dynamic changes in interand intra-specific trophic level partitioning, while detrended analysis showed that the terrestrial environment is subsidizing the aquatic environment. Regression analysis showed that Schilbe intermedius diet was driven by discharge (F = 7.03; P = 0.045; R2 = 0.58) while that of Marcusenius macrolepidotus was driven by water depth (F = 25.88; P = 0.04; R2 = 0.93). Conclusion. The terrestrial–aquatic ecotone is important in fish growth of seasonal floodplains. Energy uptake is optimised through cannibalism to ensure species survival. Furthermore, species inter-relations are dynamic due to variations in food availability driven by seasonal flooding, which shortens and lengthens the food chain periodically. This dynamic relation is pronounced at low floods when predation and competition increases within the fish community. The evidence from this study has shown that predation, cannibalism, interand intra-specific competition are regulating factors in floodplain fish communities, driven by seasonal flooding.

Gillnet selectivity for the north African catfish, Clarias gariepinus (Actinopterygii: Siluriformes: Clariidae), from the Upper Okavango Delta, Botswana.

Background. Successful fi sheries management requires estimation of gillnet selectivity for optimum exploitation of the resource. In the Okavango Delta, no study has assessed the selectivity of gillnets for Clarias gariepinus (Burchell, 1822) which is an important component of both the subsistence and the commercial gillnet fi shery catch. The aim of this study was to simulate the harvesting pattern of the commercial gillnet fi shery and provide gillnet selectivity parameters for C. gariepinus. This will help fi shery managers with information on the appropriate mesh sizes needed for sustainable utilisation of the catfi sh resource. Materials and methods. Monthly gillnet sampling was conducted over a period of 8 years from 2001 to 2009. The SELECT method was used to estimate gillnet selectivity for C. gariepinus using catch data from four mesh sizes (73 mm, 93 mm, 118 mm, and 150 mm). Results. The 93 mm and 118 mm mesh sizes were the most effi cient when capturing C. gariepinus accounting for 44.6% and 21.9% of the total catch, respectively. Mean fi sh length increased with increasing mesh size and was signifi cantly different between mesh sizes (P < 0.001). The modal fi sh lengths for the four mesh sizes were estimated at 41.63 cm, 53.23 cm, 66.35 cm, and 85.54 cm in order of increasing mesh size. Conclusion. The modal fi sh lengths for meshes 93 mm and 118 mm are greater than the size at maturity and therefore the current commercial gillnet fi shery which uses 100 mm mesh size may not be a threat to the C. gariepinus population in the Upper Okavango Delta. Future studies should aim to conduct fi shery dependent selectivity studies in the lower Okavango Delta to determine if selectivity changes with location and gear. Moreover, a comprehensive age-based stock assessment is required to establish the mesh size that optimizes yield without adversely depleting the spawning stock biomass.

Fisheries governance, management and marginalisation in developing countries: Insights from Botswana

Abstract Globally, fish is a key source of food and nutrition security for all marginalized riparian communities. This is particularly so for Sub-Saharan Africa and South Asia. Indeed, debates about power relations on fisheries governance underscore issues bordering on the quality of life and livelihood opportunities for marginalized, riparian communities. The fundamental problems impeding the ability of fisheries resources in a developing country like Botswana to contribute to food and nutrition security are governance issues and poorly-thought out management approaches. This study reviewed relevant literature and key informant interviews to elicit secondary and primary data on the management of the fisheries sector. Despite its middle income status, Botswana is still faced with food and nutrition insecurity. These can be ameliorated by increased supply of fish, especially to marginalised riparian communities. However, the fisheries sector is maligned in terms of access to human and financial resources. Governance of the sector is also misaligned between food production needs and conservation imperatives. Consequently, poor physical infrastructure (due to low government support and investment) has limited the optimal performance of the sector in enhancing people’ livelihoods. Key recommendations from this study include: (i) realigning fisheries legislation and governance, (ii) paradigm shift in management, (iii) increase funding for research and marketing, (iv) infrastructural development, (v) cultural shift in fish valuation, and (vi) participatory inclusion in decision-making. Ultimately, marginalisation can be reduced through devolution of power from the centre to the margins. This would contribute towards alleviating food and nutrition insecurity in the developing world.

Influence of the annual flood-pulse on catch per unit effort, condition and reproduction of Clarias gariepinus from the upper Okavango Delta, Botswana

Catch per unit effort (CPUE), length, weight and maturity data for Clarias gariepinus were collected during monthly gillnet surveys in the upper Okavango Delta between 2001 and 2009 to investigate their relationship with the annual flood-pulse. CPUE, condition factor (K) and the proportion of ripe-running fish (PRR) in the population followed a unimodal annual cycle that could be modelled using water temperature and flood-pulse hydrology. Increased CPUE during declining water levels was most likely a result of feeding migrations and aggregation behaviour. The observed increase in K during low floods in October and November preceded the increase in PRR, which increased mainly with increasing temperature but appeared less dependent on flow. This study provided quantitative evidence that the biology of fish in the Okavango Delta is mainly dependent on the annual flood regime and, therefore, that conservation efforts should be focused on maintaining natural flow patterns in the face of climate change and potential water extraction schemes upstream.