Indications of the effects of climate change on the pelagic fishery of Lake Kariba,Zambia–Zimbabwe

The pelagic fishery of Lake Kariba comprises the introduced clupeid, Limnothrissa miodon, from Lake Tanganyika. The annual fishery catches grew logistically from 1974, when commercial fishing began. It peaked at 37 000 tonnes (estimated mean sustainable yield = 40 000 tonnes) around 1990 and declined steadily thereafter. A piecewise regression of Limnothrissa catches against time gives a breakpoint around 1987–1988. Regressions of Limnothrissa against air temperature and lake temperature gave breakpoints of 34.8 and 28.7 °C, respectively. Other studies indicate that Lake Kariba has warmed by close to 2 °C, with accompanying changes in thermal stratification. The lake phytoplankton is now dominated by Cyanophyceae, particularly Cylindrospermum raciborskii. The lake zooplankton population has greatly diminished. These data are similar to the results obtained for Lake Tanganyika (Nature 2003; 424 , 766–8). In the Lake Tanganyika study, the declining pelagic fishery has been attributed to reduced food availability resulting from a reduced phototrophic zone depth, as well as epilimnion nutrient recharge from reduced mixing. The warming observed in Lake Kariba and its environs is consistent with the IPCC Fourth Assessment Report (AR4). While the effects of global warming have been observed in large natural lakes, this is the first study in which global warming has affected the ecosystem of a large impoundment.     

An assessment of climate change and stratification in Lake Kariba (Zambia–Zimbabwe)

Lake Kariba is a monomictic reservoir, isothermal in winter (June–August), and stratified for the rest of the year. Its temperature has increased since the 1990s, with some researchers suggesting it now has a stronger and more stable thermocline as a result. It was also claimed that the depth of the epilimnion had decreased, and nutrient depletion has led to a decrease in the depth of the euphotic zone, algal biomass and primary productivity. A re‐analysis of the data suggests this view is incorrect. The epilimnion is more homogenous now than in the 1980s, with less vertical variation in temperature and dissolved oxygen in the upper 20 m of the lake. The depth of the euphotic zone has not changed since the 1960s, and the algal biomass and primary productivity have increased. The thermocline appears to have weakened with temperature and density gradients being much smaller than in the 1980s because of a more rapid rate of warming in deeper water. The reasons for this phenomenon are not clear, although evaporative cooling at the surface may contribute to it. There was a significant increase in Birgean heat budgets over time, although there was none for stability. This may be explained by the fact that the lake is monomictic, cooling to 22°C in winter, with a new cycle of stratification beginning each year, and it may be affected by variable factors such as evaporation, rainfall, river flow and internal wave movements.     

Climate change does not explain historical changes in the pelagic ecosystem of Lake Kariba (Zambia–Zimbabwe)

Recent studies on the pelagic ecosystem of Lake Kariba identified a number of changes in its thermal regime, planktonic communities and fishery production, concluding they were the result of climate change, particularly warming. This study re‐examines these conclusions and suggests alternative explanations for these changes. Historical data suggest there was no warming of the lake until at least the 1990s. Furthermore, lack of recent data makes it difficult to conclude that the lake’s temperature has increased by 2 °C. It is also not clear that the pattern of thermal stratification has changed, or that the thermocline has risen and become more stable. Although one of the suggested effects of climate change was a decreased number of larger zooplankton, this change occurred in the 1970s, when there had been no change in the lake temperature. Rather, there is strong evidence that the zooplankton composition changed as a result of selective predation by the introduced clupeid, Limnothrissa miodon. Furthermore, the loss of large grazing zooplankton species could have affected the composition of the phytoplankton, although this phenomenon also has been attributed to climate change. Although the phytoplankton communities in the lake are not as well documented as the zooplankton communities, it is clear that many changes in the lake actually began in the 1970s. Finally, the decline in the pelagic fishery for Limnothrissa was also linked to temperature changes. However, because the fisheries on the Zambian and Zimbabwean sides exhibited very different behaviours, there is little evidence to support this conclusion. It is concluded that the impacts of climate change on Lake Kariba are likely to be complex and that possible over‐simplification in identifying these impacts will not facilitate our understanding of these complexities.     

Environmental state of Lake Kariba and Zambezi River Valley: Lessons learned and not learned

Lake Kariba, still the largest reservoir in the world by volume, is 60 years old. It has undergone changes in its thermal properties, associated with global warming, which reflect in turn on its limnology. These changes include a shallower eipilimnion, higher heat content and increased tropicality to near equatorial status. The role of Lake Kariba with regard to its energy characteristics is discussed in light of global warming findings. The lake’s water residence time has increased from 3.7 years to ≈5.7 years, attributable to a reduced inflow from the Zambezi River. The phytoplankton communities have changed towards a cyanophyceae‐dominated community, leading to a decline in entomostracan zooplankton, and a near collapse of the planktivorous Limnothrissa miodon fishery. Prolonged use of pesticides to control Glossina has led to measurable ecosystem level impacts on both terrestrial and aquatic biota. The impacts of the forced relocation of the Tonga people were still evident during this study. Siltation from resettlement areas has led to the loss of habitat and biodiversity in the inflowing streams to the lake. Unplanned shoreline development in the early history of the lake poses health problems. It is projected that global warming will cause the lake temperature to rise by ≈4 °C by the end of the century. Higher temperatures will be accompanied by windier conditions, thereby enhancing the risks from storms on the lake. The appropriateness of administrative structures intended to manage the Zambezi River Basin environment also is discussed herein. It is concluded that the management protocol is institutionally a non‐inclusive process lacking the capacity to involve other stakeholders in managing the lake’s resources, and even less so in the integrated management of the basin.     

Evaluation of artificial PVC pipe structures as fish habitat in Spring Lake,Western Illinois,USA

Three types of artificial fish habitat constructed of polyvinyl chloride (PVC) pipe were placed at three sites in Spring Lake, a drinking water reservoir in McDonough County, Illinois. Direct current (DC) electrofishing was used on a monthly basis from May through October 2008 at each site, and a corresponding control area. The fish catch per unit effort (CPUE) and mean size of fish captured were compared between the artificial structures and the control areas. The CPUE and mean size of fish captured also were compared to the number of ‘branches’ on the artificial habitat. The fish catch rates for the various structures were evaluated among the three sites on the lake. The mean CPUE was significantly higher for the PVC structures, and compared to control sites, for bluegill (Lepomis macrochirus), white crappie (Pomoxis annularis) and largemouth bass (Micropterus salmoides). This was not performed for gizzard shad (Dorosoma cepedianum). No significant differences were detected in mean size for all four species between the PVC and control sites. A linear regression indicated no significant relationship between the number or size of fish, and the number of branches on the PVC structures. Bluegills used the PVC structures significantly more in the dam area than in other areas of the lake. In contrast, gizzard shad used the PVC structures significantly more in the west end of the lake. These results suggest that PVC structures, which can be constructed of common, inexpensive materials, can be effective in attracting fish in drinking water reservoirs. Another advantage of the PVC structures is that they are non‐organic and, therefore, will not degrade water quality.     

The myths and realities in the management of the Kapenta fishery at Lake Kariba (Zimbabwe)

Biological considerations have dominated the management of the Kapenta fishery since its inception. State fisheries' managers employed the traditional scientific models that proved useful in managing temperate fisheries. This study uses both secondary and primary data to discuss the problems of using scientific models to manage tropical river‐fed lakes. Where fish stock size fluctuates in response to environmental factors such as river flows and lake level (which influence the level of nutrients in the water body), the concepts of maximum sustainable yield (MSY) and maximum economic yield (MEY) become problematic. This study demonstrated that, in an unstable environment, and where the target species is resilient to high levels of fishing effort, biological models lose their predictive power, allowing the state to determine the proportion of the resource to be distributed among its nationals. The redistribution of access rights or fishing licences (as a way of redressing a legacy of an unequal distribution of resources between emerging local entrepreneurs and companies established during the colonial era) is a major management issue in Zimbabwe. This study uses historical appraisal to illustrate how biological growth models failed to determine the optimal level of fishery exploitation upon which informed decisions could be made regarding whether to redistribute fish access rights or to broaden participation (or just recruit more fishers into the industry). This study also investigates tensions between the industry (advocating for broadening of access) and the state (seeking to redistribute existing resource use rights). Each stakeholder group has evoked institutions and discourses supporting their positions in influencing the strategies to eliminate the existing imbalances.     

洪泽湖流域污染负荷削减对生境质量的影响

运用InVEST生境质量模型分析了1980—2020年洪泽湖流域生境质量时空变化特征,通过Johnes输出系数模型计算不同情景下污染负荷输出量,利用回归模型建立污染负荷与生境质量关系,预测流域污染负荷削减措施对生境质量的改善效果。结果表明：洪泽湖流域生境质量逐年恶化,空间上呈“南优北差、东优西差”分布格局;流域污染负荷输出呈逐年上升趋势,且污染负荷在差生境质量区域有明显集聚效应,污染负荷与生境质量呈显著负相关;在自然发展(污染负荷无削减)情景下,2030年流域生境质量平均值与2020年相比较将降低1.0%～2.4%;根据流域污染负荷削减目标,在污染负荷削减率为8%情景下,流域生境质量平均值较自然发展情景提升0.9%～2.2%,在污染负荷削减率为18%情景下,流域生境质量平均值较自然发展情景提升1.3%～2.7%,流域污染负荷削减对于提高生境质量具有显著效果。     

Decline of zooplankton food resources of Limnothrissa miodon fishery in Lake Kariba: Global warming‐induced ecosystem disruption by Cylindrospermopsis raciborskii

The present study was conducted on the Lake Kariba portion of the Middle Zambezi Biosphere Reserve. Magadza (2011) previously reported the pelagic Lake Tanganyika sardine (Limnothrissa miodon) fishery introduced into Lake Kariba was declining. This phenomenon was attributed to observed climate‐change‐related warming of the lake, facilitating establishment of the invasive Cylindrospermopsis raciborskii, currently the dominant alga in the lake. Chlorophyceae contribute to less than 10% of the phytoplankton biomass. Accordingly, it was postulated that its dominance in the lake resulted in a food chain disruption because this cyanobacterium was not preferred by zooplankton grazers. Accordingly, the present study examined the feeding behaviour of L miodon with reference to available food items in the Sanyati Basin of Lake Kariba. The results highlighted a virtual absence of entomostraca zooplankton in the Sanyati Basin of Lake Kariba, in contrast to previous findings of previous studies, the avoidance of C. raciborskii in the diet of L. miodon, as measured by the Ivlev electivity index, a low stomach fullness index, the movement of L. miodon to macroinvertebrates associated with the littoral zone as a food source, and unfavourable dietary indicators of L. miodon. These results collectively suggest a food deficit for L. miodon in Lake Kariba, leading to the previously observed decline in the lake fishery.     

The relevance of species-based indicators as a tool for evaluating the structure of exploited fish assemblages: a comparative study of two tropical lakes in Mali, West Africa

A comparative study of the fish communities in two lake ecosystems in Mali (Lakes Manantali and Selingue) was based on monitoring small‐scale fishery landings for a 13‐month period. The main difference between the two lakes is the fishing pressure, being higher in Lake Selingue and lower in Lake Manantali. The effects of fishing on the structure of the fish communities in the two lakes were analysed with the use of diversity indices, rank species abundance (RSA) and abundance–biomass comparative curve (ABC) methods. The diversity differed significantly between the lakes, being higher for Lake Selingue. Analysis of the community structure, using RSA curves, indicated a more regular distribution of the weights between fish species for Lake Selingue than for Lake Manantali. The two lakes exhibited different exploited fish community structures. For Lake Selingue, the fishery was based on smaller‐sized species, compared to that for Lake Manantali. The indicators based on the species composition of exploited fish communities, in terms of abundance and life‐history traits, seemed to be more relevant in this study. Indicators such as diversity indices and RSA curves depend strongly on the exploitation strategy, being unreliable for evaluating exploited fish communities from catch data.     

水库建设影响河流鱼类生境的保护研究

由于水库的建设会对河流产生阻隔,从而影响鱼类洄游、产卵、索饵等,本文以赤峰市东台子水库为例,结合西拉木伦河流域特征、水库特性及鱼类生活习性、繁殖特征等,简要阐述了通过采取水库生态调度、鱼类增殖放流站及鱼道建设等保护措施,对河道内的鱼类生境进行有效保护,可减少水库工程建设对鱼类生境的影响。     

Diel patterns in spatial distribution of fish assemblages in lentic and lotic habitat in a regulated river

Fish assemblages in large rivers are governed by spatio‐temporal changes in habitat conditions, which must be accounted for when designing effective monitoring programmes. Using boat electrofishing surveys, this study contrasts species richness, catch per unit effort (CPUE), total biomass, and spatial distribution of fish species in the Saint John River, New Brunswick, Canada, sampled during different diel periods (day and night) and macrohabitats (hydropower regulated river and its reservoir) in the vicinity of the Mactaquac (hydropower) Generating Station. Taxa richness, total CPUE, and total biomass were significantly higher during night surveys, resulting in marked differences in community composition between the two diel periods. Furthermore, the magnitude of diel differences in catch rate was more pronounced in lentic than in lotic macrohabitats. The required sampling effort (i.e., number of sites) to increase accuracy and precision of CPUE estimates varied widely between fish species, diel periods, and macrohabitats and ranged from 15 to 185 electrofishing sites. Determining a correction factor to contrast accuracy and precision of day‐ with night‐time surveys provide useful insights to improve the design of long‐term monitoring programmes for fish communities in large rivers. The study also shows the importance of multihabitat surveys to detect differences in the magnitude of diel changes in fish community metrics.     

Response of fish assemblages to restoration of rapids habitat in a Great Lakes connecting channel

Rapids habitats are critical spawning and nursery grounds for multiple Laurentian Great Lakes fishes of ecological importance such as lake sturgeon, walleye, and salmonids. However, river modifications have destroyed important rapids habitat in connecting channels by modifying flow profiles and removing large quantities of cobble and gravel that are preferred spawning substrates of several fish species. The conversion of rapids habitat to slow moving waters has altered fish assemblages and decreased the spawning success of lithophilic species. The St. Marys River is a Great Lakes connecting channel in which the majority of rapids habitat has been lost. However, rapids habitat was restored at the Little Rapids in 2016 to recover important spawning habitat in this river. During the restoration, flow and substrate were recovered to rapids habitat. We sampled the fish community (pre- and post-restoration), focusing on age-0 fishes in order to characterize the response of the fish assemblage to the restoration, particularly for species of importance (e.g. lake whitefish, walleye, Atlantic salmon). Following restoration, we observed a 40% increase in age-0 fish catch per unit effort, increased presence of rare species, and a shift in assemblage structure of age-0 fishes (higher relative abundance of Salmonidae, Cottidae, and Gasterosteidae). We also observed a “transition” period in 2017, in which the assemblage was markedly different from the pre- and post-restoration assemblages and was dominated by Catostomidae. Responses from target species were mixed, with increased Atlantic salmon abundance, first documented presence of walleye and no presence of lake sturgeon or Coregoninae.     

Nutrient concentrations in the surface sediments of Lake Chivero,Zimbabwe: A shallow,hypereutrophic, subtropical artificial lake

A lake‐wide survey in Lake Chivero, Zimbabwe, was performed to (i) examine the spatial distribution of three sediment grain sizes; (ii) determine their nutrient concentrations; and (iii) analyse the relationships between the measured surface sediment variables in the three sediment grain sizes. The results of this study indicated sand, silt and mud sediments occupied about 24, 22 and 54%, respectively, of the lake area. All the measured physicochemical variables varied significantly among the three sediment grain sizes (P < 0.05, anova ). High values for pH, dissolved oxygen and NO₃ were observed in the sand sediments, while water depth, electrical conductivity, soluble reactive phosphorus, NH₄, total phosphorus, total nitrogen and total organic carbon were observed in the mud sediments. In the silt sediments, most measured variables showed intermediate values. Intervariable correlation indicated varied correlation patterns among the three sediment grain sizes. Information generated in this study is essential for an accurate delineation of the spatial extent of nutrient and contaminant distribution in the sediment of Lake Chivero and will facilitate the ability of lake managers to derive a comprehensive nutrient budget of the lake as a means of establishing realistic target conditions for lake restoration.     

Ecological monitoring of fish assemblages downstream of a hydroelectric dam in French Guiana (South America)

Dam impacts on the downstream section of dammed rivers are known to be important for fish because of changes in the physical and chemical river characteristics. However, the available data seem to be insufficient to draw general conclusions regarding the transformation of fish communities. A hydroelectric dam was built on the Sinnamary River in French Guiana and closed in January 1994. From December 1991 to December 1996 fish collections were made from the downstream section. The results show a rapid decrease in fish abundance in the middle downstream course immediately after dam closure. This low abundance was observed until the end of the filling phase, when the flow was maintained at a minimum level. Afterwards, abundance returned to a level comparable with that prior to dam closure. Fish collection in different zones showed that the decrease in abundance was probably due to the escape of fish to shelter areas. Another marked effect was the reduction of sample diversity due to a decrease in species richness, as well as in the regularity of the distribution of species’ relative abundance. Examination of biological traits of species suggests that under these conditions feeding strategies could have been the most important factor affecting the abundance of populations. Copyright © 1999 John Wiley & Sons, Ltd.     

Artisanal and commercial fishing gear and practices in the Lake Victoria basin drainage systems of Kenya: A photodiagrammatic verification

This study focused on providing a photodiagrammatic database illustrating various types of fishing gear and practical skills currently used in artisanal and commercial fishing in the shoreline, nearshore and offshore areas drainage systems of Kenya in the Lake Victoria drainage basin. It also identified the fish species typically caught by the fishing gear, and the gear catch per unit effort and selectivity. Four artisanal fishing gears (spear, trap, basket and hook) and two commercial gears (mosquito seine and purse seine) are used to fish in shoreline areas. Five commercial fishing gears (gill net, parallel tennis net, semicircular‐tennis net, sett net, long line) are used to fish in nearshore areas, while two commercial gears (drift net and offshore seine net) are used in offshore areas. For artisanal fishing, the basket is the most efficient gear for catching six fish species. The mosquito seine is the most efficient commercial fishing gear for catching 22 fish species. There is evidence for a chronological shift to more efficient artisanal and commercial fisher handmade fishing gear, resulting in overexploitation of diverse fish species. At the same time, the licensing and management policies for the use of various fishing gear are unclear, suggesting re‐evaluation of policies to cater for the use of handmade fishing gear.     

Effects of gravel augmentation on macroinvertebrate assemblages in a regulated California River

Enhancement projects within anadromous salmonid rivers of California have increased in recent years. Much of this work is intended as mitigation in regulated streams where salmon and steelhead spawning habitat is inaccessible or degraded due to dams, water diversions and channelization. Little research has been done to assess the benefits of spawning habitat enhancement to stream organisms other than salmon. We monitored benthic macroinvertebrates at seven spawning gravel augmentation sites in the lower Mokelumne River, a regulated stream in the Central Valley of California. Placement of cleaned floodplain gravel decreased depths and increased stream velocities. Benthic organisms colonized new gravels quickly, equalling densities and biomass of unenhanced spawning sites within 4 weeks. Macroinvertebrate species richness equalled that of unenhanced sites within 4 weeks and diversity within 2 weeks. Standing crop, as indicated by densities and dry biomass, was significantly higher in enhancement sites after 12 weeks than in unenhanced sites and remained so over the following 10 weeks. Although mobile collector/browsers initially dominated new gravels, sedentary collectors were the most common feeding category after 4 weeks, similar to unenhanced sites. These data suggest that cleaned gravels from adjacent floodplain materials, used to enhance salmonid spawning sites, are quickly incorporated into the stream ecosystem, benefiting benthic macroinvertebrate densities and dry biomass. Copyright © 2005 John Wiley & Sons, Ltd.     

Effects of anthropogenic activities on macroinvertebrate assemblages in the littoral zone of Lake Hawassa,a tropical Rift Valley Lake in Ethiopia

In developing tropical countries, the littoral region of lakes is exposed to high human pressures, and the degradation effects are more evident on riparian vegetation and macrophytes. Human impacts on other biological entities, such as plankton and macroinvertebrates, are rarely investigated and reported. The present study focuses on assessing the effects of habitat disturbance on the littoral macroinvertebrate assemblages in Lake Hawassa, Ethiopia. Samples were taken from February to November 2015/16 from 12 sampling sites purposefully selected on the basis of percentage disturbance score (PDS) and categorized into minimally disturbed (four sites), moderately disturbed (five sites) and highly disturbed (three sites). Macroinvertebrate samples were collected from each site using a D‐frame net of 500 µm, with both in situ measurement and laboratory analysis of the environmental variables being conducted. A total of 23,051 macroinvertebrates belonging to 43 families were identified. Analysis of similarities (ANOSIM) and non‐metric multidimensional scaling (NMDS) analyses revealed significantly different macroinvertebrate assemblages among the disturbance levels (ANOSIM: p = .004, R = .55). Variables such as Biological Monitoring Working Party, Shannon diversity index, Ephemeroptera and Trichoptera (ET) family richness, and family richness exhibited a flawless pattern of decrease with increasing human disturbances and a negative correlation with nutrients. The reduction of these metrics for highly disturbed sites was attributable to the deterioration of vegetation and bottom substrate qualities, increasing stormwater inputs, lack of buffer zones and well‐planned land use induced by human interventions. It was evident anthropogenic activities in and around the littoral zone of the lake contributed to the degradation of habitat quality and decreased macroinvertebrate richness and abundance. Thus, effective lake management practices are recommended to rehabilitate the degraded littoral habitat quality in order to realize the sustainable use of this natural resource.     

Bacterial abundance on the skin,gills and intestines of Cyprinus carpio in Lake Naivasha,Kenya: Implications for public health and fish quality

Cyprinus carpio is the most important fish species in the Lake Naivasha fishery, comprising 51% of the total catch in the lake. Microflora, especially enteric bacteria of human or animal origin, are the causative agent for fish contamination and spoilage. Poor sanitation standards and poor sewage treatment and disposal methods within Lake Naivasha and its catchment pose a great threat for degrading the quality of C. carpio. The potential impact is rejection of the fish in the local, regional and international markets, risking the collapse of the Lake Naivasha fishery. This study determined the bacterial quality of water and C. carpio from three different sites within Lake Naivasha, namely Malewa River mouth, sewage discharge point and a mid‐lake site, based on plate count techniques. Physicochemical parameters characterizing the lake water also were also measured in situ. This study results indicated that both the fish and water in Lake Naivasha exhibited poor bacterial quality. All the physicochemical parameters were within the recommended range for fish culture, although they also were conducive to the proliferation of bacteria. Most of the sampling sites exhibited significant spatial variation in their bacterial abundance (P < 0.05). The sewage discharge sampling site exhibited the highest mean density values for bacterial densities and clearly degrade the quality of the fish in the lake. Proper sewage treatment, and the installation of modern sanitation facilities, is recommended to improve the bacterial quality of the fish.     

A review on the status of some major fish species in Lake Victoria and possible conservation strategies

Lake Victoria, the second largest freshwater lake in the world, supports an enormous flora and fauna biomass, with a large human population around the Lake. The lake is a source of food (fish), water for domestic use and crop and livestock production, transportation and tourism, among other uses. As a result of these varied uses, human intervention within the lake and its catchment has resulted in several ecological changes in the lake in recent years, with profound effects on its fish resources. One of the most notable changes is the reduction and risk of extinction of some fish species, including the endemic tilapiine species (Oreochromis esculentus and Oreochromis variabilis), catfishes (Xenoclarias eupogon), haplochromines and cyprinids (Labeo victorianus and Barbus altinialis) in the lake. The reduced biodiversity, as well as extinctions within the lake, has been attributed to the introduction of alien fish species (e.g. Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) into the lake, habitat loss and cultural eutrophication. If left unchecked, these changes will have devastating effects on the lake's resources, as well as to people living around the lake and beyond who depend on the lake fisheries for their livelihood. Thus, there is an urgent need for management measures based on sound scientific research to be implemented in order to curb the loss of ichthyodiversity within the lake, as well as to save the livelihoods of those stakeholders who are either directly or indirectly dependent on the lake.