Spatial patterns and trophic modelling within the Lake Malawi Demersal fish community : conservation and fisheries applications

Lake Malawi has a complex, highly speciose, fish community supporting a diverse fishery of great importance as a food source for subsistence communities and to local and national economies. The fish community is also of international importance to scientists interested in the study of evolutionary mechanisms. Little information is available on the structure and ecology of the demersal fish community to help managers understand how best to both sustain fisheries productivity and maintain species diversity. Community level impacts of the demersal trawl fishery in the southern part of the lake are documented but mechanisms for the impacts are unknown. A greater understanding of species distributions and the trophic structure and function of the demersal fish community are required.The principal aim of this thesis was to improve the information base on demersal fish distributions for use in the selection and design o f aquatic reserves, and to enable a more predictive approach to management of the fish community through ecosystem modelling. Specific objectives were: describe spatial patterns of species distributions throughout the lake; determine the trophic structure of the fish community through dietary studies; model community trophic structure using the ECOPATH software, and; evaluate the ECOSIM simulation software for predicting impacts of ecosystem disturbance such as from demersal trawling and the introduction of non-native species.Species catch compositions from over four hundred demersal trawls conducted throughout Lake Malawi were analysed for spatial distribution patterns. Species assemblages were differentiated along gradients of water depth and geographic location. Depth differentiation was characterised by a combination of species replacements, differences in the abundance of shared species, and a reduction in species richness with increasing depth. Only the catfish species were shared by species assemblages from all depths. Assemblage similarity along the geographic gradient was negatively correlated with the distance between sample areas but the proportion of species shared between sample areas showed no clear spatial pattern. A number of hypotheses are proposed as possible explanations for the evolution and persistence of the observed pattern of species assemblages. In particular, the lake-wide consistency in species richness at depth is discussed in the context of species-energy limitations within the system. Finally, the potential application of information on species spatial patterns to the selection of ‘no-take’ conservation areas and aquatic reserves is discussed.Diet analysis, supported by analysis of stable isotope ratios, identified ten main trophic guilds among the demersal fish community ranging across three to four trophic levels. An additional guild was formed to incorporate those shallow water species not often encountered in the demersal trawls that were reported in the literature as feeding on epilithic algae and macrophytes. The mean trophic level o f the demersal fish community for all depths and sample areas, pooled and weighted by the respective areas of lakebed coverage, was estimated at 3.18 with the bulk of biomass (58%) concentrated at trophic level 3.0. Mean trophic levels of fish assemblages were significantly influenced by water depth and sample location. The main influence of depth was realised in the shallow water where mean trophic values in the 0 to 20 m depth band were significantly lower than at all other depths.For many taxa within the demersal fish community there appeared to be a high proportion of trophic-equivalence. It was therefore thought that the community included a number of functional analogues that might be maintained by spatial variability, or perhaps by unobserved behavioural mechanisms operating across species complements. While subtle mechanisms may be at play to support the unusually high species diversity of the lake the results suggested that elimination of a species might result in its subsequent replacement by a functional analogue. Caution should be exercised, however, in making this interpretation based on the rather broad dietary characteristics considered here. Moreover, such functional analogues could also be highly important for the maintenance of ecosystem function and stability through provision of a buffer against environmental change. Loss of these species could thus be highly damaging to the long-term resilience and ecological integrity of the community and, consequently, to the viability of the fishery.Energy flows between trophic levels were estimated through food consumption studies for a selection o f representative species. A trophic model o f the demersal fish community and a graphical representation o f the food web and its energy flows were constructed using the ECOPATH approach and software. The area of the lake modelled was restricted to the southern and western sectors o f the lake to a water depth of approximately 140m.The proposed role of the lakefly (Chaoborus edulis) in the ecosystem was somewhat clarified. Previous study showed that C. edulis was a more important food source to the pelagic fish community than originally suggested, and went on to propose that demersal fish might also rely on consumption of C. edulis larvae. Integration of the demersal and pelagic systems to form a model of the complete ecosystem showed that approximately 57 % of C. edulis production was directly consumed by pelagic fish and a further 2.4 % by demersal fish. The remaining 40 % of production either flowed to detritus, where a certain proportion was recycled through detritivores, or left the system through dispersal as flying adults, some of which returned to the lake to be consumed by surface-feeding catfish.In the demersal zone the role of C. edulis was less significant than in the pelagic. System modelling suggested that although C. edulis provided a direct link between the demersal fish community and pelagic productivity the main pathways for energy flow in the demersal domain were through the consumption of detritus and, to a lesser extent, consumption of copepods by demersal fish apparently migrating into the pelagic for feeding. Integration of the demersal and pelagic components of the Lake Malawi ecosystem indicated that, for the area of lake modelled, the system was more efficient than previously supposed as the demersal fish community was able to utilise much of the pelagic production. In summary, the demersal system appeared to rely most heavily on biomass imported from the pelagic through consumption of detritus and of copepods consumed by fish migrating vertically to feed in the pelagic.Sensitivity analyses were run for a number of ECOSIM simulations based on disturbance scenarios of relevance to the Lake Malawi demersal fish community. Simulations included increased fishing yields, initiation of a deep-water trawl fishery, and the introduction of an alien pelagic zooplanktivore. In all cases the quantitative nature of ECOSIM predictions was sensitive to input values for the ‘vulnerability’ parameter while the qualitative nature of predictions was unaffected. As available data were found insufficient to obtain robust estimates for vulnerability it was concluded that ECOSIM simulations should not be employed to predict the quantitative nature o f group biomass changes but may be used in a qualitative way to predict the direction of change.In all impact scenarios modelled ECOSIM simulations predicted a change in species composition and community trophic structure. It was suggested that the predicted changes in trophic structure might lead to unpredictable outcomes such as trophic cascades and abrupt phase shifts in community structure with associated reduction or stagnation of catches.In conclusion, this study has demonstrated the complex nature of trophic interactions within the Lake Malawi fish community. Modelling simulations have demonstrated the potential for human intervention to disrupt community structure with associated consequences of reduced productivity and stability of the fishery. A number of gaps in knowledge have been identified which, if filled, would greatly increase the robustness of the model and increase its utility as a predictive tool for managing the fishery

Future large hydropower dams impact global freshwater megafauna

Dam construction comes with severe social, economic and ecological impacts. From an ecological point of view, habitat types are altered and biodiversity is lost. Thus, to identify areas that deserve major attention for conservation, existing and planned locations for (hydropower) dams were overlapped, at global extent, with the contemporary distribution of freshwater megafauna species with consideration of their respective threat status. Hydropower development will disproportionately impact areas of high freshwater megafauna richness in South America, South and East Asia, and the Balkan region. Sub-catchments with a high share of threatened species are considered to be most vulnerable; these are located in Central America, Southeast Asia and in the regions of the Black and Caspian Sea. Based on this approach, planned dam locations are classified according to their potential impact on freshwater megafauna species at different spatial scales, attention to potential conflicts between climate mitigation and biodiversity conservation are highlighted, and priorities for freshwater management are recommended

Global patterns of freshwater species diversity, threat and cross-taxon congruence

Aim Global-scale studies are required to identify broad-scale patterns in the distributions of species, to evaluate the processes that determine diversity and to determine how similar or different these patterns and processes are among different groups of freshwater species. Broad-scale patterns of spatial variation in species distribution are central to many fundamental questions in macroecology and conservation biology. We aimed to evaluate how congruent three commonly used metrics of diversity were among taxa for six groups of freshwater species. Location Global. Methods We compiled geographical range data on 7083 freshwater species of mammals, amphibians, reptiles, fishes, crabs and crayfish to evaluate how species richness, richness of threatened species and endemism are distributed across freshwater ecosystems. We evaluated how congruent these measures of diversity were among taxa at a global level for a grid cell size of just under 1°. Results We showed that although the risk of extinction faced by freshwater decapods is quite similar to that of freshwater vertebrates, there is a distinct lack of spatial congruence in geographical range between different taxonomic groups at this spatial scale, and a lack of congruence among three commonly used metrics of biodiversity. The risk of extinction for freshwater species was consistently higher than for their terrestrial counterparts. Main conclusions We demonstrate that broad-scale patterns of species richness, threatened-species richness and endemism lack congruence among the six freshwater taxonomic groups examined. Invertebrate species are seldom taken into account in conservation planning. Our study suggests that both the metric of biodiversity and the identity of the taxa on which conservation decisions are based require careful consideration.As geographical range information becomes available for further sets of species, further testing will be warranted into the extent to which geographical variation in the richness of these six freshwater groups reflects broader patterns of biodiversity in fresh water

Monitoring extinction risk and threats of the world’s fishes based on the Sampled Red List Index

Global biodiversitytargets require us to identify species at risk of extinction and quantify status and trends of biodiversity. The Red List Index (RLI) tracks trends in the conservation status of entire species groups over time by monitoring changes in categories assigned to species. Here, we calculate this index for the world’s fishes in 2010, using a sampled approach to the RLI based on a randomly selected sample of 1,500 species, and also present RLI splits for freshwater and marine systems separately. We further compare specific traits of a worldwide fish list to our sample to assess its representativeness. Overall, 15.1% of species in the sample were estimated to be threatened with extinction, resulting in a sampled RLI of 0.914 for all species, 0.968 in marine and 0.862 in freshwater ecosystems. Our sample showed fishing as the principal threat for marine species, and pollution by agricultural and forestry effluents for freshwater fishes. The sampled list provides a robust representation for tracking trends in the conservation status of the world’s fishes, including disaggregated sampled indices for marine and freshwater fish. Reassessment and backcasting of this index is urgent to check the achievement of the commitments proposed in global biodiversity targets

Monitoring Extinction Risk and Threats of the World\u27s Fishes Based on the Sampled Red List Index

Global biodiversitytargets require us to identify species at risk of extinction and quantify status and trends of biodiversity. The Red List Index (RLI) tracks trends in the conservation status of entire species groups over time by monitoring changes in categories assigned to species. Here, we calculate this index for the world’s fishes in 2010, using a sampled approach to the RLI based on a randomly selected sample of 1,500 species, and also present RLI splits for freshwater and marine systems separately. We further compare specific traits of a worldwide fish list to our sample to assess its representativeness. Overall, 15.1% of species in the sample were estimated to be threatened with extinction, resulting in a sampled RLI of 0.914 for all species, 0.968 in marine and 0.862 in freshwater ecosystems. Our sample showed fishing as the principal threat for marine species, and pollution by agricultural and forestry effluents for freshwater fishes. The sampled list provides a robust representation for tracking trends in the conservation status of the world’s fishes, including disaggregated sampled indices for marine and freshwater fish. Reassessment and backcasting of this index is urgent to check the achievement of the commitments proposed in global biodiversity targets

Assessing the cost of global biodiversity and conservation knowledge

Knowledge products comprise assessments of authoritative information supported by stan-dards, governance, quality control, data, tools, and capacity building mechanisms. Considerable resources are dedicated to developing and maintaining knowledge productsfor biodiversity conservation, and they are widely used to inform policy and advise decisionmakers and practitioners. However, the financial cost of delivering this information is largelyundocumented. We evaluated the costs and funding sources for developing and maintain-ing four global biodiversity and conservation knowledge products: The IUCN Red List ofThreatened Species, the IUCN Red List of Ecosystems, Protected Planet, and the WorldDatabase of Key Biodiversity Areas. These are secondary data sets, built on primary datacollected by extensive networks of expert contributors worldwide. We estimate that US$160million (range: US$116–204 million), plus 293 person-years of volunteer time (range: 278–308 person-years) valued at US$14 million (range US$12–16 million), were invested inthese four knowledge products between 1979 and 2013. More than half of this financingwas provided through philanthropy, and nearly three-quarters was spent on personnelcosts. The estimated annual cost of maintaining data and platforms for three of these knowl-edge products (excluding the IUCN Red List of Ecosystems for which annual costs were notpossible to estimate for 2013) is US$6.5 million in total (range: US$6.2–6.7 million). We esti-mated that an additional US$114 million will be needed to reach pre-defined baselines ofdata coverage for all the four knowledge products, and that once achieved, annual mainte-nance costs will be approximately US$12 million. These costs are much lower than those tomaintain many other, similarly important, global knowledge products. Ensuring that biodi-versity and conservation knowledge products are sufficiently up to date, comprehensiveand accurate is fundamental to inform decision-making for biodiversity conservation andsustainable development. Thus, the development and implementation of plans for sustain-able long-term financing for them is critical