A comparative analysis of ecosystem models of Lake Victoria (East Africa)

The advent of ecosystem-based fisheries management (EBFM) in recent years has expanded the scope of fisheries management, where policies are aimed not only at maximizing production (or biological yield) but also maintaining an optimal balance between socio-economic and conservation objectives when utilizing aquatic living resources. Ecosystem models help to assimilate diverse information on the drivers of ecosystem change, thereby providing integrated assessment and advice that is needed for EBFM. Different ecosystem models, however, often provide different predictions, and this uncertainty is one of the major challenges impeding their use in EBFM. This thesis compares ecosystem models of Lake Victoria (East Africa), focusing on Ecopath with Ecosim (EwE) and Atlantis modelling frameworks, to get insights into sensitivity of fishing policy scenarios to model choice. The thesis is based on six papers. Paper I highlights the history of ecosystem modelling, both in African lakes and globally. Paper II describes temporal changes in food web of Lake Victoria using Ecopath models. Paper III compares a historic Ecopath model of Lake Victoria, fitted to time series data, to recent models that are not fitted to time series data to show how reliable none-fitted models are in terms of evaluating long-term ecosystem impacts of fishing. Paper IV compares EwE and Atlantis models of Lake Victoria to show how ecosystem impacts of fishing may be sensitive to model structure and complexity. Paper V explores socio-ecological trade-offs in Lake Victoria using both EwE and Atlantis models. Paper VI explores spatio-temporal variation in fishing patterns and fishing pressure in Lake Victoria in relation to the balanced harvesting approach (where fishing mortality is distributed across taxa or species in proportion to biological productivity). Overall, these studies contribute to understanding of how advice from ecosystem models can be affected by model structure, data assumptions and quality. The findings in this thesis suggest that simpler models may also support robust qualitative advice, which is vital for long-term strategic planning, but key ecological mechanisms explaining the differences in fishery responses between models (simple vs. complex, data-rich vs. data-poor) must be identified before hand. In view of this finding, the importance of multi-model simulations as an aid to guarding against the effects uncertainty on strategic management decisions can not be overstated. This further implies that the lack of model diversity observed on most African lakes requires urgent attention.Notkun vistkerfisnálgunar við stjórn fiskveiða á undanförnum árum hefur breikkað svið fiskveiðistjórnunar með því að ákvarðanir miðast ekki einvörðungu við hámörkun heildarafla eða afraksturs heldur er einnig tekið tillit til félagslegra og hagrænna þátta við nýtingu lifandi auðlinda sjávar. Vistkerfislíkön draga saman fjölbreytt upplýsingasöfn um hvað ræður vistkerfisbreytingum og gefa þannig samræmt heildarmat á ástandi og þá ráðgjöf sem þarf að nota fyrir vistkerfisnálgun við stjórn fiskveiða (ecosystem-based fisheries management, EBFM). Ólík vistkerfislíkön geta hins vegar gefið talsvert ólíkar spár um þróun. Slíka líkan-óvissa er talsverð hindrun við notkun þessara líkana við EBMF. Þær rannsóknir sem hér eru kynntar rýna í áhrif flækjustigs líkana og óvissu í gögnum á ákvarðanir um auðlindanýtingu í ljósi EBFM og eru niðurstöður kynntar í sex greinum. Í þessari ritgerð eru borin saman vistkerfislíkön fyrir Viktoríuvatn í Austur-Afríku, mest með Ecopath with Ecosim"(EwE) og Atlantis, til ˛ að sýna áhrif líkanavals á ályktanir um afleiðingar stjórnvaldsaðgerða. Ritgerðin byggir á 6 greinum. Í Grein I er gefið yfirlit um notkun vistkerfislínana á vötn í Afríku. Grein II lýsir líkani fyrir fæðuvef Viktoríuvatns stöðulíkani (Ecopath). Grein III ber saman sögulegt EwE líkan fellt að eldri tímaraðargögnum við jafnstöðu líkan lagt að nýrri gögnum. Tilgangurinn er að skilja áreiðanleika jafnstöðulíkana sem ekki nota formlega aðlögun að gögnum þegar þau eru notuð til að spá fyrir um afleiðingar stjórnvaldsákvarðana. Grein IV ber saman EwE og Atlantis líkön til að auka skilning á hvernig líkagerð, flækjustig og forsendur hafa áhrif á mat á stjórnvaldsaðgerðum. Grein V fjallar um mat á félagslegum og efnahagslegum áhrifum stjórnunaraðgerða með Atlantis og EwE. Grein VI metur breytileika í veiðimynstri og sókn í tíma og rúmi í samhengi við aðferðar jafnrar sóknar (balanced harvest strategy) þar sem sókn er sett sem hlutfall af framleiðslu tegundar. Sú heildarmynd fæst úr þessum tilraunum að ráðgjöf byggð á ólíkum líkönum getur orðið talsvert háð innri byggingu hvers líkans, gagnanýtingu og gæðum gagna. En niðurstöður benda líka til þess að einföld líkön geti veitt trausta almenna ráðgjöf, sem er mikilvæg fyrir langtíma áætlanagerð. Lykileiginleika vistkerfis, sem útskýra mun á spá einfaldra og flókinna líkana, þarf hins vegar að auðkenna fyrirfram. Í ljósi þessa er mikilvægi þess að prófa mörg líkön augljóst og nauðsynlegt að beina sjónum að fæð þeirri sem auðkennir breytileika í líkanagerð fyrir vötn í Afríku.United Nations University Fisheries Training Programme (UNU-FTP), Iceland

High-resolution bathymetries and shorelines for the Great Lakes of the White Nile basin

This article is licensed under a Creative Commons Attribution 4.0 International License.HRBS-GLWNB 2020 presents the first open-source and high-resolution bathymetry, shoreline, and water level data for Lakes Victoria, Albert, Edward, and George in East Africa. For each Lake, these data have three primary products collected for this project. The bathymetric datasets were created from approximately 18 million acoustic soundings. Over 8,200 km of shorelines are delineated across the three lakes from high-resolution satellite systems and uncrewed aerial vehicles. Finally, these data are tied together by creating lake surface elevation models collected from GPS and altimeter measures. The data repository includes additional derived products, including surface areas, water volumes, shoreline lengths, lake elevation levels, and geodetic information. These data can be used to make allocation decisions regarding the freshwater resources within Africa, manage food resources on which many tens of millions of people rely, and help preserve the region’s endemic biodiversity. Finally, as these data are tied to globally consistent geodetic models, they can be used in future global and regional climate change models.ECU Open Access Publishing Support Fun

Assessment of exploited fish species in the Lake Edward System, East Africa

The unknown status of inland fish stocks hinders their sustainable management. Therefore, increasing stock status information is important for sustainable inland fisheries. Fisheries reference points were estimated for five exploited fish species (11 stocks) in the Lake Edward system, East Africa, which is one of the most productive inland water systems. The aim was to ascertain the status of the fisheries and establish reference points for effective management. The reference points were based on four linked stock assessment approaches for data‐limited fisheries. Estimates showed poor stock status with the stocks defined as either collapsed, recruitment impaired or overfished. However, higher catches could be obtained under sustainable management. Estimates of maximum sustainable yield (MSY) and supporting biomass (Bmsy) are provided for 10 of the stocks as targets for rebuilding plans. The immediate target of management should be rebuilding biomass to Bmsy. Applicable measures include shifting length at first capture to the length that maximizes catch without endangering size structure and biomass, and livelihood diversification out of fisheries

The Consequences of Anthropogenic Stressors on Cichlid Fish Communities: Revisiting Lakes Victoria, Kyoga, and Nabugabo

Lakes Victoria, Kyoga, and Nabugabo (“the Lake Victoria region”) are remarkable for hosting one of the largest assemblages of cichlid fishes among the African inland lakes. Here, we review the role and severity of anthropogenic and environmental stressors on the cichlid communities in the Lake Victoria region to understand the mechanisms leading to the persistence and resurgence of some of the cichlid fishes. Our review suggests that (1) the native Oreochromis species populations primarily collapsed due to overfishing and that the introduced species and habitat change suppressed their ability to recover; (2) without primary triggers associated with change in the environment and habitat conditions, particularly eutrophication and associated anoxia and reduced water transparency, Nile perch (Lates niloticus) predation alone may not have caused the massive loss of species diversity; and (3) the resurgence of haplochromine cichlids is due to a combination of general improvement in the environment and reduction in L. niloticus abundance, with additionally possibly some rapid ecological adaptations. We conclude that environmental stressors will likely continue to shape the ecosystems in which the remaining endemic cichlid fish diversity continue to evolve, clearly involving genetic exchange between species. If water clarity can be improved again, it is possible to maintain a diverse assemblage of endemic species

Ecosystem Modelling of Data-limited Fisheries: How Reliable are Ecopath with Ecosim Models without Historical Time Series Fitting?

Long-term time series data are not available for many of the African Great Lakes. This precludes fitting ecosystem model parameters to time series data, and we do not know how reliable non-fitted models are compared to fitted ones in terms of predicting consequences of alternative management strategies. To investigate this, we generate a historical Ecopath with Ecosim (EwE) model for Lake Victoria (East Africa), fitted to time series data (1980–2015), and a present-day EwE model (representing average conditions for the period 2010–2015). We do scenario simulations using the present-day model and the comparable 2015 end-state of the historical model, and test if incorporating information on short-term biomass trends by adjusting biomass accumulation (BA) parameter in the present-day model increases its reliability. We find that there are differences in model predictions, but those differences can be lessened by adjusting BA terms in the present-day model to reflect biomass trends from short-term empirical data. We also compare the models with and without fitted vulnerability parameters. The models generally give comparable results for the dominant commercial fisheries at low fishing pressure; when fishing mortality is increased, the models give variable predictions. This study adds to the current understanding of the limitations of EwE models that are not challenged to reproduce long-term historical fishery responses to perturbations. We conclude that for the less productive groups, as well as groups that suffer heavy mortality (either due to predation or fisheries), it may be appropriate to use negative BA as first draft assumption in present-day models

Ecosystem Models of Lake Victoria (East Africa): Exploring the Sensitivity of Ecosystem Effects of Fishing to Model Choice

Ecosystem simulation models are valuable tools for strengthening and promoting ecosystem-based fisheries management (EBFM). However, utility of these models in practical fisheries management is often undermined by lack of simple means to test the effect of uncertainty on model outputs. Recently, the use of multiple ecosystem models has been recommended as an ‘insurance’ against effects of uncertainty that comes with modelling complex systems. The assumption is that if models with different structure and formulation give consistent results, then, policy prescriptions are robust (i.e. less sensitive to model choice). However, information on the behaviour of trends from structurally-distinct ecosystem models with respect to changes in fishing conditions is limited, especially for freshwater systems. In this study, we compared outputs of two ecosystem models, Ecopath with Ecosim (EwE) and Atlantis, for Lake Victoria under different fishing pressure scenarios. We compared model behaviour at the ecosystem level, and also at a level of functional groups. At functional group level, we determined two questions: what is the change in the targeted group, and what are the consequent effects in other parts of the system? Overall results suggest that different model formulations can provide similar qualitative predictions (direction of change), especially for targeted groups with similar trophic interactions and adequate data for parameterization and calibration. However, considerable variations in predictions (where models predict opposite trends) may also occur due to inconsistencies in the strength of the aggregate multispecies interactions between species and models, and not necessarily due to model detail and complexity. Therefore, with more information and data, especially on diet, and comparable representation of feeding interactions across models, ecosystem models with distinct structure and formulation can give consistent policy evaluations for most biological groups

Simulating Trade-offs Between Socio-economic and Conservation Objectives for Lake Victoria (East Africa) Using Multispecies, Multifleet Ecosystem Models

Most small scale inland fisheries worldwide are open access, and fishing provides the only source of employment and livelihood for the riparian communities. Management of these fisheries requires information on trade-offs between fish production, profits from fishing, employment, and conservation objectives. We use the non-linear optimization procedure in Ecopath with Ecosim (EwE) modelling package to determine long-term, gear-specific fishing effort that can maximize economic, social, and conservation objectives for Lake Victoria (East Africa). Then, the resulting “optimal fishing effort” levels are applied in both EwE and Atlantis models to simulate long-term changes in the ecosystem. Results show profit maximization to be more compatible with conservation objectives than is the maximization of catch (or employment). However, maximizing economic value, while maintaining ecosystem structure, would require a reduction in fishing effort of almost every fishing gear. This trade-off can be severe (high social cost) for fishing communities with limited alternative livelihoods. This study provides an understanding of relative risks and benefits of various management objectives, which will enable stakeholders and the public to conduct informed discussions on future management policies

Ecosystem Models of Lake Victoria (East Africa): Exploring the Sensitivity of Ecosystem Effects of Fishing to Model Choice

Ecosystem simulation models are valuable tools for strengthening and promoting ecosystem-based fisheries management (EBFM). However, utility of these models in practical fisheries management is often undermined by lack of simple means to test the effect of uncertainty on model outputs. Recently, the use of multiple ecosystem models has been recommended as an ‘insurance’ against effects of uncertainty that comes with modelling complex systems. The assumption is that if models with different structure and formulation give consistent results, then, policy prescriptions are robust (i.e. less sensitive to model choice). However, information on the behaviour of trends from structurally-distinct ecosystem models with respect to changes in fishing conditions is limited, especially for freshwater systems. In this study, we compared outputs of two ecosystem models, Ecopath with Ecosim (EwE) and Atlantis, for Lake Victoria under different fishing pressure scenarios. We compared model behaviour at the ecosystem level, and also at a level of functional groups. At functional group level, we determined two questions: what is the change in the targeted group, and what are the consequent effects in other parts of the system? Overall results suggest that different model formulations can provide similar qualitative predictions (direction of change), especially for targeted groups with similar trophic interactions and adequate data for parameterization and calibration. However, considerable variations in predictions (where models predict opposite trends) may also occur due to inconsistencies in the strength of the aggregate multispecies interactions between species and models, and not necessarily due to model detail and complexity. Therefore, with more information and data, especially on diet, and comparable representation of feeding interactions across models, ecosystem models with distinct structure and formulation can give consistent policy evaluations for most biological groups

Automated classification of schools of the silver cyprinid Rastrineobola argentea in Lake Victoria acoustic survey data using random forests

The dagaa classification reported here was supported specifically by several Scottish Funding Council Global Challenge Research Fund (GCRF) grants from the University of St Andrews and the University of Strathclyde, by a GCRF Networking Grant to ASB and RJK from the UK Academy of Medical Sciences (GCRFNG\100371), and a Royal Society International Collaboration Award to ASB and Rhoda Tumwebaze, LVFO (ICA\R1\180123).Biomass of the schooling fish Rastrineobola argentea (dagaa) is presently estimated in Lake Victoria by acoustic survey following the simple "rule" that dagaa is the source of most echo energy returned from the top third of the water column. Dagaa have, however, been caught in the bottom two-thirds, and other species occur towards the surface: a more robust discrimination technique is required. We explored the utility of a school-based random forest (RF) classifier applied to 120kHz data from a lake-wide survey. Dagaa schools were first identified manually using expert opinion informed by fishing. These schools contained a lake-wide biomass of 0.68 million tonnes (MT). Only 43.4% of identified dagaa schools occurred in the top third of the water column, and 37.3% of all schools in the bottom two-thirds were classified as dagaa. School metrics (e.g. length, echo energy) for 49081 manually classified dagaa and non-dagaa schools were used to build an RF school classifier. The best RF model had a classification test accuracy of 85.4%, driven largely by school length, and yielded a biomass of 0.71 MT, only c. 4% different from the manual estimate. The RF classifier offers an efficient method to generate a consistent dagaa biomass time series.Publisher PDFPeer reviewe