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

Lake Bottom (Strata), Lake Victoria, Vector Point and polygon, ~2017

Lake Bottom (Strata), Lake Victoria, Vector Point, ~2017 Reference Information and Units: Projected Coordinate System: Africa Lambert Conformal Conic ESRI:102024 (https://epsg.io/102024) GCS: GCS_WGS_1984 File Naming Convention: LV_Strata_Points_V1.shp Data Origin: Admiral Bathymetric Maps: Description: British admiral bathymetry maps of the lake. Maps were scanned and georeferenced. Once georeferenced the points were digitized and the strata was recorded. Points: Approximately 541 Year: 1990-1955 Grab Samples: Description: Description: Points were taken to determine the strata of the lake by using a Simrad EK60 echo sounder. The points were read into Arc using X,Y coordinates Points: Approximately 46 Year: 2009 Data Development: Strata points were obtained using admiral bathymetry charts and points taken out in the field. The points from the admiral bathymetry charts were digitized. The points taken in the field were brought into ArcMap using X, Y coordinates. The strata was recorded for each point in the following fields “Strata1”, “Strata2”, “Strata3”, and “Stra_srce”. Field descriptions can be found below. Field: Z- Depth in Meters Strata1- the strata type. Mud/Sand/Rock/Gravel/Humus Example: m Strata2- the texture of the strata type. Soft/Medium/Hard Examples: sft Strata3- the color of the strata. Black Examples: blk Stra_srce- source where the strata came from. Admiral Bathymetric Charts Example: Admirality Legend: Please see the attached metadata PDF to view the Admiral Bathymetric Charts legen

Modelling and in situ observation of broadband acoustic scattering from the Silver cyprinid (Rastrineobola argentea) in Lake Victoria, East Africa

Funding: YY was funded by the China Scholarship Council. This work was supported by a grant from the UK Royal Society under the “International Collaboration” theme to ASB and RJK.Lake Victoria is the second-largest freshwater lake in the world, and fish from the lake are a vital food resource for millions of people living around it. The silver cyprinid (Rastrineobola argentea), a small schooling pelagic species known in Tanzania as “dagaa” contributes ca. 55% to the total annual catch (ca. 0.51 million tonnes (MT) in 2014). The acoustic target strength (TS, dB re 1 m2) of dagaa, a key factor for biomass estimation, is however not well described, and is a major source of uncertainty in biomass estimation. In this study, we developed a Kirchhoff-ray mode (KRM) model to predict the TS of dagaa at standard fisheries survey frequencies. The model was based on the morphology of the body and the dual-chambered swimbladder, as obtained from X-ray images of fish ranging in total length (TL) between 2.8 and 5.4 cm. The results suggested that the swimbladder (which comprises 2.6 to 8.2% of body volume) accounts for ca. 65 to 90% of the total backscattering at 120 kHz. The predicted TS was highly dependent on tilt angle, varying by 14.0 dB at 120 kHz across the tilt range 65–115˚ (where 0˚ is head up and 180˚ is tail up), and TS variability with tilt generally increased with increasing frequency. The tilt angle of acoustically tracked individual fish indicated a distribution of tilt angles with a mean and s.d. of 93.5 and 15.1°. Our model suggested a new tilt-averaged TS–TL relationship for dagaa [TS120 kHz=19.49log(TL)−70.3], which leads to a TS 1.5 dB higher than the value in the relationship presently used to estimate stock biomass. The new relationship will lead to a substantial reduction (by ca. 30%) in estimated biomass. The discrepancies between the mean relative frequency response of the in situ measurements of backscatter from dagaa and the KRM model predictions were in the range of -2.9–3.1 dB at frequencies from 45 to 250 kHz. The KRM modelling and in situ broadband measurements of dagaa will be beneficial for acoustic identification and behavioural studies of dagaa, and will enable improved biomass assessment, thereby underpinning sustainable long-term management.Publisher PDFPeer reviewe

Bathymetry Points, Lake Victoria, vector points, 1900 - 2017, V7

Bathymetry Points, Lake Victoria, vector point, 1900-2017 Reference Information and Units: Projected Coordinate System: Africa Lambert Conformal Conic ESRI:102024 (https://epsg.io/102024) GCS: GCS_WGS_1984 File Naming Convention: LV_Bathymetry_Points_V7.shp Data Origin: Admiral Bathymetric Maps: Description: British admiral bathymetry maps of the lake. Maps were scanned and georeferenced. Once georeferenced the depth points were digitized Points: Approximately 5,212 Year: 1900-1955 Depth Sounder: Description: Depth readings taken from a depth sounder were read into ArcMap using X, Y coordinates Points: Approximately 5,888 Year: 2013 German Bathymetry Map: Description: A German bathymetry map was scanned and georeferenced. Once georeferenced the depth points were digitized. Points: Approximately 50 Year: 1913 Acoustic Surveys from Nafirri: Description: Instruments used are two Depth Profiling systems (a submersible Conductivity Temperature-Depth profiling system (CTD, Sea and Sun Technologies) and a YSI 650 multiparameter Sonde). Readings were read into ArcMap using X, Y coordinates. Date: 11/2015: Approximately 55 points Date: 8/2016 Approximately 50 points Limnology Survey from Nafirri: Description: Limnology survey using acoustic equipment and GPS Points: Approximately 10 Date: 2007/2008 Sounding Data: Description: Depth readings taken from a depth sounder were read into ArcMap using X, Y coordinates Points: Approximately 17,200 Date: 2009 Acoustic Sounder Strata Data: Description: Points were taken to determine the strata of the lake by using a Simrad EK60 echo sounder. These points contained Z values. The points were read into ArcMap using X, Y coordinates Points: Approximately 45 Date: 2009 Lake Victoria Bathymetric Survey For EAC/LVBC: Description: Points were taken using a single beam echo sounder 200kHz and were provided from Ola at MMT. These points are concentrated in three bays, Mwanza Gulf (TZ), Kisumu (KY), and Port Bell (UG). Points: Approximately 120,000 Date: 2007 Lake Victoria Regional Hydro-acoustics Working Group: Description: The survey was conducted by the Lake Victoria Regional Hydroacoustics Working Group under coordination of LVFO with NSF project funds. It was conducted using Tanzania's RV. Lake Victoria Explorer. Points: Approximately 3,945,260 Date: 09/12/2017 – 10/5/2017 Data Development: The final bathymetry points were created by converting all sources to the same unit(meters). Once all sources were in the same unit they were combined into the same file. When all points were in the same file a spatial query was ran to select all the bathymetry points that fell outside of the Lake Victoria shoreline polygon. Any point that was located outside of the shoreline polygon was removed from the file. The next step was to remove the redundant points. PostgresSQL was used to find all the points that were within 100 meters from another point (SQL Code 1). One of the points would be removed from the final set of points to clean up the data. The points that were obtained from the survey conducted for the EAC/LVBC were not included in the removing of points within 100 meters of one another. Any point from version 6 that was located within 100 meters of a point from the Lake Victoria Regional Hydro-acoustics Working Group dataset, was removed. All points from the acoustic survey conducted by Lake Victoria Regional Hydro-acoustics Working Group were kept regardless of their distance from each other. PostgresSQL was also used to check the depths of points in relation to points around them to determine outliers (SQL Code 2). Outliers were double checked and removed if not deemed plausible. Points that were denoted as “greater than” the recorded depth were removed if the depth was less than 60.96 meters. SQL Code 1: Finds all points within 100 meters of another point select a.objectid, b.objectid from lv_bathymetry_points_v5 as a, lv_bathymetry_points_v5 as b where st_distance(a.geom,b.geom) < 100 and a.objectid != b.objectid SQL Code 2: Checks for points with depths that are outliers compared to neighbor points select a.objectid, b.objectid, (@(a.z - b.z)) as z_diff from lv_bathymetry_points_v5 as a, lv_bathymetry_points_v5 as b where (@(a.z - b.z)) > 5 and st_distance(a.geom, b.geom) < 200 <br

Acoustic Monitoring of Blast Fishing: Pilot Study - Dar es Salaam

<p><strong>Technical report: WWF Tanzania Country Office - Marine Programme</strong></p> <p>Blast fishing is illegal fishing method according to Fisheries Act (2003) due to its negative impact especially on coral reefs. The method is widely used and it is a serious problem along the coast of Tanzania mainland. The most affected area is Dar es Salaam, where existing regulations are not effectively enforced to address the problem. Inadequate data on the frequency of occurrence and distribution of blast activity prevent effective control measures.</p> <p>Using Digital Spectrogram Long-Term Acoustic Recorders (DSG), we performed two deployments. One was a two-day long deployment to identify the acoustic characteristics of blasts, and the second was a two- month long deployment to monitor blast activity off Dar es Salaam coast.</p> <p>In the blast identification phase we deployed one DSG over two days on Milliards Bank recording sound at 80 kHz, we analysed the acoustic characteristics of 45 underwater explosions to identify and characterize blasts used during illegal fishing operations. We developed a neural- network that semi-automatically identifies explosion-like recordings. Ninety-five percent of the explosion-like events identified by the neural network are attributable to blast fishing, while the other five percent were other unidentified transient signals.</p> <p>During the monitoring phase two DSGs were deployed, however we were able to retrieve only one of them. Weather, currents, and possibly vandalism might be the causes for the loss. The retrieved DSG was deployed on Mbudya patches between 15 April and 30 May, 2014 recording at a sampling frequency of 40kHz during five minutes every ten minutes between 0600 and 1800 hrs. Using this additional data the neural network improved its classification accuracy to 98%. We estimate an average of 19 blasts per day during daytime off Mbudya, with most of the blasts occurring in the morning before 1300 hrs.</p> <p>To avoid further loss of instruments, we recommend the use of acoustic releases in further deployments. However care must be taken to avoid potential problems caused by biofouling as well as preventing interference from noise near the DSGs caused by floats and moving elements part of the release.</p> <p>In order to better understand blast fishing patterns as well as factors influencing underwater noise production we also recommend recording environmental variables related to weather and currents.</p> <p>This work represents the first steps of a potential future monitoring program off the Tanzanian coast. We provide methods and recommendations for future monitoring of blast fishing</p> <p> </p

Changes in the Diet of Synodontis victoriae and Synodontis afrofischeri in Lake Victoria, Tanzanian waters.

The diet of Synodontis victoriae and S.afrofischeri was investigated from samples collected for stomach analysis in May 2013, October 2013, and April 2014 in the Tanzanian waters of Lake Victoria. The diet of S. victoriae was dominated by freshwater shrimps Caridina nilotica followed by unidentified fish remains, the cyprinids Rastrineobola argentea and Enteromius profundus as well as insects, molluscs, haplochromines and worms. The diet of S. afrofischeri was dominated by R. argentea followed by insects, fish remains, worms, molluscs, C. nilotica, algae and haplochromines. There was considerable variation in the diets of both species collected at different times and they displayed considerable plasticity in their diet. Both species exhibited a wider range of diet, utilizing food items that may not have been available before the changes in the lake that followed the Nile perch upsurge in the 1980s.Keywords: Caridina nilotica, Diet expansion, Food and Feeding habits, Rastrineobola argentea, Seasonal variatio

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

Mind the gaps for the best practices: Enhancing the management of Lake Victoria fisheries resources

Fisheries resources face a confluence of socio-ecological challenges, the resolution of which requires interdisciplinary scientific information for sustainable utilization and management. The present study assessed gaps and challenges in Lake Victoria fisheries resources management for better research focus, policy formulation and improved governance of the fishery towards sustainability. Using key informant interviews and a plenary discussion with trans-disciplinary experts regarding Lake Victoria fisheries research, management and policy sectors, the present study identified current management challenges, gaps and priorities. The present study results indicate a constantly increasing fishing effort, poor enforcement of existing regulations and pollution and invasive weeds pose the greatest threats to the sustainability of Lake Victoria\u27s fisheries resources. Policy gaps include a lack of regulations on emerging technologies (e.g. cage culture) and an absence of implementation guidelines and framework for some existing policies. The aspects and gaps of each challenge are discussed, using available literature for the lake, with policy and capacity interventions recommended under each section for sustainable management of Lake Victoria fisheries resources