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

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

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

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.</p

Need for harmonized long-term multi-lake monitoring of African Great Lakes

To ensure the long-term sustainable use of African Great Lakes (AGL), and to better understand the functioning of these ecosystems, authorities, managers and scientists need regularly collected scientific data and information of key environmental indicators over multi-years to make informed decisions. Monitoring is regularly conducted at some sites across AGL; while at others sites, it is rare or conducted irregularly in response to sporadic funding or short-term projects/studies. Managers and scientists working on the AGL thus often lack critical long-term data to evaluate and gauge ongoing changes. Hence, we propose a multi-lake approach to harmonize data collection modalities for better understanding of regional and global environmental impacts on AGL. Climate variability has had strong impacts on all AGL in the recent past. Although these lakes have specific characteristics, their limnological cycles show many similarities. Because different anthropogenic pressures take place at the different AGL, harmonized multi-lake monitoring will provide comparable data to address the main drivers of concern (climate versus regional anthropogenic impact). To realize harmonized long-term multi-lake monitoring, the approach will need: (1) support of a wide community of researchers and managers; (2) political goodwill towards a common goal for such monitoring; and (3) sufficient capacity (e.g., institutional, financial, human and logistic resources) for its implementation. This paper presents an assessment of the state of monitoring the AGL and possible approaches to realize a long-term, multi-lake harmonized monitoring strategy. Key parameters are proposed. The support of national and regional authorities is necessary as each AGL crosses international boundaries

Need for harmonized long-term multi-lake monitoring of African Great Lakes

To ensure the long-term sustainable use of African Great Lakes (AGL), and to better understand the functioning of these ecosystems, authorities, managers and scientists need regularly collected scientific data and information of key environmental indicators over multi-years to make informed decisions. Monitoring is regularly conducted at some sites across AGL; while at others sites, it is rare or conducted irregularly in response to sporadic funding or short-term projects/studies. Managers and scientists working on the AGL thus often lack critical long-term data to evaluate and gauge ongoing changes. Hence, we propose a multi-lake approach to harmonize data collection modalities for better understanding of regional and global environmental impacts on AGL. Climate variability has had strong impacts on all AGL in the recent past. Although these lakes have specific characteristics, their limnological cycles show many similarities. Because different anthropogenic pressures take place at the different AGL, harmonized multi-lake monitoring will provide comparable data to address the main drivers of concern (climate versus regional anthropogenic impact). To realize harmonized long-term multi-lake monitoring, the approach will need: (1) support of a wide community of researchers and managers; (2) political goodwill towards a common goal for such monitoring; and (3) sufficient capacity (e.g., institutional, financial, human and logistic resources) for its implementation. This paper presents an assessment of the state of monitoring the AGL and possible approaches to realize a long-term, multi-lake harmonized monitoring strategy. Key parameters are proposed. The support of national and regional authorities is necessary as each AGL crosses international boundaries