Predicting the Edibility of Fish in the Flag Boshielo System
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PREDICTING THE EDIBILITY OF FISH IN THE FLAG BOSHIELO SYSTEM Report to the WATER RESEARCH COMMISSION by JOSEPH R. SARA, SEAN M. MARR, WILMIEN J. LUUS-POWELL, WILLEM J. SMIT and ABRAHAM ADDO-BEDIAKO University of Limpopo WRC Report No. 2544/1/21 ISBN 978-0-6392-0262-4 July 2021 Obtainable from Water Research Commission Private Bag X03 Gezina, 0031 PRETORIA [email protected] or download from www.wrc.org.za The publication of this report emanates from a project titled Investigation of large-scale drivers of seasonal fluctuation on water chemistry and toxicant levels in the Flag Boshielo System (WRC Project No: K5/2544) DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. © Water Research Commission PREDICTING THE EDIBILITY OF FISH IN THE FLAG BOSHIELO SYSTEM EXECUTIVE SUMMARY RATIONALE Despite South Africa being food secure at national level, Statistics South Africa found that almost 21% of South African households experience food insecurity (Statistics South Africa, 2017b). South Africa’s National Development Plan aims to eliminate poverty and increase food security in rural areas by 2030. South Africa has more than 4 700 impoundments constructed primarily for domestic, agricultural and industrial water supply. The Department of Agriculture, Forestry and Fisheries have proposed that these water storage and distribution impoundments be developed for inland fisheries and aquaculture in order to improve rural livelihoods and increase food security in South Africa. Fish is a vital source of food for many of the world’s people, especially low-income groups, as they are a rich source of protein, micronutrients and essential fatty acids, cheaper than other protein sources, and available from local lakes, rivers and impoundments. A geographic information system model based on the morpho-edaphic index of fisheries potential identified the north-eastern regions of South Africa as having the highest fisheries potential and estimated the total national potential yield for inland fisheries from all South Africa’s impoundments to be ~15 000 t/yr (WRC project K5/1954/4); a fraction of South Africa’s marine fishery that yields more than 600 000 t/yr. The relatively low production potential of South Africa’s inland water bodies precludes the development of industrial or large-scale commercial fisheries on inland waters, however, a combination of recreational, small-scale subsistence and artisanal fisheries for livelihood purposes was considered the optimal model to maximise socio-economic benefit of inland fisheries in South Africa. The Department of Agriculture, Forestry and Fisheries is currently developing an Inland Fisheries Policy. However, the consumption of fish harvested from contaminated inland waters could result in detrimental long-term health impacts. Flag Boshielo Dam on the main-stem of the Olifants River had been proposed as a suitable location for the development of a fishery and a WRC Project has commenced to determine the fishery potential of this impoundment (WRC project K5/2497/4). WRC project (K5/1929), conducted between 2009 and 2012, identified that the concentrations of selected metals in fish from impoundments in the Olifants River system exceeded safe consumption limits and were potentially detrimental to the health of communities who regularly consumed fish from these impoundments. The Department of Agriculture, Forestry and Fisheries heeded the warnings generated from this work and ensured that the draft Inland Fisheries Policy makes provision to “establish product quality and safety programmes for freshwater fisheries foods which conform, as far as possible, with relevant local standards and requirements and, as far as possible, with international standards or requirements.” However, understanding of the dynamics of metal accumulation in fish muscle tissue and the environmental processes that drive is insufficient to construct models to allow the fisheries managers to predict when the fish captured from an impoundment would be safe for human consumption. There is, therefore, a need to investigate the seasonal dynamics of metal concentrations in fish muscle tissue and to identify environmental drivers of the observed patterns. iii PREDICTING THE EDIBILITY OF FISH IN THE FLAG BOSHIELO SYSTEM OBJECTIVES The aim of this project was to evaluate seasonal fluctuations in the metal concentration in fish muscle tissue in Flag Boshielo Dam in order to establish guidelines for the safe consumption of fish from contaminated impoundments. The results of the project are expected to provide information to inform policy for the development of inland fisheries on South Africa’s water storage impoundments. The results of this project should directly inform the Inland fisheries Policy, which refers directly to the requirement that the fish products from inland fisheries should be monitored to determine whether they are suitable for human consumption. In order to achieve the stated aim of the project, the following objectives have been outlined for the project: • Monitor seasonal fluctuations in selected water quality parameters at Flag Boshielo Dam during drought and flood cycles (if possible, within the duration of the project) • Monitor seasonal fluctuations in the metal concentration in fish muscle at Flag Boshielo Dam • Investigate whether environmental and limnological factors can be identified as drivers of the concentration of metals in fish muscle tissue at Flag Boshielo Dam using linear regression and distance-based linear modelling, and • Use the findings of the study to inform the Inland Fisheries Policy regarding the safe utilisation of fish captured from Flag Boshielo Dam for human consumption. METHODOLOGY This project used a combination of analysis of historical data, two-and-a-half years of field data collection, and multivariate regression to evaluate seasonal fluctuations in the metal concentration in fish muscle tissue in Flag Boshielo Dam. Three fish species, Oreochromis mossambicus, Labeo rosae and Schilbe intermedius were chosen for the study due to their abundance in Flag Boshielo Dam and because these species occupy different niches within the system, with the first two species currently targeted by subsistence fishers. Historical data from the Department of Water and Sanitation’s Resource Quality Services for the Olifants and Elands rivers and Flag Boshielo Dam was compiled and analysed to establish whether patterns were evident. The dynamics of river flow and relationships between selected water physico-chemical parameters were explored. Eleven field surveys of Flag Boshielo Dam were conducted between February 2016 and April 2018. During each survey the water physico-chemical properties were measured and samples of the water, sediment and fish muscle tissue collected for metal content analysis at Waterlab, a SANAS accredited laboratory in Pretoria. A desktop Human Health Risk Assessment, based on US EPA methodology, was conducted for fish muscle tissue from Flag Boshielo Dam over the study period. iv PREDICTING THE EDIBILITY OF FISH IN THE FLAG BOSHIELO SYSTEM Finally, regression analyses were used to determine whether drivers of the metal concentrations in fish muscle tissue could be identified. A suite of environmental and water chemistry parameters was initially selected for the analyses but no clear set of drivers could be identified to predict the metal concentrations in fish muscle tissue. Therefore, a reduced set of ten “limnological drivers” that could be measured in the field and did not require chemical analysis were selected: physico-chemical parameters (water temperature (°C), pH, dissolved oxygen (DO) and electrical conductivity (EC)), lake limnological parameters (depth of thermocline and centre of buoyancy (CB)), river discharge (at the survey time and averaged over the month prior to the survey), dam level (change in level over the month proceeding the survey) and the number of days since the last flood flow of > 20 cumecs. Two approaches were used: Generalised linear models and Distance-based redundancy analysis. Generalised linear models were developed for a reduced set of eight metals, selected based on having the highest number of non-zero records: Al, Ba, Cu, Mn, Se, Sr, Ti and Zn. Generalised linear models were established for each of the eight metals for each study species. Multi-model inference (Burnham and Anderson, 2002) was used to identify the frequency of the variables in the top models, models that differed by less than 4 AIC points from the best model. Distance-based redundancy analysis (dbRDA) (Anderson et al., 2008) was conducted for the three fish species using a matrix of the 19 metal concentrations in the fish muscle tissue for each fish analysed for the long-term monitoring study as the independent variable and a matrix of the environmental parameters recorded during each field survey matched to the fish data as the explanatory variables. The dbRDA determined the environmental drivers that best describe the patterns in the independent variable matrix. The results were plotted and the importance of the variables was projected over the dbRDA plot. The analyses were conducted for both the environmental and limnological drivers. RESULTS The Olifants River system appears to cycle between dry periods of low flow and