An assessment of fish and fisheries in impoundments in the upper Orange-Senqu River Basin and Lower Vaal River Basin

[摘要] South Africa is an arid country that, as a result of the need to store water for domestic,industrial and agricultural use, has invested heavily in the construction of impoundmentswhich now cover a total of approximately 3 000 km2. Faced with high levels of poverty andunemployment, South Africa is increasingly considering developing fisheries in freshwaterimpoundments to provide economic opportunities and food security in rural areas. To beeffective, such development needs to be guided by information on current utilisation, fishspecies composition and abundance, as well as the impact of potential harvest methods.Unfortunately, inland fisheries have received limited interest in a country with large marinefisheries and a history of failures in developing inland commercial fisheries. As a result, there is a general lack of even basic information on fish communities in impoundments and theirutilisation. This thesis attempts to address this paucity of information in the Free StateProvince (FSP) by conducting a rapid appraisal of the fisheries potential of impoundmentsusing empirical approaches; collating a 35 year time series of catch returns from recreationalangling tournaments and commercial fisheries; conducting surveys to determine fish species composition in 21 impoundments and testing a new fishing gear.The FSP has the largest inland water surface area (145 677 ha) in South Africa and is situatedcentrally within the Orange-Senqu River Basin that is drained by the Orange-Senqu and VaalRiver Systems. As a result of complete lack of data on inland fisheries in the FSP, a scoringsystem (based on empirical estimations of potential fish yield and water level fluctuations)was developed to provide information on the potential of impoundments for the developmentof commercial fisheries. Applying different models to Morphoedaphic index (MEI) dataprovided the first estimates of yields ranging from very conservative to highly opportunistic(e.g. for Gariep Dam from 5.0 to 58.5 kg ha-1 y-1). As predicted by most MEI-based models,impoundments situated at high altitudes with large average depth, with least fluctuation inwater levels, had the least potential for the establishment of fisheries.Data on commercial fisheries were collated from various field stations and digitised. Thisprovided the first complete database of commercial fisheries yields in South Africa. Between1979 and 2014, a total of 9 036 tons of fish were harvested by commercial fishery enterprisesoperating sporadically at seven impoundments. Commercial fisheries yields were dominatedby Bloemhof Dam (73%) and Kalkfontein Dam (23%) with sporadic attempts to developcommercial fisheries in the five other impoundments, contributing only 4% to the total yieldover the 35 year period. The gear used to harvest fish determined which species were caught,with the indigenous cyprinids, namely Orange River mudfish Labeo capensis, moggel Labeoumbratus and smallmouth yellowfish Labeobarbus aeneus dominating catches when gill netswere used, while common carp Cyprinus carpio dominated when seine nets were used. Onlytwo commercial ventures operated at one impoundment (Bloemhof Dam) on a continuousbasis for more than 32 years. Prior knowledge, skills and experience and most importantly apre-existing and self initiated market were identified as determinants of success.Data from license sales demonstrate that the recreational fishery sector in the FSP comprises7 710 licensed recreational anglers. Of these, 748 are affiliated with 30 angling clubs. Ananalysis of historical data indicated that there has been a decrease of 76% in the number ofrecreational anglers since 1971 (based on license sales). Over the period 1974 to 2014available data show that at least 4 817 angling tournaments were held at 17 impoundments.Historical analysis of participation indicates a decline in angling tournament participation.This was in support of the trends observed using license sales data. The total recorded catchfor these tournaments was 414 tons. The recreational fishery was dominated by C. carpio thatcontributed 81% to the total weight and 77% of the total number of fish landed.Experimental fishing surveys conducted during this study, sampled 23 831 fish from the 21impoundments surveyed. Sixteen species were sampled, with the bulk of the catchesconsisting of four large cyprinid species (L. capensis, L. umbratus, L. aeneus and C. carpio),and the sharptooth catfish Clarias gariepinus. On average, 7.6 ± 1.4 and 6.0 ± 1.1 specieswere sampled per impoundment in the Vaal River and Orange-Senqu River Systemsrespectively. Fish assemblages in different impoundments were similar (average Jaccard'sIndex of Similarity of 64.2%). Examination of population structure (using length frequencies) indicated that most species sampled were established with evidence of several year classes ofadults as well as young of year.An assessment of fyke nets as a potential harvesting gear demonstrated that this gear mightbe a suitable passive gear for small-scale fisheries. Eleven fish species were caught with fykenets, with catches dominated by four large cyprinid species (L. capensis, L. umbratus, L.aeneus and C. carpio), and C. gariepinus of which most are important angling species, exceptL. umbratus. The preliminary assessment of the suitability of fyke nets as gear for smallscalefisheries has revealed a number of research questions and once these have beenaddressed, fyke nets might become the gear of choice in future inland fisheries developmentin central South Africa.With the new interest in the development and promotion of inland fisheries in South Africa toaddress government's policy objectives of job creation and poverty alleviation, this thesiswill contribute to the knowledge base, as for the first time in South Africa such acomprehensive study on an assessment of fish and fisheries within impoundments has beendone. This thesis will provide baseline information which may support and inform the policydevelopment process for inland fisheries in South Africa that was initiated during February2015 by the National Department of Agriculture, Forestry and Fisheries.

[发布机构] University of the Free State