Aquaculture practices in irrigation reservoirs of the Western Cape Province of South Africa in relation to multiple resource use and socio-ecological interaction

Salie, Khalid
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Stellenbosch : Stellenbosch University
ENGLISH ABSTRACT: Aquaculture has proven to be a viable operation in multi-used irrigation reservoirs (also referred to as farm dams) in the Western Cape province (WCP) of South Africa. Many studies found that the fitness-for-use of these reservoirs for both net cage culture of fish and irrigation of crops is feasible. However, practising intensive fish farming in existing open water bodies can increase the nutrient levels of the water through organic loading, originating from uneaten feeds and fish metabolic wastes. Under such conditions the primary (irrigation) and secondary (drinking water and recreation) usage of the dam could be compromised by deteriorating water quality. Rainbow trout (Oncorhynchus mykiss) farming is done in Mediterranean climatic conditions of the WCP. This type of climate presents short production seasons with fluctuating water quality and quantity. The study investigated the dynamics of water physico-chemical parameters and assessed the long term impact of rainbow trout farming on irrigation reservoirs. Furthermore, associated land-use in the catchment of such integrated aqua-agriculture systems is described, and mitigation to minimise the impact of fish farming evaluated. The investigation concluded with assessing the contribution of aquaculture to rural and peri-urban communities. The aim is to present an integrated, socio-ecologically balanced farming system for irrigation reservoirs with associated aquaculture activities. A total of 35 reservoirs, including both fish farming and non-fish farming ones, were selected as research sites. They were located in three geographical regions namely, Overberg (Grabouw/Caledon), Boland (Stellenbosch/Franschhoek) and Breede River (Ceres/Worcester). Reservoirs were <20 ha in surface area and the volume ranges from 300 000 to 1 500 000 m3. Water samples were collected monthly and seasonally for the different investigations and analysed for a range of water quality parameters, including: transparency (Secchi disc), temperature, dissolved oxygen (DO), pH, sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), chloride (Cl), carbonate (CO3), bicarbonate (HCO3), manganese (Mn), copper (Cu), zinc (Zn), boron (B), total phosphorous (TP), orthophosphate (PO4), total ammonia nitrogen (TAN), nitrate-nitrogen (NO3-N), nitrite-nitrogen (NO2-N), aluminium (Al), total suspended solids (TSS), total dissolved solids (TDS), alkalinity, hardness and sulphate. Phytoplankton samples were also collected, genera identified and biomass calculated. The water quality data were analysed in terms of surface and bottom strata in both fish farming and non-fish farming reservoirs based on repeated measurements at the same site location at different times using the procedure General Linear Models of Stastical Analysis System (SAS, 2012). Values p<0.05 were considered as statistically significant. A Principal Component Analysis (PCA) biplot was used to graphically depict all the sites and measured water quality variables with the purpose of trying to see whether the fish farming and non-fish farming ones showed any groupings and how the sites were related to the measured variables. Structured questionnaires and informal discussions were used to collect additional information on the water use, production data and socio-economic effects on fish farmers. Categorical data gathered from the interviews (21 aquaculture projects) were analysed for frequency of occurrence using the Statistical Product and Service Solutions (SPSS) computer programme (SPSS Systems for Windows, Version 12.0). Results are presented in publication form with research chapters focusing on the subject areas of water quality impact, catchment land-use, potential mitigation measures and aquaculture contribution. Results for the water quality analyses indicated that as a collective, the farm reservoirs’ overall minimum, mean and maximum values for the physico-chemical parameters were fit-for-use for trout farming. The depth of the reservoirs ranged from 1.2 - 21.6 m with the low value taken during the summer season. Values lower than 5.0 m can cause management problems for floating cages that require a minimum of 4.0 m for net suspension and 1.0 m of free space below for adequate lateral flow. The Secchi disc reading of the reservoirs ranged from 10 – 510 cm. Higher transparencies were recorded after the winter rains when sand, silt and clay settled. Trout feeding is dependent on visibility and transparencies of more than 50 cm are required for good feeding conditions. The dissolved oxygen (DO) ranged from 0.3 – 16.4 mg/L with values below 5.00 mg/L recorded during summer when extraction and temperatures were high and provided conditions unable to sustain trout farming. The situation reverses with the onset of winter when the dams fill and DO rises above 5.00 mg/L as required for trout farming. The phosphorous (P) levels ranged from 0.001 – 0.735 mg/L. Higher concentrations were recorded during the winter turnover phase when bottom and surface waters mixed. Concentration above 0.01 mg/L can cause eutrophication of the water bodies. Total ammonia nitrogen (TAN) ranged from 0.015 - 6.480 mg/L. Higher concentrations were recorded during summer when temperatures were high and depths were low. TAN can be toxic to fish when the pH and temperature are high. The generally low least square means (LSM) for TAN were indicative of minor environmental impact of trout farming operations conducted during the colder, winter rainfall months. Trout farming coincided with conditions where the water temperatures were low, dam levels were high and dams were overflowing. The difference in bottom and surface water quality of reservoirs and the site location were found to be more important than the absence or presence of fish farming. The difference in bottom and surface water is directly linked to the ecological status of the sediment, which serve as nutrient sinks. In monomictic dams found in Mediterranean areas, mixing occurs during the winter turnover phase. Nutrients are released due to surface and bottom water mixing, brought about by torrential rains and wind turbulence. The concentration of organic material in the sediment and bottom waters is a function of the nutrient loading over time, irrespective whether the non-point sources were fish farming or agricultural activities and therefore it is difficult to partition causes and effects. In cases where reservoirs were already eutrophic due to past agricultural practices, implementing aquaculture could exacerbate the poor water quality status of the reservoir. There was a statistically significant difference between fish farming and non-fish farming for phosphorous, Secchi disc, total suspended solids and nitrite-nitrogen (p<0.05) and no statistically significant difference between fish farming and non-fish farming for dissolved oxygen, total ammonia nitrogen and nitrate-nitrogen (p>0.05). There was a statistically significant difference between surface and bottom waters for P and TAN (p<0.05). One reason for higher P and TAN concentrations in bottom waters is the accumulation of both in the sediment and subsequent release in the water column when the water mixes. A two-dimensional scatter plot was generated using the score for the first two principal components. The first two principal components accounts for 40 and 17 % of the total variance respectively, and the two groups of fish farming and non-fish farming did not separate well based on the first two principal components. The occurrence and distribution of phytoplankton biomass fluctuated with dam water levels and nutrient concentrations. The prevailing phytoplankton communities are important to fish farmers for two reasons: 1. It leads to fluctuations in dissolved oxygen concentrations via users (respiration and decomposition) and producers (photosynthesis). 2. It could lead to algal taint of fish flesh when geosmin-producing phytoplankton species are present. The frequency of occurrence indicated that the Group Chlorophyta (including genera, Chlamydomonas, Closterium, Oocystis, Scenedesmus, Staurastrum, Tetraedron, etc) had the most occurrences (n=371) with Chrysophyta (including genera, Dinobryon, Mallomonas, Synura, etc) the least (n=34). There was a statistically significant difference between genera occurrence and season (p<0.05). The geographical location of sites had no significance influence on the frequency of phytoplankton occurrence. There was no direct link between water quality and production yield (p>0.05). The fish yield of farms were linked mainly to the quality of fingerlings and the feed conversion ratio (FCR) achieved (p<0.05). Land-use patterns in the catchment where fish farming dams were located have shown that the dams are multiple-used systems. The ecological integrity of the farm dam ecosystem is dependent on the base volume. The dam is primarily for irrigation and fish farming can be compromised when higher demand for water is required during the dry season. The dams receive about 20 % of its water from rainfall and the rest from runoffs. Farmers could not provide accurate extraction rates making it difficult to predict water levels for future fish production. Four potential mitigation measures to reduce nutrient loading were described namely, feed management (quantity, frequency, type, etc.), feeding method (demand feeders, hand feeding), feed ingredients (formulation) and floating gardens. Both feed management procedures and demand feeders were evaluated as to the efficiency of reducing feed wastage and optimising FCR’s. The small-scale fish farmers were producing approximately 6 tons and had an average FCR of 1.96:1 ± 1.15. If farmers could improve their FCR’s by 0.1 (i.e. from 1.96 to 1.86), it would translate into a reduction of 100 kg feed for every ton of fish produced and result in 5% decrease in nutrient loading. The results of the water analysis and visual assessment of faecal length and colour showed no statistically significant difference between treatments for the guar-gum based binder (p>0.05). In addition, the level of binder did not influence digestibility of the experimental diets. The floating garden study indicated that it was feasible to construct a low cost raft system that is easy to manage and can produce plant crops as a hydroponic system in conjunction with fish farming cages. The lettuces grown on farm dam water provided support for the premise that the water quality can be improved via extraction of nutrients for crop production. For the production of 3.5 kg/m2 lettuce, a ratio of 1.09 plants/fish equal to 1.84 g feed/day/plant would reduce the accumulation of soluble nutrients around floating net cage farming system. The socio-economic evaluation of the contribution of fish farming to the welfare of rural and peri-urban farming communities supported the notion that aquaculture can lead to the upliftment of participating communities. Seventy-one percent (71%) of the respondents indicated that their motivation for exploring aquaculture is to supply fish to the wholesale market in order to generate income. Sixty-one percent (61%) of the respondents conducted the sales themselves or co-opted family members to assist them. The contribution of aquaculture provided direct benefits through improvement in household income, subsistence food supply and skills development. Indirect benefits included providing an information hub for other emerging farmers, elevation of the fish farmer’s status in the community through greater wealth and knowledge creation and promoting sector diversification through new products and technology. The three main constraints to the promotion and growth of aquaculture were listed as lack of government support, insufficient market intelligence and access, and limited choice in the availability of suitable candidate aquaculture species. Irrigation reservoirs in the WCP have a history of enrichment through external sources supplying water via agricultural runoff (fertilisers and pesticides), catchment runoff (leaf litter and organic debris) and stormwater effluent (grey and black water). The incorporation of aquaculture into such dams adds extra nutrients to the water column and management is crucial to limit the nutrient loading and ensure environmental sustainability. Such an approach will ensure that commercial land-based crop farmers’ irrigation regime and water distribution operations would not be negatively affected. Therefore future research needs should focus on; firstly the prevention and minimisation of pollution deriving from aquaculture through improved production management and technology transfer, secondly the monitoring and evaluation of the catchment ecosystem as a continuum with all the external factors affecting the ecology of farm dams and thirdly, evaluating the sediment processes and dynamics as sinks for nutrient accumulation.
AFRIKAANSE OPSOMMING: Akwakultuur het getoon dat dit ‘n lewensvatbare inisiatief is vir meerdoelige-gebruik van besproeiingsdamme (ook genoem plaasdamme) in die Wes-Kaap provinsie (WKP) van Suid-Afrika. Vele studies het bewys dat die geskiktheid-vir-gebruik van die reservoirs haalbaar is vir beide visproduksie sowel as besproeiing van landbougewasse. Nieteenstaande, die beoefening van intensiewe visboerdery in bestaande buitelug watersisteme kan lei tot ‘n toename in nutriëntvlakke van die water as gevolg van organiese belading afkomstig van ongevrete voere en metaboliese afvalstowwe van die vis. Onder sulke omstandigthede kan die primêre- (besproeiing) en die sekondêre (drinkwater en rekreasie) gebruik van die dam in gedrang kom weens ‘n afname in waterkwaliteit. Reënboogforel (Oncorhynchus mykiss) boerdery word beoefen in die omliggende Mediterreense klimaat van die WKP. Die tipe klimaat verskaf kort produksie-seisoene met wisselvallige water kwaliteit en kwantiteit. Die studie het die dinamika van water se fisies-chemiese parameters ondersoek en het die impak van forelboerdery op besproeiingdamme oor die langtermyn beskryf. Verder het die studie die geassosieerde landgebruik in die opvangsgebied met geïntegreerde akwa-landbou sisteme beskryf, asook moontlike toetrede (mitigasie maatreëls) geëvalueer wat die impak moontlik kan verlaag. Die ondersoek is afgesluit deur die bydrae wat akwakultuur lewer aan landelike en semi-stedelike gebiede, te beskryf. Die hoofdoel is die daarstelling van ‘n geïntegreerde, sosio-ekologiese gebalanseerde sisteem vir besproeiingdamme met gesamentlike akwakultuuraktiwiteite. ‘n Totaal van 35 besproeiingsdamme, insluitend die met visboerdery en nie-visboerdery, is gekies as navorsingspersele. Dit is hoofsaaklik geleë in drie geografiese gebiede naamlik, Overberg (Grabouw/Caledon), Boland (Stellenbosch/Franschhoek) en Breederivier (Ceres/Worcester). Die reservoirs is almal < 20 ha in oppervlakarea en die volumes het gewissel van 300 000 – 1 500 000 m3. Watermonsters is maandeliks sowel as seisoenaal versamel vir die onderskeie ondersoeke en ontleed vir ‘n reeks van parameters, insluitend: sigbaarheid (Secchi disc), temperatuur, opgeloste suurstof (OS), pH, natrium (Na), kalium (K), kalsium (Ca), magnesium (Mg), yster (Fe), chloor (Cl), karbonaat (CO3), bikarbonaat (HCO3), mangaan (Mn), koper (Cu), sink (Zn), boor (B), totale fosfor (TP), ortofosfaat (PO4), totale ammoniak stikstof (TAN), nitraat-stikstof (NO3-N), nitriet-stikstof (NO2-N), aluminium (Al), totale gesuspendeerde vaste stowwe (TGV), totale opgeloste vaste stowwe (TOV), alkaliniteit, hardheid en sulfate. Phytoplanktonmonsters is ook versamel, genera geïdentifiseer en die biomassa bepaal. Die waterkwaliteitsdata is ontleed in terme van oppervlak- en bodemstrata vir beide visboerdery en nie-visboerdery reservoirs en was gebaseer op herhaalde metings by dieselfde perseel op verskillende tye deur gebruik te maak van die Algemene Liniêre Model van Statistiese Analitiese Sisteem (SAS, 2012). Waardes p<0.05 is oorweeg as statisties beduidend. ‘n Hoofkomponentanalise bi-stipping (HKA) is toegepas om die persele en veranderlikes grafies voor te stel en te bepaal of die visboerdery en nie-visboerdery s’n enige groeperinge vorm asook hoe die persele assosieer met die veranderlikes. Gestruktureerde vraelyste en informele besprekings is onderneem om inligting in te samel op watergebruik, produksie-data, en die sosio-ekonomiese invloed wat akwakultuur bied aan visboere. Kategoriese data wat deur die onderhoude (21 akwakultuurprojekte) ingesamel is, is ontleed vir die frekwensie van aanwesigheid deur die gebruik van Statistiese Produk en Dienste-oplossings (SPDO) rekenaarprogram (SPSS Systems for Windows, Version 12.0). Die resultate vir die verskeie ondersoeke is beskryf en saamgestel in publikasie-vorm met die navorsingshoofstukke wat gefokus het op die areas van waterkwaliteitsimpak, opvangsgebied landgebruik, toetrede-meganismes en die bydrae van akwakultuur. Die resultate vir die waterkwaliteitsanalises het getoon dat gesamentlik die reservoirs se oorhoofse minimum, gemiddelde en maksimum waardes vir die verskillende fisies-chemiese parameters geskik is vir forelboerdery. Die diepte van die reservoirs het gewissel van 1.2 - 21.6 m, met die laagste waarde aangeteken gedurende die somermaande. Waardes laer as 5.0 m kan bestuursprobleme vir drywende hokstelsels versoorsaak want ‘n minimum van tenminste 4.0 m vrye spasie onder die hokke word benodig vir voldoende laterale vloei. Die Secchi-skyf lesing (sigbaarheid) van die reservoirs het gewissel van 10 – 510 cm. Hoër sigbaarheidswaardes is aangeteken na winterreëns wanneer sand-, slik- en klei deeltjies uitgesak het. Forel voer op sig en sigbaarheid van > 50 cm word benodig om goeie voeding te handhaaf. Die OS het gewissel van 0.3 – 16.4 mg/L met waardes onder 5 mg/L aangeteken gedurende somer wanneer wateronttrekking en temperature hoog was. Dit het gelei tot ongunstige toestande vir forelboerdery. Die situasie swaai om met die begin van winter wanneer die damme vol reën en die OS bo 5 mg/L styg soos benodig vir forelboerdery. Die P-vlakke het gewissel van 0.001 – 0.735 mg/L. Hoër waardes is aangeteken gedurende die winteromkeerfase wanneer die bodem en oppervlak se water meng. Konsentrasies bo 0.01 mg/L kan tot eutrofikasie van watersisteme lei. TAS het gewissel van 0-015 – 6.480 mg/L. Hoër konsentrasies is aangeteken gedurende die somer wanneer temperature hoog is en damvlakke laag. By hoë pH’s en temperature kan TAS toksies wees vir vis. The algemene lae kleinste kwadaat gemiddelde (KKG) waarde vir TAS het getoon dat daar ‘n klein impak op die omgewing was wanneer forelboerdery bedryf word gedurende die koue, winter reënvalmaande. Forelboerdery val saam met omstandigthede wanneer die watertemperature laag is, damvlakke hoog en die reservoirs oorloop. Die verskil in die bodem- en oppervlak water in die besproeiingsdamme en die ligging van die perseel is vasgestel om meer belangrik te wees as die teenwoordigheid of afwesigheid van visboerdery. Die verskil in die bodem en oppervlak is toe te skryf aan die toestand van die sediment waar nutriënte kan opgaar. In monomiktiese damme soos gevind in Mediterreende areas, vind vermenging plaas gedurende die winteromkeerfase. Nutriënte word vrygestel a.g.v. die vermenging van die oppervlak en bodem se water wat dan veroorsaak word deur harde reën en windturbulensie. Die konsentrasie van organiese materiaal in die sediment en bodem water is ‘n funksie van die nutriëntlading met tyd, ongeag of dit afkomstig was vanaf visboerdery of landbou-aktiwiteite. Dit is dus moelik om die spesifieke oorsaak van besoedeling af te baken. In gevalle waar die reservoirs alreeds eutrofies is a.g.v. aangewese landbouaktiwiteite, kan die toestand van die waterbron vererger indien akwakultuur toegepas word. Daar is ‘n statistiese noemenswaardige verskil tussen visboerdery en nie-visboerdery vir P, Secchi-skyf, totale gesuspendeerde vaste stowwe en nitrite-stikstof (p<0.05), en geen statistiese noemenswaardige verskil tussen visboerdery en nie-visboerdery vir OS, TAS en nitraat-stikstof (p>0.05). Daar is ‘n statistiese noemenswaardige verskil tussen oppervlak- en bodem water vir P en TAS (p<0.05). Een moontlike rede vir hoër P en TAS konsentrasies in die bodemwater, is die akkumulasie van beide parameters in die sediment en gevolglike vrystelling in die waterkolom wanneer die water gemeng word. ‘n Twee dimensionele spreidingstipping is geprodueer deur die waardes te gebruik van die eerste twee hoofkomponente. Die eerste twee hoofkomponente dra by 40 % en 17 % van die totale variansie onderskeidelik, en die twee groepering van visboerdery en nie-visboerdery het nie duidelik getoon nie. Die voorkoms en verspreiding van phytoplankton biomassa het gewissel met die verandering in damvlakke en nutriëntkonsentrasies. Die aanwesige phytoplanktongemeenskappe is belangrik vir die visboer vir twee redes: 1. Dit kan wisselende OS-vlakke versoorsaak deur die verbruik (respirasie en dekomposisie) en produksie (fotosintese) daarvan. 2. Dit kan lei tot alge na-smake van vis wanneer geosmin-produserende phytoplankton spesies aanwesig is. The frekwensie van voorkoms het getoon dat die Groep Chlorophyta (insluitend die genera, Chlamydomonas, Closterium, Oocystis, Scenedesmus, Staurastrum, Tetraedron, ens.) die meeste voorkom (n=371), met Chrysophyta (insluitend die genera, Dinobryon, Mallomonas, Synura, ens.) die minste (n=34). Daar is ‘n statistiese noemenswaardige verskil tussen genera voorkoms en seisoen (p<0.05) vir phytoplankton. Die geografiese ligging van die perseel het geen noemenswaardige invloed op die frekwensie van phytoplankton voorkoms nie. Daar is geen statistiese noemenswaardige verbintenis tussen waterkwaliteit en visproduksieopbrengste nie (p>0.05). Die visopbrengste by plase is hofsaaklik afhangende van die kwaliteit van die vingerlinge en die voeromsettingsverhouding (VOV) wat bereik is (p<0.05). Die landgebruikspatrone in die opvangsgebied waar visboere gesetel is, het aangedui dat die besproeiingsdamme meeldoelige sisteme is. Die ekologiese integriteit van die plaasdam-ekosisteem is afhanklik van die basisvolume. Die dam is hoofsaaklik daar vir die besproeiing en visboerdery kan in gedrang kom wanneer daar ‘n hoër aanvraag vir water gedurende die droë seisoen is. Die damme het omtrent 20 % van die water vanaf reënval ontvang en die res van aflope. Boere kon nie akkurate inligting verskaf van waterontrekking nie wat dit moeilik gemaak het om te voorspel wat die beskikbare watervlakke in die toekoms sou wees vir visproduksie. Vier potensiële toetrede meganismes om die nutriëntlading te verminder, is beskryf naamlik voedingsbestuur, (kwantiteit, frekwensie, tipe, ens.) voermetodes (aanvraagvoeder, handvoeding), voerbestandele (formulasies) en drywende tuine. Beide voedingsbestuur prosedure en aanvraagvoeders is geëvalueer as ‘n metode om die voervermorsing te verminder en die VOV te verbeter. Die kleinskaalse visboere het ongeveer 6 ton produseer met ‘n gemiddelde VOV van 1.96:1 ± 1.15. Indien die visboere hul VOV’s met 0.1 kan verbeter (bv. van 1.96 tot 1.86), sal dit beteken dat daar ‘n vermindering van 100 kg voer bewerkstellig word vir elke ton vis geproduseer. Dit kan ook lei tot ‘n vermindering van 5 % in die nutriëntlading. Die resultate van die wateranalises en die visuele waarneming van faeceslengte en kleur het geen statistiese noemenswaardige verskil tussen die behandelinge vir die guar-gom binder getoon nie (p>0.05). Verder, die hoeveelheid van die binder het nie die vertering van die eksperimentele diëte beïnvloed nie. Die studie op die drywende tuine het getoon dat dit haalbaar is om ‘n lae-koste sisteem te bou wat maklik is om te bestuur en gewasse kan produseer soos in ‘n hidroponiese sisteem tesame met visproduserende hokstelsels. Die kropslaaie se groei het getoon dat die waterkwaliteit van besproeiingsdamme kan verbeter word deur die opname van nutriënte wanneer plante verbou word. Vir die produksie van 3.5 kg/m2 kropslaaie, sal ‘n verhouding van 1.09 plante/vis of 1.84 g voer/dag/plant die akkumulasie van opgeloste nutriënte rondom die hokstelsels verminder. Die sosio-ekonomiese evaluasie van die bydrae van visboerdery tot die welvaart van die landelike en semistedelike plaasgemeenskappe ondersteun die feit dat akwakultuur verbetering kan bewerkstellig, veral onder deelnemende gemeenskappe. Een-en sewentig persent (71 %) van die respondente het getoon dat hul oorweging vir die bedryf van akwakultuur is om vis te voorsien aan die grootmark en daarvolgens geld te maak. Een-en-sestig persent (61 %) van die respondente het aangedui dat hulself die vis verkoop of vir familie-lede vra om met die verkope te help. Die bydrae van akwakultuur het direkte voordele aan die deelmers voorsien deur ‘n verbetering in huishoudelike inkomste, voedselvoorsiening vir selfgebruik en die ontwikkeling van vaardigthede. Indirekte voordele sluit in dat die deelmers ‘n bron van inligting geword het vir opkomende boere, hul status in die gemeenskap het verbeter omdat hul kennis verbreed het en dit het verder gelei tot diversifisering in die sektor a.g.v. die skepping van nuwe produkte en tegnologie. Die drie hoof struikelblokke wat die groei en bevordering van akwakultuur belemmer is o.a., ‘n tekort aan staatsondersteuning, onvoldoende markinligting en toegang en ‘n beperkte keuse in spesies vir boerdery. Besproeiingsdamme in die WKP het ‘n geskiedenis van verryking deur eksterne bronne wat water voorsien vanaf landbou-afloop (bemestingstowwe en pesbestrydingsmiddels), opvangsgebied-afloop (blare en ander organiese debris) en stormwateruitlaat (gruis- en swart water). Die implementering van akwakultuur in sulke damme voeg addisionele nutriënte tot die waterkolom en bestuur is krities om die lading te verminder en te verseker dat omgewingsvolhoubaarheid behou word. Indien die regte praktyke en bestuur toegepas word, sal dit beteken dat die kommersiële boer se besproeiing en waterverspreiding nie negatief beïnvloed word nie. Vervolgens moet toekomstige navosingsbehoeftes fokus op eerstens, die voorkoming en vemindering van besoedeling afkomstig van akwakultuur deur verbeterde produksie-bestuur en tegnologie-oordrag, tweedens, die monitoring en evaluering van die opvangs-ekosisteem as ‘n kontinuum met al die eksterne faktore wat die ekologie van die plaasdam kan beïnvloed en laastens, die ondersoek en evaluering van die sediment se prosesse en dinamika as ‘n sisteem wat nutriënte ophoop.
Thesis (PhD)--Stellenbosch University, 2014.
Aquaculture -- Environmental aspects, Aquaculture -- Irrigation reservoirs, Aquaculture -- Western Cape Province -- South Africa, UCTD, Theses -- Agriculture, Dissertations -- Agriculture