Effects of rainbow trout (Oncorhynchus mykiss) cage culture on Western Cape irrigation reservoirs.

Maleri, Monika (2011-03)

Thesis (PhD (Conservation Ecology and Entomology))--University of Stellenbosch, 2011.


ENGLISH ABSTRACT: Water storage in reservoirs forms an integral part of the agricultural landscape in the Western Cape Province, South Africa. A few large reservoirs serve primarily as a drinking and industrial water supply, while on private farms, small reservoirs provide irrigation water for the dry summer period. Protection of water quality to secure irrigation and drinking water quality, and the extension of water use efficiency are priority issues in the Western Cape. In the current study, the suitability of rainbow trout (Oncorhynchus mykiss) cage farming as a non-abstractive water use was investigated. The current study concentrated on the identification, and where possible quantification of aquaculture impacts, the identification of successful sites and a description of requirements in which net-cage aquaculture has none or a very low negative impact on water quality (e.g. <15 % change from previous water quality conditions for phosphorus concentrations). In order to study the effects of 5 t trout cage production units in reservoirs <15 ha in area, the general ecology of the irrigation reservoirs was assessed. Sixteen reservoirs without aquaculture production (reference reservoirs) were compared to 26 reservoirs with aquaculture production (production sites with varying production histories). Catchment characteristics were also monitored. Water from different depths (0 m, 2 m, 6 m and near bottom) was tested for physical and chemical qualities as well as nutrient concentrations. Phytoplankton and zooplankton biomass and species composition was monitored. In addition to the general phytoplankton findings, cyanophytes were evaluated for their contribution to algal taint problems that emerged at a number of production sites. Sediments were tested for total phosphorus content and phosphorus release capacity. Indicators and minimum conditions to avoid the most common production problems were formulated. In order to determine long-term production success, which prevents trophic level changes of reservoirs, a mass balance approach (nutrient budget) was employed to indicate the limits for nutrients that can be added. The phosphorus balance indicated long term trends for reservoirs with and without aquaculture. The detailed mass balance approach was compared to a “ready to use” carrying capacity model that estimated the maximum fish load each reservoir could support. The anthropological input of phosphorus into the reservoirs causes a decreasing water quality in the studied reservoirs and this development was also reported for lowland rivers. Twenty percent of the studied reservoirs are in a condition that could be an immediate threat to fish or water bird health (e.g. free ammonia concentrations and pH). Harmful algal blooms were not observed. Aquaculture production evoked changes in water chemistry and ecology in most of the studied reservoirs. Adverse effects of aquaculture sites versus non-aquaculture sites were: increased phytoplankton biomass and species shifts towards sizes >80 μm. The increased phytoplankton abundance influenced pH maxima to values >9 at mid-day. The high pH fluctuations were greatly influenced by the naturally low alkalinity and hence low calcium buffering capacity of Western Cape waters. The deoxygenation of the hypolimnion during stagnation (summer) occurred faster in reservoirs of certain character, greatly dependent on elevation and surface area, with consequent acidification of the hypolimnion, as well as ammonia and total phosphorus (TP) accumulation. In this context, a diversity of each reservoir with and without aquaculture production, with a similar ratio of undisturbed reservoirs to reservoirs with influence of e.g. agriculture, were compared to each other. When grouping the respective differences from the average reference reservoir (i.e. no trout production), a low impact on water quality was manifested at four sites (15 %) with <15 % increase of bottom TP and ammonia, while eight sites (31 %) showed medium effects (59 % mean increase), and a high impact was found at 54 % of the sites (312 % mean increase). In reservoirs without aquaculture, the extent of incoming phosphorus (which could represent an influence by runoff from agricultural land) was very high. However, in small reservoirs (<5 ha), these values were exceeded by the incoming phosphorus from aquaculture practices. In the case of small reservoirs where the carrying capacity was clearly surpassed, effects caused by aquaculture were severe and the assimilation of waste by the system was not possible (in extreme cases aquaculture waste delivered 60 to 90 % of all incoming phosphorus – two to nine times the phosphorus brought in by rivers and runoff). With regards to sediment, only indirect conclusions could be drawn. Aquaculture production increased hypolimnetic anoxia and the latter was shown to increase potential phosphorus release from sediments. This implies that not only will aquaculture increase the phosphorus concentration of surface waters directly, but it will also increase internal loading. The sedimentation rate was increased with cage aquaculture which affects a hypothesized area of approximately 0.2 to 1.0 ha depending on reservoir hydrology. The composition of the sediment increased organic components which can impact on sediment processes. It can be postulated that increased sedimentation of aquaculture waste and extended anoxic conditions impact on macrozoobenthos. Hydrological and nutrient mass exchange of the reservoirs indicated that no annual increase of phosphorus was achieved with low nutrient input (good inflowing water quality) or good water exchange (>5 times per year), and sometimes with extraction of hypolimnetic water during the stagnation period (summer). A model developed by Beveridge (1984) showed similar results to the mass balance approach and can therefore serve as a more ready model to determine suitable stocking rates. The small (man-made) reservoirs in the Western Cape are in a eutrophication process which far exceeds the speed of natural eutrophication (trophic states indicating highly eutrophic or hypertrophic conditions after approximately 10 to 20 years following construction of the reservoirs) and this process is triggered by agricultural practice (indicated by literature – not a subject of this study). However, it is concluded from the data of this study that trout-cage aquaculture duplicated the total phosphorus already present (independent of continuation of the projects, the phosphorus introduced was trapped in the closed systems the reservoirs represent) in only 1 to 2 years of production - which means a significant acceleration of the eutrophication process already in place. There are positive exceptions where trout-cage production is possible without negative effects. Careful site selection is the most important step in successful and sustainable trout production. No impact of aquaculture was recorded at four reservoirs (15 % of the investigated reservoirs) which shared the characteristics of good water exchange (>3 times per year) and a minimum surface area of 5 ha. Additionally, criteria that reduced the risk of algal taint included a minimum water depth of 6 to 7 m in a reservoir at its lowest water point (to avoid intermediate mixing during the stagnation period) as well as cold hypolimnetic conditions (<17 °C) to minimize cyanophyte cyst remobilization. Further improvement of food conversion ratio (feeding management) and feed quality are the next (after site selection) two most important components that determine if a reservoir can be utilised for cage production without any long-term changes. There is potential in advances in feed quality, feed management and waste collection systems. These measures (e.g. the cage size could be decreased to efficate feeding management) can increase the number of sustainable sites and achieve multiplication of water use without water quality deterioration.

AFRIKAANSE OPSOMMING: Die stoor van water in reservoirs vorm ‘n integrale deel van die landboulandskap in die Westelike Kaap gedeelte van Suid-Afrika. ‘n Aantal groter reservoirs voorsien primêr water primer vir drinkdoeleindes en vir aanwending deur industrieë, terwyl kleiner reservoirs op private plase gebruik word vir besproeiing gedurende die droë somerperiode. Die beskerming van die waterkwaliteit en die uitbreiding van watergebruik se doeltreffendheid, word bestempel as ‘n prioriteit vir die Wes-Kaap. In die huidige studie is die geskiktheid van forel hokkultuur as ‘n bykomende watergebruik, maar sonder waterverbruik, ondersoek. Die huidige studie fokus op die identifisering en waar moontlik die kwantifisering van die impak van akwakultuur op besproeiingsreservoirs; die identifisering van potensiële reservoirs vir akwakultuur; en die beskrywing van toestande waarin akwakultuur geen of ‘n baie lae negatiewe uitwerking op die waterkwaliteit (bv. < 15 % verandering vanaf oorspronklike waterkwaliteitstoestande vir fosforkonsentrasies) sal hê. Om die impak van 5 t forel hokproduksie eenhede in <15 ha reservoirs te bestudeer, is die algemene ekologie van die besproeiingsreservoirs bepaal. Watermonsters, geneem op verskillende dieptes (0, 2, 6 en naby die bodem), is getoets vir fisiese- en chemiese kwaliteit sowel as vir die nutriëntkonsentrasies. Die biomassa en spesiesamestelling van fitoplankton sowel as zooplankton is gemonitor. Sestien verwysingsreservoirs is vergelyk met 26 produksiereservoirs (met wisselende vlakke van produksiegeskiedenis). Die karakeristieke eienskappe van die opvangsgebied is ook gemonitor. Tesame met die algemene fitoplankton bevindinge, is blougroen alge ook geëvalueer vir hul bydrae tot algae besmettingsprobleme wat voorgekom het by ‘n aantal produksie persele. Sedimente is getoets vir die totale fosforinhoud en fosforvrystellingskapasiteit. Perseelseleksie is geidentifiseer as een van die mees belangrikste faktore vir suksesvolle hokproduksie. Indikatore en minimum toetstande wat vereis word om die mees algemene produksieprobleme te verhoed, is geformuleer. Om die langtermyn produksie sukses te bepaal, wat trophiese vlakke se verandering in die reservoirs verhoed, is ‘n massa balans benadering (nutriënt balans) gebruik. Die fosfor balans het langtermyn tendense aangetoon en in die geval van die produksie reservoirs, is die akwakultuurimpak gekwantifiseer. Die gedetaileerde massa balans benadering is vergelyk met ‘n “gereed om te gebruik” drakapasiteitsmodel wat die maksimum vis wat die reservoir kan onderhou, geskat het. Die antropogeniese toevoer van fosfor na die reservoirs veroorsaak ‘n afname in die waterkwaliteit van die reservoirs wat bestudeer is. Die verskynsel van ‘n afname in waterkwaliteit is ook vir verskeie laagliggende riviere geraporteer. Sowat 20 % van die besproeiingsreservoirs wat bestudeer is, is tans in ‘n toestand wat die onmiddelike gesondheid van vis en watervoëls kan bedreig. Skadelike algebbloei is nie waargeneem nie. Akwakultuurproduksie het negatiewe effekte getoon in ‘n aantal reservoirs. Die nadelige uitwerking van akwakultuur waar digthede te hoog was, is: toename in fitoplankton biomassa en spesiesverskuiwinge. Die toename in fitoplankton teenwoordigheid het die pH maxima beïnvloed tot waardes >9 teen 12 uur middag. Die hoë pH fluktuasies is grootliks beïnvloed deur die natuurlike lae alkaliniteit en gevolglike kalsium bufferkapasitiet van die Wes-Kaap se waterbronne. Die deoksiginasie van die hipolimnion gedurende stagnasie (somer) het vinniger plaasgevind in oorbelaaide reservoirs, met gevolglik ‘n versuring van die hipolymnion, sowel as die akkumelering van ammoniak en totale fosfor. In hierdie konteks word reservoirs met en sonder landbouproduksie, met ‘n gelyke verhouding van onversteurde reservoirs tot reservoirs wat deur landboubedrywighede beinvloed word, met mekaar vergelyk. By vier persele (15 %) is ‘n lae impak vasgestel (<15 % toename in bodem TF en ammoniak), terwyl agt persele (31 %) ‘n medium impak getoon het (59 % gemiddelde toename in bodem TF en ammoniak) en ‘n hoë impak is opgemerk by 54 % van die persele (31 % gemiddelde toename in bodem TF en ammoniak). In reservoirs sonder akwakultuur, was die omvang van fosforinvloeiing baie hoog en is moontlik veroorsaak deur die invloei van afloopwater uit omliggende landbougebiede. Alhoewel in klein reservoirs (<5 ha) was hierdie waardes oorskadu deur die invloeiing van fosfor deur akwakultuur praktyke.In die geval van klein reservoirs, waar die drakrag duidelik oorskry is, was die gevolge, soos veroorsaak deur akwakultuur ernstig, en die assimilasie van afval deur die sisteem nie moontlik nie. In die uiterste gevalle het akwakultuurafval 60 % - 90 % van alle inkomende fosfor gelewer - twee tot nege maal die fosfor wat deur riviere en afloopwater ingevloei het. Wat die sediment aanbetref, kon slegs indirekte gevolgtrekkings gevorm word. Akwakultuurproduksie het hipolimnetiese anoksia laat toeneem en die laasgenoemde verskynsel, het die potensiaal aangedui vir die toename in die vrystelling van fosfor vanaf die sediment. Dit dui daarop dat akwakultuur nie alleen die fosforkonsentrasie in die oppervlaktwater laat toeneem nie, maar sou ook die interne belading laat kon toeneem. Die sedimentasie tempo het toegeneem met die teenwoordigkeid hokkultuur en het ‘n hipotetiese area van ongeveer 0.05 tot 1.00 ha, afhangende van reservoir hidrologie, beïnvloed. Die samestelling van die sediment het toegeneem in teme van die organiese komponente wat die sedimentasie prosesse kon beïnvloed. Dit kan gepostuleer word dat die toename in sedimentasie van akwakultuurafvalprodukte tesame met verlengde anoksiese toestande, ‘n invloed op die makrosoöbentiese organismes het. Hidrologiese en nutriënt massa uitruiling van die reservoirs het aangetoon dat geen jaarlikse toename in fosfor verkry kan word met lae nutrient toelae (kwaliteit van invloeiende water) of met goeie waterverplasing nie, en soms met die ekstraksie van hipoliminetiese water gedurende die stagnasie periode (somer). Die Beveridge model het soortgelyke resultate getoon tot die massabalans benadering en kan daarom dien as ‘n meer aanvaarbare model om gepaste beladingstempo vas te stel. Kleiner mensgemaakte reservoirs in die Wes-Kaap is onderhewig aan ‘n eutrofikasie proses wat die spoed van natuurlike eutrofikasie (trofies verwys na ‘n hoogs eutrofiese of hipertrofiese toestand ongeveer 10 tot 20 jaar na reservoir konstruksie) oorskry. Literatuur (nie ondersoek in die huidige studie) dui aan dat hierdie versnelde eutrofikasie proses meegebring word deur landbouaktiwiteite in die opvangsarea van die reservoirs. Resultate van die huidige studie het getoon dat forelproduksie in hokstelsels, die konsentrasie van total fosfor wat reeds beskikbaar was, verdubbel het. Die toename in fosforkonsentrasie het binne 1 tot 2 jaar na die aanvang van forelproduksie in die betrokke reservoirs, plaasgevind. Daar is egter uitsonderings waar forelproduksie in hokstelsels moontlik was, sonder die gepaardgaande afname in die waterkwaliteit. Die belangrikste stap vir suksesvolle en volhoubare forelproduksie is deeglike perseelseleksie. Daar is geen impak van akwakultuur waargeneem by vier persele (15 %) wat die eienskappe van goeie waterverplasing (>3 keer per jaar) en ‘n minimum oppervlakarea van 5 ha gehad het nie. Bykomend, sluit kriteria wat die risiko van algbesmetting laat afneem, ‘n minimum waterdiepte van 6 tot 7 m in ‘n reservoir by die laagste punt in (om te verhoed dat intermediêre vermenging plaasvind gedurende die stagnasieperiode) sowel as koue hipolimnetiese toestande (<17 °C) om sianobakterieë sist remobilisasie te minimaliseer. Verdere verbetering van die VOH (voeromsettingsverhouding onder voedingsbestuur) en voerkwaliteit is na perseeleleksie, die volgende komponente wat kan aandui of ‘n perseel gebruik kan word vir hokkultuur sonder enige impak. Vordering met voerkwaliteit en voedingsbestuur kan die aantal volhoubare persele laat toeneem en daardeur meer effektiewe watergebruik teweeg bring, sonder die verwante waterkwaliteit verswakking.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/6680
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