Fouling by non-indigenous marine species - impacts on biodiversity and mariculture

Havenga, Brendan Stephen (2014-04)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Alien fouling species are most likely to be introduced into Saldanha Bay via two vectors: the high shipping volume experienced in the Bay and the intensive mariculture operations that take place in the system. The invasive ascidian Ciona intestinalis was first recorded in South African waters in 1955 and has since become a common fouling species in Saldanha Bay. Despite this ascidian being known to impact species richness elsewhere, its ecological impacts have not been considered in South Africa. The first chapter of this thesis aims to assess the impact of this species on indigenous fouling communities and considered the role of water movement and depth in moderating any effects. The results from this study revealed that water movement and depth affected settlement of C. intestinalis, with individuals recorded only under conditions of low water movement and only on deep experimental plates (i.e. 3.1 m depth). Unexpectedly, no effect on community structure or diversity was found where C. intestinalis settled. The second chapter aims to document seasonal trends in the fouling communities that affect oyster farms in Saldanha Bay, and assess the prevalence of alien species in these communities. Community structure differed significantly between seasons and depths. The orientation (i.e. the top versus bottom side of oyster cages) only affected the settlement of mussels. Deep cages supported greater fouling biomass than shallow cages. Although there were fewer alien fouling species than indigenous species, alien species supported a greater biomass. At these high densities, alien filter-feeding species may have negative impacts on cultured oysters. The last chapter follows on from this and investigates the impact of C. intestinalis fouling on the growth of cultured oysters, assessing the benefits of four week versus nine week intervals between cage cleaning. During this work the settlement rate of C. intestinalis was unexpectedly low. Results showed that at these low abundances, this species had no effect on growth, shell density or condition of the oysters. In fact cleaning at a four weekly interval was detrimental to the growth of the cultured oysters. It is thus suggested that oyster farms maintain their current nine week cleaning regimes.

AFRIKAANSE OPSOMMING: Die invoer van uitheemse bevuilingspesies na Saldanhabaai geskied waarskyklik via twee vektore: die groot omvangs inskeping wat in die Baai ervaar word, sowel as marikultuur aktiwiteite wat binne die sisteem bedryf word. Die indringer ascidian Ciona intestinalis is in 1955 vir die eerstekeer in Suid Afrikaanse waters waargeneem waarna dit 'n algemene bevuilingspesie in Saldanhabaai geword het. Al is dié ascidian elders daarvoor bekend om spesiesrykheid te beïnvloed, is die ekologiese impakte wat dit in Suid-Afrika mag hê nog nie oorweeg nie. Die eerste hoofstuk van die tesis het beoog om die impak wat dié spesie op inheemse bevuilingsgemeenskappe mag hê te beraam en neem ook verder die invloed van waterbeweging en diepte op hierdie impak in ag. Die studie se resultate onthul dat waterbeweging en diepte beide die vestiging van C. intestinalis beïnvloed. Individue is slegs tydens lae water beweging en op experimentele plate geleë in diepwater (i.e. 3.1m diepte), waargeneem. Daar is geen effek op gemeenskapstruktuur of -diversiteit gevind waar C. intestinalis gevestig is nie. Die tweede hoofstuk het beoog om seisonale patrone binne die bevuilingsgemeenskappe wat oesterplase in Saldanhabaai beïnvloed, aan te teken en om die algemeenheid van uitheemse spesies binne dié gemeenskappe te assesseer. Daar was 'n beduidende verskil in gemeenskapstruktuur tussen seisoen en diepte. Die ligging van die oesterhokke (i.e. die boonste teenoor die onderste kant van die hokke) het slegs die vestiging van mossels beïnvloed. 'n Hoër bevuilingsbiomassa was op die diepgeleë hokke teenwoordig. Alhoewel daar minder uitheemse bevuilingspesies as inheemse -spesies teenwoordig was, het uitheemse spesies bygedra tot 'n groter biomassa. Uitheemse filtreervoedende spesies kan tydens hoë digtheid potensiële negatiewe impakte vir gekweekte oesters inhou. Die laaste hoofstuk het die impak van C. intestinalis bevuiling op die groei van gekweekte oesters geondersoek en het terselfdertyd die potensiële voordele van vierweeklikse teenoor negeweeklikse intervalle tussen hok skoonmaak, geassesseer. Die vestigingskoers van C. intestinalis was onverwags laag gedurende dié studie. Resultate het daarop gedui dat dié spesie tydens „n verminderde teenwoordigheid, geen effek op die groei, skulp digtheid of toestand van die oesters gehad het nie. Daar is verder gevind dat hok skoonmaak op 'n vierweeklikse interval wel nadelige vir die groei van oesterkulture is. Dit word dus voorgestel dat oesterplase hul huidige skoonmaak roetine behou.

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