Browsing by Author "Havenga, Brendan Stephen"

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    Fouling by non-indigenous marine species - impacts on biodiversity and mariculture
    (Stellenbosch : Stellenbosch University, 2014-04) Havenga, Brendan Stephen; Robinson, Tamara Bridgett; Jackson, Susan; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    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.