Marine alien species of South Africa : threats and opportunities
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University of Cape Town
Until recently, marine bioinvasions have received little attention in South Africa, and the status of intertidal marine aliens was last assessed in 1990. I thus investigated the present distribution and status of three intertidal alien species (Mytilus galloprovineialis, Care in us maenas and Careinus aestuarii) and documented the presence and status of a previously unknown invasive, the Japanese oyster, Crassostrea gigas. M galloprovincialis was recorded along the entire west coast of South Africa, with populations extending up to central Namibia, and along the south coast of South Africa to 40 km west of East London. Along South African shores a total stock of 35 403.7 tons (± 7 241.4 SE) was recorded, with the vast majority of the stock occurring on the west coast (31 054.5 tons ± 6 274.1 SE). C. maenas supports a substantial population of 133 568 individuals (95 % confidence range == 97 694 - 166 862) in Table Bay Harbour which appears to be acting as in invasion incubator for surrounding areas. A new population of 9 180 individuals (95 % confidence range 5 870 - 12 003) was recorded in Hout Bay Harbour. This represents a range extension along the Cape Peninsula. The low intertidal abundance of this crab between these localities is thought to reflect the inability of C. maenas to inhabit exposed habitats. C. aestuarii was not recorded during this study despite previous documentation of its presence. In 2003, naturalised populations of C. gigas were documented in the low shore zone of the Breede, Goukou and Knysna Estuaries, but not on the open coast. This may again be indicative of the limiting effect of wave action on species alien to South African shores. As such, the presently undetermined impact of this species may well be focused on estuarine habitats. To quantify the threat posed to intertidal communities by M galloprovincialis, changes in community structure on the rocky shore of Marcus Island and the sandy shore of Langebaan Lagoon were measured. In both habitats, the invasion significantly altered community composition. On Marcus Island the effects were focused within the mid-to-low intertidal zones, where habitat complexity was enhanced and patchiness was decreased, resulting in dramatic changes in invertebrate density, species number, richness and diversity. On sandy shores, M galloprovincialis beds changed available habitat structure, resulting in a concurrent invasion by indigenous rocky shore species. In the sediment below the mussel beds, soft-sediment species were excluded due to anoxia. Following a die-off of the mussel beds the rocky shore species disappeared, but were not replaced by sandy shore organisms as the sediment remained uninhabitable. The subsequent removal of the dead mussel shells appears to have recovered, but community composition is still to return to the preinvasion state. In order to consider the biological viability of a fishery for A1. galloprovincialis in the Northern Cape, a harvesting project operated by two impoverished coastal communities was initiated. Harvesting took place on a rotational basis and twelve sites, nested within four harvesting locations, were each exposed to spectrum of harvesting intensities (F=O, F=30%, F=60% and F=90%). A dynamic biomass-based fisheries model predicted monthly Maximum sustainable yield (MSY) estimates of 1 560 kg per 100 m of shore in March-April and September-October, but two orders of magnitude less (15 kg) during the remaining months. These peaks correspond to spawning periods of this mussel along the South African west coast. Experimental manipulations revealed high recruit densities recorded at low harvesting intensities (2 000 20 000 per 0.01 m exceeding levels required for population maintenance. However, if adult mussel beds are eliminated or significantly reduced (which occurred at F=30% or above), recruitment may limit stock replenishment. Indirect effects on non-target species were also considered. Intertidal communities changed dramatically in response to harvesting, with increased algal dominance and shifts in the distribution of grazers between primary- and secondary-substrates. Community composition did not return to the pre-harvest state after four months of no harvesting, even in areas which were only harvested at an intensity of F=30%. Northern Cape intertidal communities are thus considered to have low resilience and elasticity in response to harvesting of A1. galloprovincialis. It is thus recommended that a harvesting intensity of between 10% and 30% be employed if A1. galloprovincialis stocks in the Northern Cape are to be harvested on a commercial basis. This would protect stock replenishment and minimise effects on intertidal communities. In addition, harvesting should be focused within the two spawning seasons spanning peak MSY estimates so as to maximise yield and aid community recovery between harvests. If implemented, this would represent the first instance in South Africa of a marine invasive species being utilised in a way that employs dynamic fisheries management to achieve socioeconomic goals, while taking into account the effects on other elements of the biotic community.
Robinson, T. 2005. Marine alien species of South Africa : threats and opportunities. University of Cape Town.