The effect of gracilaria verrucosa on the growth rate of pacific oyster (crassostrea gigas)

The Namibia mariculture industry is focused on the cultivation of shellfish, especially oysters, and unpolluted nutrient rich Benguella current waters greatly enhance the suitability for culture. The oyster producer at Walvis Bay uses a land-based system in which water is pumped from the sea into nan-made ponds and the oysters are put in bags that hang from ropes or wooden structures. This allows vild Gracilaria verrucosa to grow within the oysters. Both organisms utilize the rich marine water pumped into the ponds, thus the production rates of oysters are affected by various environmental conditions mcluding toxic sulfur compounds, unfavorable temperature gradients and salinity . Graci/aria verrucosa has not yet been identified ·with certainty as a culprit that might partly cause a reduction in production rates. Oyster rafters from Walvis Bay have frequently been observed to be intergrown with Graci/aria verrucosa.

In natural aquatic systems, there are ecological balances between different species of organisms and individuals. However in culture conditions the systems are usually meant to favour the preferred organisms, but still other organisms might invade and share in the utilization of resources. Gracilaria verrueosa i an unwanted plant that grows in marine cultured beds along the Namibian coast and this experiment showed that it adversely affects the growth performance of the Pacific oyster ( Crassostrea gigas). The decrease in growth of the oyster in rafts was directly linked to the presence of Graci/aria verrucosa. This experiment quantitatively showed that invasion quantities of Graci/aria verrucosa influences the percentage growth reduction rates of oyster. The various seeding rates (0, 400, 800 g) of Graci/aria verrucosa resulted in significantly (P>0.05) reduced growth rates of oysters by 21 % and 37¾ in medium and high seeding rates of the seaweed, respectively. This was shown to be significant using the t-test and Wilcoxon Sum Rank Test (P>0.05). Although conducted over a short growing period of 6 weeks, the experiment demonstrated detrimental invasion of Graci/aria verrucosa in oyster rafts