SHELLFISH CULTURE IN THE BENGUELA SYSTEM - A CARBON BUDGET OF SALDANHA BAY FOR RAFT CULTURE OF MYTILUS-GALLOPROVINCIALIS

A carbon budget of Saldanha Bay (South Africa) was constructed as a fi rst step in estimating carrying capacity of the bay for floating raft culture of the mussel Mytilus galloprovincialis. The bay is part of th e Benguela upwelling system and is characterized by subsurface nitrate input and high chlorophyll levels for most of the year. On the basis of existing mussel yield, a top-down approach was used to estimate the phytoplankton carbon consumed by mussel production. These calculation s were compared with a bottom-up approach using observed phytoplankton production and water exchange to calculate potential production of mu ssels. These results indicate that mussel rafts require an exchange ra te of only 10% of the estimated rare, even allowing for frictional flo w inhibition by the rafts. Within the 1,000 ha of proposed culture; mu ssel production would be the dominant carbon sink, requiring 7.5% of t he advected food supply compared with suspension-feeding benthos (3%): fouling organisms on the mussel lines (2.5%), and zooplankton (2%). T hese estimates are for an idealized case wherein phytoplankton are ren ewed via exchange of water from outside the culture area. The ultimate constraint on mussel production is new primary production for the who le bay (4,480 ha), and calculations on this basis indicate that second ary production by suspension-feeding benthos is dominant (40% of new p roduction), followed by zooplankton (24%), mussels (21%), and fouling organisms (7%). There is a small carbon surplus of -8% of new producti on, which could potentially be channeled into further mussel culture, although approximations used in the carbon budget make it difficult to gauge the error on this estimate. We recommend doubling the raft dens ity from 1 to 2 ha(-1) in several hectares of culture and comparing gr owth with present density culture areas as a means of testing whether the system is close to carrying capacity. Although these calculations can be made on an annual basis, interannual variation in upwelled forc ing of new production remains a significant regulator of system-wide p roduction in Saldanha Bay.