In situ biodeposition rates of Pacific oysters (Crassostrea gigas) on a marine farm in Southern Tasmania (Australia)

Abstract

In situ biodeposition rates of farmed Pacific oysters (Crassostrea gigas) were determined over 2–3 consecutive days in November 1995 and June 1996 as a means of assessing biodeposit loadings. The study was conducted on a 10 ha commercial intertidal oyster farm located in a marine lagoon in southern Tasmania. The farm had approximately 9.5 km of racks and at the time of study was being used to on-grow and condition oysters at a density of 360 oysters/m of racking. The hydrological characteristics of the lagoon were calculated using tide data recorded from gauges located within the lagoon.Average daily biodeposition rates differed between the two time periods with 180.5 g m−2 day−1 in November and 39.6 g m−2 day−1 in June, although background sedimentation rates were similar (7.28 and 8.00 g m−2 day−1, respectively). Biodeposit load from the lease at each sampling time was calculated based on the estimated percentage of baskets containing oysters. A daily average of 494 kg (dry wt) from 10 ha was calculated for the November period and 185 kg (dry wt) for June. Sediment samples collected showed low organic matter content (1.9–2.5%) and suggested that minimal biodeposits were accumulating under the racks, indicating that they were being transported and deposited elsewhere. Average residence time of water in the lagoon was approximately 1.5 days with an average exchange ratio of 0.53 and maximum of 0.97, indicating that at times there is near complete turnover of lagoon water within a tidal cycle. The average difference in total volume and low water volume was 1.38 million m3, with a mean tidal prism of approximately 73% of the total volume.Variation in biodeposition rates were attributed to the time of year and stocking density. Minimal enrichment of the sediment was found under the culture structures indicating that tidal water movement within the intertidal culture area is sufficient to disperse biodeposits. However, the impact of this material being transported and accumulating elsewhere in the lagoon remains unknown.