I Mitchell

Effects of shellfish farming on the benthic environment

The benthic environment under and near three shellfish farms in Tasmania, Australia, which had had a relatively high level of production over many years was investigated. Benthic samples were collected along transects which ran across the farms, generally from 100 m upstream to 100 m downstream. Sediment deposition, redox values, sediment sulphide concentrations, organic carbon content and water turbidity levels near the bottom were significantly different between the farms but not between sites outside the farm, at the boundary and sites within the farm. Video recordings at one farm showed dense coverage of fine filamentous algae and patchy bacterial mats directly under some longlines and this algae is thought to have fallen off the mussel longlines. At another farm dense beds of seagrass were observed in the videos both under trays of oysters and outside the farm. The benthic infauna did not show clear signs of organic enrichment, and neither univariate nor multivariate measures of benthic infauna were significantly different between sites inside and outside the farm, although they were different between farms. It was concluded from these results that shellfish farming is having little impact, and much less than salmon farming, on the benthic environment in Tasmania. Thus extensive monitoring of shellfish farms would appear to be not necessary.

The degradation of fish-cage waste in sediments during fallowing.

Abstract The composition and rate of degradation of organic waste deposited in sediment underneath and adjacent to fish cages in the Huon Estuary, Tasmania, Australia, were investigated. Sediment samples from two near-adjacent sites, but with different sediment types and depths, were analyzed for total organic matter, lipids (fatty acids and sterols), %C, δ 13 C, %N, δ 15 N, and redox potential during a 12-month fallowing period. Additionally, representative samples of fish food and fish faeces were analyzed. It was found that most of the accumulation of organic matter was confined to an area directly underneath the fish cages, but at 30 m from the center of the cage, indicators of fish cage waste (faeces and fish food) were still elevated compared with reference sites. As both fish food and faeces have distinctive fatty acid profiles, the relative proportion of food and/or faeces deposited on the sediment could be determined. After 12 months fallowing, fish-farm-derived organic matter in surface sediment at the center of the cage remained greater than at 30 m distance, even though redox potentials indicated that normal oxic conditions had returned.

Flat oyster (Ostrea angasi) growth and survival rates at Georges Bay, Tasmania (Australia)

An in situ study of growth and survival rates of flat (native) oysters (Ostrea angasi) was conducted on beds of oysters in Georges Bay, Northeast Tasmania, from February 1994–February 1995. Shell length and whole weight of three size groups of tagged flat oysters were measured at 1–3 month intervals at three sites. Mean increase in shell length ranged from 7 to 28 mm and live weight from 40.3 to 47.0 g for large to small oysters, respectively. Significant differences were shown in the increase in shell length and live weight between sites and size classes. Instantaneous mortality rates were variable between sites and sizes with percentage mortality ranging from 6.43% to 22.95%. The data were also analysed using Fabens parameterisation of the von Bertalanffy growth equation for tag recapture data. Growth curves generated were significantly different with calculated L∞ and k values ranging from 103.5 to 118.8 mm and 0.560 to 0.661 year−1, respectively. Length–weight equations were also calculated for each site. Most shell growth occurred during the warmer months from late spring to autumn. Growth rates found in this study were slightly greater at two sites than those obtained from studies on flat oysters in Victoria.