C Crawford

Oyster Reefs at Risk and Recommendations for Conservation, Restoration, and Management

Native oyster reefs once dominated many estuaries, ecologically and economically. Centuries of resource extraction exacerbated by coastal degradation have pushed oyster reefs to the brink of functional extinction worldwide. We examined the condition of oyster reefs across 144 bays and 44 ecoregions; our comparisons of past with present abundances indicate that more than 90% of them have been lost in bays (70%) and ecoregions (63%). In many bays, more than 99% of oyster reefs have been lost and are functionally extinct. Overall, we estimate that 85% of oyster reefs have been lost globally. Most of the worlds remaining wild capture of native oysters (> 75%) comes from just five ecoregions in North America, yet the condition of reefs in these ecoregions is poor at best, except in the Gulf of Mexico. We identify many cost-effective solutions for conservation, restoration, and the management of fisheries and nonnative species that could reverse these oyster losses and restore reef ecosystem services.

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.

Detection of organic enrichment near finfish net-pens by sediment profile imaging at SCUBA-accessible depths

Sediment profile images (SPI) of cores collected by SCUBA diver were obtained using a modified Hargrave corer from fish farm sites in the Bay of Fundy, Canada and southeastern Tasmania, Australia. Shipboard and land based photography were used to obtain the SPI with a tripod mounted digital camera and image analysis by commercially available software. Computer images were analyzed to determine the variables used by Nilsson and Rosenberg [Mar. Ecol., Prog. Ser. 197 (2000) 139], modified to account for non-equilibrium conditions, to assess successional stages of organic enrichment. To validate the method, we concurrently sampled macrofaunal species composition and abundance and measured profiles of redox potentials and total sulphides by ion analysis. In each case, the null hypothesis that sediments collected directly under an active salmon net-pen were indistinguishable from a nearby reference site was rejected. The SPI method can successfully detect organic enrichment where impacts occur in soft sediments in geographically diverse locations.

Environmental management of marine aquaculture in Tasmania, Australia

Marine farming is an important rural industry in coastal bays and estuaries of Tasmania. The two main species cultured are the introduced Pacific oyster, Crassostrea gigas, and Atlantic salmon, Salmo salar. Legislation has been introduced to assist the development of aquaculture, and this includes requirements for environmental management, such as baseline assessments and routine monitoring of leases. Local impacts on the seabed around salmon farms are monitored using video footage, analysis of benthic invertebrate infauna, and chemical measures (redox and organic matter). Monitoring of shellfish farms is minimal because our research has shown that shellfish culture is having little impact on the environment. Research related to management of aquaculture wastes is ongoing. Studies include investigating appropriate inexpensive measures for an industry-wide long-term monitoring program. Mitigation measures against excessive loadings of organic matter from fish farms, mainly by fallowing, i.e. rotating the position of fish pens around a lease, are currently being researched. Rates of recovery of a heavily impacted salmon lease area after the removal of fish have also been studied. A new project is investigating system-wide effects of salmon farming on the environment, in particular, increased release of nutrients into waterways. This includes monitoring dissolved oxygen, nutrients and phytoplankton, modelling the system, and investigating ecological indicators of eutrophication.

Qualitative risk assessment of the effects of shellfish farming on the environment in Tasmania, Australia

Community concerns about the detrimental effects of shellfish farming on the environment have been increasing over the last decade in many shellfish producing countries, including in Tasmania, Australia. Environmental effects of shellfish farming can be assessed and managed using risk management processes, which have been developed for a wide range of human activities. In this study the risk of shellfish farming activities having detrimental impacts on the ecology of the Tasmanian marine environment was assessed using standard risk assessment methodology. This was based on local environmental information and on the level of production, stocking densities, and known husbandry practices in Tasmania compared to farm operations and levels of impact observed in other countries. The risk management process used in this study appears to be applicable to a variety of aquaculture activities. The qualitative risk assessment of detrimental impacts of shellfish farming rated the risk of spread of introduced pests and/or pathogens as high. However, this high risk rating would also apply to many other activities in the marine environment, such as commercial and recreational fishing and sea transport. The level of risk due to habitat disturbance was rated as moderate within the lease area, but would not be expected to extend outside the farm. Risks of organic enrichment of the seabed and reduced food resources for other filter feeders were both rated as low.

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.

Preliminary results of experiments on the rearing of Tasmanian flounders, Rhombosolea tapirina and Ammotretis rostratus

Abstract Tasmanian species of flounder were cultured successfully following stripping and fertilizing eggs after hormone-induced ovulation. The larvae were fed rotifers followed by Artemia nauplii. Survival rates of larvae from first-feeding to metamorphosis, which were as high as 94–98% for R. tapirina and 65% for A. rostratus , indicate that both species can be readily cultured in captivity.

Australian shellfish ecosystems: past distribution, current status and future direction

We review the status of marine shellfish ecosystems formed primarily by bivalves in Australia, including: identifying ecosystem-forming species, assessing their historical and current extent, causes for decline and past and present management. Fourteen species of bivalves were identified as developing complex, three-dimensional reef or bed ecosystems in intertidal and subtidal areas across tropical, subtropical and temperate Australia. A dramatic decline in the extent and condition of Australia’s two most common shellfish ecosystems, developed by Saccostrea glomerata and Ostrea angasi oysters, occurred during the mid-1800s to early 1900s in concurrence with extensive harvesting for food and lime production, ecosystem modification, disease outbreaks and a decline in water quality. Out of 118 historical locations containing O. angasi-developed ecosystems, only one location still contains the ecosystem whilst only six locations are known to still contain S. glomerata-developed ecosystems out of 60 historical locations. Ecosystems developed by the introduced oyster Crasostrea gigas are likely to be increasing in extent, whilst data on the remaining 11 ecosystem-forming species are limited, preventing a detailed assessment of their current ecosystem-forming status. Our analysis identifies that current knowledge on extent, physical characteristics, biodiversity and ecosystem services of Australian shellfish ecosystems is extremely limited. Despite the limited information on shellfish ecosystems, a number of restoration projects have recently been initiated across Australia and we propose a number of existing government policies and conservation mechanisms, if enacted, would readily serve to support the future conservation and recovery of Australia’s shellfish ecosystems.

A database of chlorophyll a in Australian waters

Chlorophyll a is the most commonly used indicator of phytoplankton biomass in the marine environment. It is relatively simple and cost effective to measure when compared to phytoplankton abundance and is thus routinely included in many surveys. Here we collate 173, 333 records of chlorophyll a collected since 1965 from Australian waters gathered from researchers on regular coastal monitoring surveys and ocean voyages into a single repository. This dataset includes the chlorophyll a values as measured from samples analysed using spectrophotometry, fluorometry and high performance liquid chromatography (HPLC). The Australian Chlorophyll a database is freely available through the Australian Ocean Data Network portal (https://portal.aodn.org.au/). These data can be used in isolation as an index of phytoplankton biomass or in combination with other data to provide insight into water quality, ecosystem state, and relationships with other trophic levels such as zooplankton or fish.