Bivalve molluscs (shellfish) are filter-feeders meaning they feed by filtering water through their gills. Because of their effective filtration mechanism, shellfish may accumulate microbiological, chemical and biological contaminants from the water. Bioaccumulation of contaminants by shellfish is a human health issue and is indicative of declining water quality and ecosystem integrity.
Some other shellfish-related indicators are:
Photo 1. A selection of seafood.
Shellfish beds require stricter land use and pollution control measures than many other environments because contaminated shellfish constitute a serious threat to human health and to ecosystem integrity. Consumption of contaminated seafood can cause a broad range of toxicological effects in organisms, or even their death. The ingestion of algal biotoxins through shellfish consumption results in a dramatic suite of symptoms and syndromes in humans (e.g. paralytic, neurotoxic, amnesic and diarrhetic shellfish poisonings). In the ecosystem, chemical pollutants may accumulate and magnify up through the food chain in a process called biomagnification. This lowers the reproductive abilities of organisms at higher trophic levels and renders them more susceptible to infectious disease. Economic impacts are many and varied but include costs of medical care and of closure of harvesting areas (see more detailed discussions under the economic consequences of algal blooms and commercial fisheries). The latter have implications for producers, processors and retailers.
See 'monitoring and design strategies' and 'analysis and interpretation' under Indicators 4.4 and 6.2 in the State of the Environment report for estuaries and the sea .
Shellfish quality assurance programs exist in all states, and growers are encouraged to report events such as algal blooms, fish kills, water discolouration (including turbidity), incidences of increased bird numbers and heavy rainfall to local authorities. Emergency closures of shellfish waters are often made on the basis of significant environmental events such as rainfall, faecal coliform counts or the occurrence of algal blooms. Depuration confers additional quality assurance in areas subjected to intermittent contamination, but is ineffectual in heavily polluted waters.
Government institutions and state agencies responsible for managing coastal ecosystems keep records of shellfish closures and have data on contaminants in relevant taxa. Commonwealth health and fishing authorities have some data on seafood quality although not all of it is publicly available.
For more information consult the Australian Shellfish Sanitation Control Program; Operations Manual Ist Edition 10/4/97. The Canadian Shellfish Quality Resource website has some general marine biotoxin links.
Default water quality targets for turbidity and nutrient levels to protect human consumption of aquatic foods and aquaculture values for different bioregions are provided in Water Quality Targets Online.
Kerrod Beattie, Queensland Department of Primary Industries.