degraded by chemical and biological processes in soil and water, and do not accumulate to any large extent, and are rapidly eliminated from fish. Some studies have measured the uptake and loss of several agricultural chemicals by/from fish, and showed that these chemicals had low accumulation coefficients and short half-lives of the order of hours. In one study, a single dose of parathion was added to a pond. Within two days, the fish concentrated this pesticide about 100-fold compared with the concentration in water, but ‘depurated’ it to very low levels by a month after exposure (Howgate, 1998). More difficult to control is chronic contamination. In aquaculture, the main routes of chronic contamination are the use of polluted water, leaching of agricultural or industrial chemicals from treated or contaminated soil into surface waters, and deposition from the atmosphere. Many chlorinated compounds are discharged into, or are present in, the aquatic environment, but three groups in particular are of concern: 1) chlorinated insecticides such as DDT, dieldrin, lindane and their degradative products, 2) PCBs and 3) polychlorinated dibenzo-
p-dioxins (PCDDs) and –difurans (PCDFs). Summaries and reviews of environmental impacts and the fate and significance to human health, of chlorinated organic compounds and other contaminants in the aquatic environment have been provided by several authors cited by Howgate (1998). According to Howgate (1998), a hazard that apparently hasn’t been investigated in aquacultural products is the presence of persistent organochlorines. High concentrations of these contaminants have been found in fish from some freshwater environments. For example, there is official advice against consuming fish from some parts of the Great Lakes because of high levels of organochlorines. By analogy, it is possible that fish in freshwater aquaculture could be affected similarly. There are theoretical reasons related to the physical properties of these contaminants and to aquacultural practices, for fish from freshwater aquaculture to pose only a low risk of harm to humans – however, measurements are needed for confirmation of this point. The flux of
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