Environmental+Toxicology+Tox+2000+notes (2)

The catch suffered and by 1840 only a total of 500 kg

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Unformatted text preview: the early 1900s, fish ladders were pro vided to allow fish past dams and, with a general reduction in pollution, the Atlantic Salmon began to return. By 1950, 200 rivers had good fishing stocks of salmon. F igure 3-231 Atlantic salmon In the early 1950s forests in Eastern Canada were attacked by spruce budworm and DDT began to be extensively used as a forestry insecticide. It was known that DDT was toxic to fish, but assays on adult fish indicated that it would not cause harm at the co ncentrations which would result from direct overspray of forests. The unknown factor at the time was the sensitivity of the young fish. The spruce budworm spray program took place in the spring when the young fish were in their most susceptible stage and were killed by the DDT. Slightly older fish or those that escaped direct treatment were also affected because their sources of food (the invertebrates and other organisms that inhabit streams) were killed by the DDT spray. Not surprisingly, catches of A tlantic salmon declined and, in some cases, fish were eradicated from certain streams and rivers. With the banning of DDT in 1969 and restocking programs, the Atlantic salmon has begun to return to the rivers of Eastern Canada. This problem has, however , highlighted the need to assess both the impact of pesticides on all parts of the ecosystem and their toxicity in the most sensitive stages of organisms. 3.3.2 Biomagnification of DDT As discussed in Chapter 1, biomagnification of substances in the environment is dependent on the persistence of a compound in the environment and high lipid solubility, which almost always goes hand in hand with greater log KOW. This results in the storage of the substance in fat reserves in organisms. These tissues usually have small turnover rates and any substance stored in them will be less exposed to degradative enzymes than in other tissues. How ever, mobilization of fats, which may occur during stress situations, will often r esult in the release of potentially toxic concentrations of the substance. The biomagnifica tion of DDT is a good example of the above prin ciple. DDT (Figure 3-24) is chemically stable and also lipid soluble. For an insecticide, these are useful properties because they reduce the need for repeated applications and reduce the degree to which the pesticide will wash off treated surfaces. However it is also for this reason that it accumulates in fa tty tissues and moves up the food chain from one trophic level to another. 3.3.2.1 SOLVING THE PROBLEM H Cl C C Cl DDT Cl Cl Cl H C H3 O C C Cl Cl O C H3 Cl Methoxychlor Figure 3-242 DDT and methoxychlor Solving the problem was relatively easy from a chemical point of view. Methoxychlor ( Figure 3-24) is almost as toxic to insects as DDT bu t it is much more easily broken down by the mixed function oxidase en zymes (see Chapter 1 a nd Chapter 2) and is not biomagnified to the same extent at DDT. Unfortunately, methoxychlor is not as persistent as DDT which makes it less useful in certain uses such for the control of malar...
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This document was uploaded on 04/07/2014.

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