A SYNOPSlS O F T H I C L'ESTIC'l1)15 I'ICOULUM
99
quantities of oil (c. 3 000 000 tons) which are discharged into the oceans
each year by tankers may help the dispersal of these pesticides.
Pesticides must also be dispersed by living agents, for migrant birds
have been shown to contain organochlorine insecticide residues.
Most work on the dispersal of pesticides has been on the organochlorine insecticides. Recent work in Sweden suggests that organomercury compounds may also be widely disseminated in that country
(Borg et al., 1866).
To conclude, it is now known that persistent organochlorine insecticides can become disseminated by movements of air, water and living
organisms. It is not yet possible to draw up a balance sheet to show the
relative importance of these different methods of dispersal.
2. Concentmtiow
It has long been known that DD'1' and other organochlorine insccticides can be stored in body fat, but it was not until Barker (1968) and
Hunt and Bischoff (1960) published their findings on the results of
spraying trees in Illinois and water at Clear Lake in California respectively that the ecological significance of the earlier work was fully
appreciated. Hunt and Rischoff showed that an application of TDE
( = DDD) which produced a concentration of about 0.02 ppm in water,
led to concentrations of hundreds and in some cases thousands of parts
per million in predatory fish and fish-eating birds. The population of the
principal predator of fish, the western grebe (Aechmophorus occidentcrlis),
declined catastrophically in the years following t!he apjdication of TDE
and has begun to increase since its use was discontinued and applications of parathion a t very low concentrations were used instead. Therc
is very little doubt that the decline of the grebe was due to the ingestion
of high levels of TDE in its prey. In these studies species at a higher
trophic level (i.e. at the end of the food chains) contained larger
organochlorine insecticide residues than those species a t a lower trophic
level. Subsequent residue surveys have shown that these were not
exceptional instances. For example, in a study of insecticide residues
in avian tissues and eggs collected from areas throughout the British
Isles, Moore and Walker ( 1 064) and Moore (l965b) showed that predatory species on average contained much the highest residues in terrestrial, freshwater and marine environments (see Fig. 3). The results
obtained from a number of recent studies in aquatic systems are summarized in Table IV. It will be seen that the concentration rate varies
greatly in different systems. The highest concentration rates were in the
one water body that was sprayed intentionally (Clear Lake) and in
which, as a result, the initial concentration in the water was relatively
high. In general, concentration rates appear to be greater in aquatic
99
quantities of oil (c. 3 000 000 tons) which are discharged into the oceans
each year by tankers may help the dispersal of these pesticides.
Pesticides must also be dispersed by living agents, for migrant birds
have been shown to contain organochlorine insecticide residues.
Most work on the dispersal of pesticides has been on the organochlorine insecticides. Recent work in Sweden suggests that organomercury compounds may also be widely disseminated in that country
(Borg et al., 1866).
To conclude, it is now known that persistent organochlorine insecticides can become disseminated by movements of air, water and living
organisms. It is not yet possible to draw up a balance sheet to show the
relative importance of these different methods of dispersal.
2. Concentmtiow
It has long been known that DD'1' and other organochlorine insccticides can be stored in body fat, but it was not until Barker (1968) and
Hunt and Bischoff (1960) published their findings on the results of
spraying trees in Illinois and water at Clear Lake in California respectively that the ecological significance of the earlier work was fully
appreciated. Hunt and Rischoff showed that an application of TDE
( = DDD) which produced a concentration of about 0.02 ppm in water,
led to concentrations of hundreds and in some cases thousands of parts
per million in predatory fish and fish-eating birds. The population of the
principal predator of fish, the western grebe (Aechmophorus occidentcrlis),
declined catastrophically in the years following t!he apjdication of TDE
and has begun to increase since its use was discontinued and applications of parathion a t very low concentrations were used instead. Therc
is very little doubt that the decline of the grebe was due to the ingestion
of high levels of TDE in its prey. In these studies species at a higher
trophic level (i.e. at the end of the food chains) contained larger
organochlorine insecticide residues than those species a t a lower trophic
level. Subsequent residue surveys have shown that these were not
exceptional instances. For example, in a study of insecticide residues
in avian tissues and eggs collected from areas throughout the British
Isles, Moore and Walker ( 1 064) and Moore (l965b) showed that predatory species on average contained much the highest residues in terrestrial, freshwater and marine environments (see Fig. 3). The results
obtained from a number of recent studies in aquatic systems are summarized in Table IV. It will be seen that the concentration rate varies
greatly in different systems. The highest concentration rates were in the
one water body that was sprayed intentionally (Clear Lake) and in
which, as a result, the initial concentration in the water was relatively
high. In general, concentration rates appear to be greater in aquatic