Pesticide Residues in Fish - Pesticides: Evaluation of Environmental Pollution - page 365

direct uptake of pesticides through the gills during respiration, and (3) orally, by drink-
ing pesticide-contaminated water or feeding on pesticide-contaminated prey. Poisoning
by consuming another animal that has been poisoned by a pesticide is termed “second-
ary poisoning.” For example, fish feeding on dying insects poisoned by insecticides may
themselves be killed if the insects they consume contain large quantities of pesticides or
their toxic by-products.
Exposure of fish and other aquatic animals to a pesticide depends on its biological
availability (bioavailability), bioconcentration, biomagnification, and persistence in the
environment.

13.3.1 Bioavailability

Bioavailability refers to the amount of pesticide in the environment available to fish and
wildlife. Some pesticides rapidly undergo degradation after application. Some bind tightly
to soil particles suspended in the water column or to stream bottoms, thereby reducing
their availability. Some are quickly diluted in water or rapidly volatize into the air and are
less available to aquatic life. Bioconcentration is the accumulation of pesticides in animal
tissue at levels greater than those in the water or soil to which they were applied. Some
fish may concentrate certain pesticides in their body tissues and organs (especially fats) at
levels 10 million times greater than in the water.

13.3.2 Biomagnification

Biomagnification is the accumulation of pesticides at each successive level of the food
chain. Some pesticides can bioaccumulate (build up) in the food chain. For example, if a
pesticide is present in small amounts in water, it can be absorbed by water plants that are,
in turn, eaten by insects and minnows. These organisms also become contaminated. At
each step in the food chain, the concentration of the pesticide increases. Fish, such as bass
or trout, repeatedly consume contaminated animals, and they bioconcentrate high levels
in their body fat. Fish can pass these poisons on to humans (Figure 13.2).
In other words, biomagnification occurs when the toxic burden of a large number of
organisms at a lower trophic level is accumulated and concentrated by a predator in a
higher trophic level. Phytoplankton and bacteria in aquatic ecosystems, for instance, take
up heavy metals or toxic organic molecules from water or sediments. Their predators—
zooplankton and small fish—collect and retain the toxins from many prey organisms,
building up higher concentrations of toxins. The top carnivores in the food chain—game
fish, fish-eating birds, and humans—can accumulate such high toxin levels that they
suffer adverse health effects. One of the first known examples of bioaccumulation and

DDT accumulation in food chain

DDT in water DDT in zooplankton DDT in small fish DDT in large fish DDT in fish-eating birds

FIGURE 13.2
Bioaccumulation and biomagnification.