DDT is still used in other countries. When the agricultural soil is plowed, the airborne soil dust containing DDT encircles the globe and settles out in the USA and elsewhere. The oceans are contaminated with all sorts of chemicals including DDT and other pesticides. Algae, plankton, clams and other shell fish filter much ocean water each day and absorb the toxic chemicals into their bodies. Little fish eat the lower marine life forms and concentrate the pesticides in their bodies.
Big fish eat the little fish and concentrate the pesticides to a higher level in their bodies.
As a result, many fish caught in the oceans contain significant levels of pesticides. Over half of the fish catch is made into fish meal which is then added to animal feed for cows, pigs and poultry. This process concentrates the pesticides even further. As a result, the nation's meat and dairy products are contaminated with pesticide residues. Rain washes the dioxin out of the air and into the soil where it is absorbed by plants or into the oceans where it is absorbed by fish.