DDT, like other organochlorine pesticides enter the marine environment mainly through inputs from water and air as a result of their use in agriculture. Although the use of DDT has been forbidden since the 1970's, they are still detected in the marine environment due to it's extreme stability, to illegal use or to use elsewhere (third world countries). DDT is metabolized into dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD), which are equally toxic. Therefore to asses the risk of DDT exposure, the sum of all 3 contaminants needs to be taken into account. Furthermore, this means that if you encounter a high percentage of DDT, the contamination must be a recent one.