U.S. officials say they will toughen measures to prevent consumption of fish tainted by a huge deposit of DDT on the ocean floor.

The pesticide has contaminated Santa Monica Bay's Palos Verdes Shelf for a half-century and earned a Superfund designation.

"This is the largest DDT contamination site in the country," the Environmental Protection Agency's Keith Takata said Wednesday. "The first step to protect the public health is to make sure people aren't eating white croaker contaminated with dangerous levels of DDT."

Additionally, the EPA in July will launch a three-month pilot project to cap 180 acres of ocean floor along the Los Angeles coast with sand and silt to prevent the spread of the pesticide.

The $5 million experiment will take place in waters about 200 feet deep, said Fred Schauffler, the EPA's project manager.

The contaminated area has an estimated 100 tons of DDT spread across 17 square miles of ocean floor. There are also about 10 tons of PCBs.

Chemical manufacturer Montrose Corp. allegedly dumped residue, including DDT, into the sewage system from 1947 until 1971. The area was declared a Superfund environmental cleanup site in 1996.

DDT, which was banned in the U.S. in 1973, has been linked to cancer and reproductive problems in humans.

Because contamination levels of DDT are the highest in bottom-feeding fish, such as the white croaker, the EPA will call for more aggressive enforcement of the catch limit and the commercial fishing ban in the area, Schauffler said.

The agency also will launch an educational campaign and monitor contamination levels of white croaker sold in markets, Schauffler said.

Montrose and other companies responsible for the pollution have challenged the EPA's capping plan as being too risky.

"We ... disagree with the Montrose folks who claim there's no significant human health risk and the problem is going away on its own by virtue of biodegradation of DDT," Schauffler said. "Here we are 30 years after the last significant discharge and we still have a very major problem on our hands."