A toxic paint additive used to protect the bottoms of boats has been found in Lake Tahoe and three other lakes in California in amounts dangerous to aquatic organisms, the state Department of Fish and Game said. The chemical is tributyltin, commonly known as TBT. The department announcement said TBT at a level of 1.2 parts per billion has been measured in water at Tahoe Keys marina on the south side of Lake Tahoe. At that level, TBT can kill fresh-water clams and shrimp, officials said. TBT at a level of .01 ppb was also found in Lake Berryessa in Napa County, and samples of water from two locations in Clear Lake show TBT presence ranging from .01 ppb to .03 ppb. Samples taken from Millerton Lake on the San Joaquin River indicated a TBT presence of .01 ppb.