Rates at which toxaphene concentrations decrease in lake trout from the Great Lakes

Toxaphene is a complex mixture of at least 600 hexa- to decachlorinated bor nanes and bornenes, which was used as an insecticide in the United States f rom the late 1950s to the mid-1980s. A previous study in our laboratory sho wed that the levels of toxaphene in lake trout collected in 1982 and 1992 f rom Lake Superior had remained about the same but that the concentrations i n lake trout from the other Great Lakes had decreased during this same time period. These observations in Lake Superior trout were counter-intuitive g iven that toxaphene had been banned in 1982. We have reinvestigated this is sue using more samples from both Lake Superior and northern Lake Michigan a nd using an improved analytical method. The level of toxaphene was constant in all of the trout samples from Fake Superior during the period 1977-1992 , while the level decreased by a factor of 1.4-5 in trout from the other Gr eat Lakes. These results suggest that the toxaphene concentrations in very large and very cold lakes (such as Lake Superior) decrease much more slowly than they do in relatively small and warm lakes (such as Lake Ontario).