CONTAMINANT TRENDS IN LAKE TROUT AND WALLEYE FROM THE LAURENTIAN GREAT-LAKES

Trends in PCBs, DDT, and other contaminants have been monitored in Gre at Lakes lake trout and walleye since the 1970s using composite sample s of whole fish. Dramatic declines have been observed in concentration s of PCB, Sigma DDT, dieldrin, and oxychlordane, with declines initial ly following first order loss kinetics. Mean PCB concentrations in Lak e Michigan lake trout increased from 13 mu g/g in 1972 to 23 mu g/g in 1974, then declined to 2.6 mu g/g by 1986. Between 1986 and 1992 ther e was little change in concentration, with 3.5 mu g/g observed in 1992 . Sigma DDT in Lake Michigan trout followed a similar trend, decreasin g from 19.2 mu g/g in 1970 to 1.1 mu g/g in 1986, and 1.2 mu g/g in 19 92. Similar trends were observed for PCBs and Sigma DDT in lake trout from Lakes Superior, Huron and Ontario, Concentrations of both PCB and Sigma DDT in Lake Erie walleye declined between 1977 and 1982, after which concentrations tr,ere relatively constant through 1990. When ori ginally implemented it was assumed that trends in the mean contaminant concentrations in open-lake fish would serve as cost effective surrog ates to trends in the water column. While water column data are still extremely limited it appears that for PCBs in lakes Michigan and Super ior, trends in lake trout do reasonably mimic those in the water colum n over the long term. Hypotheses to explain the trends in contaminant concentrations are briefly reviewed, The original first order loss kin etics used to describe the initial decline do not explain the more rec ent leveling of contaminant concentrations. Recent theories have exami ned the possibilities of multiple contaminant pools. We suggest anothe r hypothesis, that changes in the food web may have resulted in increa sed bioaccumulation. However, a preliminary exploration of this hypoth esis using a change point analysis was inconclusive.