Forecasting Contaminant Transport to Great Lakes Tributaries by Introduced Pacific Salmon

Pacific salmon (Oncorhynchus spp.), which have been stocked in the Great Lakes since the 1960s, have the potential to transport large quantities of contaminants to streams during spawning migrations.  In an inter-basin survey of Great Lakes streams, we found that concentrations of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene (DDE) were up to 100 times higher in resident fish from streams receiving salmon spawners than in streams without salmon.  To assess the broader implications of salmon-mediated contaminant dispersal, we used a simple forecasting model to predict contaminant levels in several species of stream-resident fish throughout the Upper Great Lakes.  Estimates of resident fish contaminant levels were based on salmon spawner biomass and contaminant concentrations of salmon tissue from the nearest sampled location.  Results show considerable variation among lake basins; resident fish in Lake Michigan tributaries receiving salmon are predicted to be 34 times more contaminated than in reaches lacking salmon, 12 times higher for Lake Huron, and less than 2 times higher for Lake Superior.  PCB comparisons among fish species within Lake Michigan streams predict that brook trout (Salvelinus fontinalis) and blacknose dace (Rhinichthys atratulus) are likely to be three times more contaminated than sculpin (Cottus spp.).  Differences may be related to direct versus indirect consumption, as gut content analysis of brook trout revealed salmon egg consumption as high as 4.5 g wet mass per individual, while little to no salmon material was found in sculpin.  The ability of salmon to deliver harmful contaminants to new environments has implications for both watershed management and human health.  Our results suggest that dam removal may facilitate the spread of contaminants by allowing salmon to invade upstream habitats, and elevated contaminant levels in stream fish, such as brook trout, may warrant additional consumption advisories.  Predicted PCB levels in stream-resident trout from Lake Michigan and Lake Huron are high enough (average of 132 ng/g wet weight) to warrant a consumption advisory of no more than one meal per week (>60 ng/g wet weight).  Thus, introduced Pacific salmon appear to be a key distributor of contaminants to stream ecosystems in the Great Lakes, and human consumption of resident fish from salmon streams may in some cases need to be restricted.