Fishes are among the most intentionally introduced organisms on planet Earth to support sportfishing, aquaculture, commercial harvest, and biological control. The proliferation of introduced fish that have annual spawning migrations, such as salmonids and clupeids, delivers millions of kilograms of labile resources to streams, rivers, wetlands, and lakes globally. However, these subsidies carry with them a milieu of toxic chemicals, such as metals and synthetic organic compounds, that can be spread 100s of kilometers from their source. The Laurentian Great Lakes provide a classic example of intentional and unintentional fish introductions that have transformed the watershed in dramatic and unanticipated ways. Some 40 species of non-indigenous fishes have been introduced into the Great Lakes, resulting in novel phenomena, such as extensive spawning migrations into naïve tributaries, and the reshaping of the native food web. The delivery and cycling of macro- and micro-nutrients from the excreta, gametes, and decomposing bodies of introduced fishes, such as Pacific salmon (Oncorhynchus spp.), is less important in the Great Lakes than in their native, oligotrophic habitats (e.g., Alaska). However, our research shows that adult salmon also transport organic contaminants in their bodies that are subsequently transferred to stream food webs of the Great Lakes watershed and, potentially, human consumers. We demonstrate that PFAS, or ‘forever chemicals’, acquired in the lakes are concentrated in the bodies and gametes of salmon and then released into spawning streams and subsequently accumulated by aquatic and riparian biota. This ‘dark web’ of salmon subsidies has serious implications for both wildlife and humans that consume these fish, particularly for individuals reliant on this protein source. The synergistic nexus of fish introductions, chemical pollution, and fish behavior can result in unintended consequences for ecosystems already challenged with emerging contaminants and other environmental changes.