Human food production activities can dominate natural systems, altering ecological and evolutionary aspects of the environment. Disease-mediated interactions are of particular concern. For example, parasites may "spill-over" from farms to wildlife. Parasites isolated on farms can evolve resistance to treatment chemicals , but "spill-back" from wildlife to farms may alter evolutionary dynamics. Here, we consider exchange of parasites (Lepeophtheirus salmonis) between wild (Oncorhynchus gorbuscha) and farmed salmon. We derive and analyze discrete-time models that implicitly include wild salmon migrations. First, we extend a standard fisheries model to show parasite exchange affects "line-dominance" in the population ecology of salmon. Second, we extend a classic population genetics model to show that wild salmon can theoretically provide an "ecosystem service" by delaying the onset of chemical resistance in parasites on farms. This service, however is affected by a nonlinear feedback if farm parasites spill-back to affect wild salmon.