Multiple stressors (e.g. climate change, chemical contamination, exotic species, etc.) have become of concern in aquatic ecosystems. Their impacts are reflected as alterations in the acquisition and assimilation of energy by organisms (i.e. bioenergetics).  Recent research has indicated that polychlorinated biphenyls (PCBs) are excellent in situ tracers of bioenergetic processes in freshwater fish species as they are passively accumulated through the diet and do not exhibit toxicological effects at environmentally relevant concentrations.  PCBs acquired over a lifetime represent an integrated signal that incorporates the variability associated with metabolic parameters such as temperature regime and activity level.  To demonstrate the use of PCBs as an in situ bioenergetic tool, bluegill sunfish (Lepomis macrochirus) and cisco (Coregonus artedii) were collected from a remote Canadian Shield lake.  PCB accumulation patterns demonstrated that despite having substantially lower temperature-driven metabolic costs, ciscoes required substantially more energy than bluegills to attain a similar body size.  This increased energy requirement may be due to the higher metabolic costs associated with diel migrations to forage in energetically expensive warm upper waters.  This research demonstrates that PCBs, as an in situ bioenergetic tool, can quantify ecological differences among species and can be used in the development of new bioenergetic models.