For wild sockeye salmon, the trip upriver from the ocean to their spawning grounds is fraught with peril and hardship. But quantifying exactly how obstacles along the way, fluctuations in water temperature and other factors impact fish survival has long eluded researchers. New advances in biological sensor tags are now allowing scientists to precisely measure animals’ energetics, their interactions with humans, and their responses to rapidly changing environments.In 2014, for example, Nicholas Burnett and colleagues used accelerometer tags to measure how salmon needed to swim in order to traverse a dam in the Seton-Anderson watershed of British Columbia, Canada and how likely they were to survive the remainder of their journey. They found that when salmon resort to strenuous anaerobic swimming, they were significantly more likely to die days or even hours later.