When an unusually strong marine heat wave warmed the ocean off the West Coast from late 2014 to 2016, the effects reverberated through the marine ecosystem. One of the telltale changes was in copepods, tiny crustaceans that provide essential food for juvenile salmon as they first enter the ocean. Instead of energy-rich copepods that help the fish grow quickly, leaner copepods with less energy began to dominate. That left young salmon facing tougher odds in the ocean.