Warming rivers and the uncanny ability of northern pike to navigate saltwater have enabled the freshwater invaders to lay siege to Southcentral Alaska’s salmon and trout.By Paula DobbynWILLOW, Alaska—Corey Ercolani pulled a northern pike from a gillnet and slit its belly with a knife. Inside its guts lay fresh evidence of a growing biological crime: a dead juvenile salmon. A coho, or silver salmon, to be exact.