Coho and sockeye juveniles occupy fresh water for one or two years, defending territory or rearing in a lake. Chum take a different route. After emerging from the spawning gravel the fry move downstream promptly, often within days, and enter estuarine water while still small, completing most of their growth at sea.
The pattern relieves a constraint. A species whose young must survive a year in a river depends on river productivity, summer flow and freshwater rearing habitat. A species whose young leave immediately can use short, cold, unproductive coastal streams and the lower reaches of very long rivers. That is a large part of why chum has the broadest distribution of any Pacific salmon, spawning from California to the Arctic and thousands of kilometres up the Yukon.
The trade is a heavy dependence on estuaries. Chum fry are small and vulnerable when they arrive, and estuarine habitat quality and the timing of plankton production bear directly on survival. How much of the early marine mortality is set by that first estuarine month, and how it interacts with the enormous numbers of hatchery chum released around the North Pacific, is actively debated.