Oncorhynchus gorbuscha has the shortest and most fixed life cycle of the Pacific salmon. Fry go to sea almost immediately after emerging, spend roughly eighteen months feeding, return, spawn and die at age two. Deviations from the two-year schedule are rare enough to be negligible for population structure.
The consequence is temporal isolation. A fish spawning in 2025 produces offspring that return in 2027; a fish spawning in 2026 produces offspring returning in 2028. The odd-year and even-year lines occupy the same streams and the same ocean but are reproductively separated by phase, and genetic studies find them to be distinct, sometimes more different from each other than from the same-phase population in another region.
The two lines are also frequently unequal. In many river systems one phase is dominant and the other sparse or effectively absent, and the pattern can persist for decades. Why a particular parity comes to dominate a given river, and how easily a depleted line can be re-established, is not well understood, which matters where pink salmon have been introduced outside their native range, as in northern Norway and Russia.