Chapter 4: Ocean Ecology of Coho Salmon
Richard J. Beamish, Laurie A. Weitkamp, Leon D. Shaul, and Vladimir I. Radchenko
Present day salmonids evolved from an ancestor that became polyploid through a duplication of all or most genes about 50 million years ago (Ohno 1970; Allendorf and Thorgaard 1984). The common ancestor of all Salmo and Oncorhynchus was diadromous and thus migrated between the sea and fresh water. The ancestral species evolved into the Atlantic lineage (Salmo) and the Pacific lineage (Oncorhynchus) in northern Asia during the Miocene (Regan 1920; Stearley 1992). Devlin (1993) and Behnke (1992) proposed that Pacific salmon separated from Atlantic salmon between 10 and 15 million years ago. Montgomery (2000) proposed that marine cooling during the Tertiary may have facilitated anadromy in Pacific salmon, but the various species evolved in fresh water as a consequence of major changes in topography caused by tectonic activity, according to Stearley (1992). Individual species are proposed to have evolved sympatrically (Dimmick et al. 1999). Coho Salmon are believed to have evolved before other North American Pacific salmon (Stearley 1992).