Data from: Persistent reproductive isolation between sympatric lineages of fall Chinook salmon in White Salmon River, Washington

Populations of fall Chinook salmon Oncorhynchus tshawytscha in the
Columbia River are divided among two evolutionarily significant units:
lower Columbia River fall Chinook salmon (or “tules”) in the lower portion
of the river and upriver “brights” (URBs) in the upper portion. The two
lineages migrate together through portions of the lower Columbia River but
spawn allopatrically. Little White Salmon National Fish Hatchery has been
releasing URBs adjacent to what was historically exclusively tule spawning
habitat in the White Salmon River for approximately 22 years. Tules and
URBs now spawn in sympatry in the adjacent White Salmon River, and
potential introgression between these two lineages has been identified as
a genetic risk of the hatchery program. To assess the extent of
introgression, we analyzed 13 microsatellite loci in juveniles taken from
the White Salmon River in three consecutive years. Assignment tests
revealed that juveniles leaving the White Salmon River from March to early
May resembled tules, while those leaving from late May to June resembled
URBs. Model-based hybrid detection revealed that between 4.3% and 15.0% of
the juveniles in each year were tule × URB hybrids. No hybrid adults were
detected in any of the collections examined. The divergence between the
tule and URB populations in the White Salmon River was comparable to that
between allopatric populations representing the two lineages. Although
hybrid juveniles are produced in the wild, we found no evidence that they
survive to return as adults or successfully cross back into the parental
populations. The separation between the two fall Chinook salmon lineages
thus appears to be based on intrinsic as well as extrinsic factors.