The Spawning Success of Early Maturing Resident Hatchery Chinook Salmon in a Natural River System

Hatchery propagation of spring Chinook Salmon Oncorhynchus tshawytscha has been shown to increase the proportion of males maturing as minijacks (age 2) or microjacks (age 1) relative to those proportions in wild populations. However, little is known about the success of early maturing males when they spawn in the wild. A captive broodstock program for spring Chinook Salmon in the White River (a tributary of the Wenatchee River, Washington) has a high rate of early male maturity. We used genetic parentage analysis to evaluate the spawning success of anadromous males in comparison with inferred early maturing resident, hatchery-origin males that spawned naturally. Based on samples of juvenile offspring (n = 1,007–1,368 fish/year) and a nearly complete sample of the potential anadromous parents, we found that during 2006–2009, 26–45% of the progeny did not have a male parent in the anadromous sample. In contrast, 0–23% of the progeny did not have a female parent represented in the sample. Using grandparentage analysis, we eliminated wild resident fish as a likely source of the unsampled male parents; thus, we concluded that those male parents were most likely early maturing resident fish that had been released from the captive broodstock program. The inferred spawning success of the unsampled resident males was significantly lower than that of the anadromous males. The typical mating pattern was for an anadromous female to produce about two-thirds of her offspring with one or two anadromous males and the remaining one-third with as many as 12 or more apparently resident males. To our knowledge, this is the first study to present evidence of successful reproduction by early maturing resident, hatchery-origin Chinook Salmon in the wild. The conservation implications of this finding are complex and will depend upon the genetic basis of early maturity and its causes in hatchery settings.