Changes in run timing and natural smolt production in a naturally spawning coho salmon (Oncorhynchus kisutch) population after 60 years of intensive hatchery supplementation

Supplementing natural fish populations with artificially propagated (hatchery) fish is a common practice. In evaluating supplementation, it is important to assess the relative fitness of both hatchery-produced and naturally produced fish when they spawn together in the wild and to evaluate how the absolute fitness of the natural population changes after many generations of supplementation. We evaluated the relative fitness of naturally produced and hatchery-produced coho salmon (Oncorhynchus kisutch) in Minter Creek, Washington, USA. We also evaluated long-term changes in natural smolt production in this stream after several decades of intensive hatchery supplementation. Total smolt production was estimated to be 14 660 and 19 415 in 2002 and 2003, respectively, compared with the average value of 28 425 from 1940 to 1955. We found no significant difference in relative fitness between hatchery and natural fish, probably because the natural population consists largely of fish produced from the hatchery a generation or two previously. There has been a long-term trend for adults to return to the stream earlier in the spawning season. We estimated standardized selection differentials on run timing, with results indicating stabilizing selection with an optimum run timing later than the mean contemporary run timing but earlier than the historical mean run timing.