Artificial Selection and Environmental Change: Countervailing Factors Affecting the Timing of Spawning by Coho and Chinook Salmon

—Spawning date is a crucial life history trait in fishes, linking parents to their offspring, and it is highly heritable in salmonid fishes. We examined the spawning dates of coho salmon Oncorhynchus kisutch and chinook salmon O. tshawytscha at the University of Washington (UW) Hatchery for trends over time. We then compared the spawning date patterns with the changing thermal regime of the Lake Washington basin and the spawning patterns of conspecifics at two nearby hatcheries. The mean spawning dates of both species have become earlier over the period of record at the UW Hatchery (since the 1950s for chinook salmon and the 1960s for coho salmon), apparently because of selection in the hatchery. Countering hatchery selection for earlier spawning are the increasingly warmer temperatures experienced by salmon migrating in freshwater to, and holding at, the hatchery. Spawning takes place even earlier at the Soos Creek Hatchery, the primary ancestral source of the UW populations, and at the Issaquah Creek Hatchery. Both species of salmon have experienced marked shifts towards earlier spawning at Soos Creek and Issaquah Creek hatcheries despite the expectation that warmer water would lead to later spawning. Thus, inadvertent selection at all three hatcheries appears to have resulted in progressively earlier spawning, overcoming selection from countervailing temperature trends. Compared with most other fishes, salmonids produce large eggs with a protracted incubation period. Spawning date is the primary factor controlling the date when offspring emerge from the gravel in the spring, and it is an adaptation to the prevailing ecological conditions during incubation and emergence, influencing juvenile survival and growth (Brannon 1987; Brännäs 1995; Webb and McLay 1996; Einum and Fleming 2000; Quinn et al. 2000). The timing of adult migration and reproduction differs greatly among salmonid populations, but within populations, timing varies only slightly among years (Ricker 1972; Brannon 1987; Groot and Margolis 1991). Timing of migration and reproduction is largely under genetic control in a variety of salmonid species (Siitonen and Gall 1989; Hansen and Jonsson 1991; Su et al. 1997; Smoker et al. 1998; Quinn et al. 2000). Dates of * Corresponding author: [email protected] 1 Present address: U.S. Department of Agriculture, Agricultural Research Service, National Center for Cool and Cold Water Aquaculture, 11876 Leetown Road, Kearneysville, West Virginia 25430, USA. 2 Present address: Aquaculture Research Institute, University of Idaho, Post Office Box 442260, Moscow, Idaho 83844-2260, USA. Received May 29, 2001; accepted January 9, 2002 migration and spawning seem to reflect selection for adult passage (Quinn and Adams 1996) and incubation of embryos (Brannon 1987), and timing diverges in populations transplanted outside their range (Quinn et al. 2000). The high heritability of spawning date means that it can be affected rapidly by artificial selection in hatcheries as well as by natural selection. Practices in hatcheries can directly select for the timing of maturation if early-maturing fish are spawned and late-maturing fish are discarded. Indirect selection for early maturation may also occur if the progeny of late-maturing fish are (1) culled as too small, (2) cannot compete in the hatchery with the larger progeny of early spawners, (3) fail during the smolt transformation process, or (4) have lower survival rates at sea. Deliberate selection for spawning date in steelhead Oncorhynchus mykiss in Washington resulted in markedly earlier spawning, allowing hatchery staff to grow the fish to smolt size in 1 year rather than 2 years (Ayerst 1977; Crawford 1979). Progressively earlier spawning has also been documented in lower Columbia River coho salmon O. kisutch populations (Flagg et al. 1995). The timing of salmon migration and reproduction is thus affected by environmental conditions and artificial selection, but how might the fish re-