The overall average daily discharge of the Willapa River during hatchery steelhead migration and spawning (December through February) of brood years 1996 through 2007 was high (42.1 m s−1), and annual average daily discharge varied fivefold among years (15.2 76.9 m s−1, Fig. 3).
Although it was assumed that the weir spanning Forks Creek at the hatchery prevented upstream migration of most if not all adult steelhead (Mclean et al. 2003, 2004), we discovered that marked (hatchery-produced) steelhead spawned in the wild every year (Dauer et al. 2009), likely bypassing the weir during the moderate to high stream flows that are common in winter and spring.
Adult hatchery ancestry steelhead migrated early, with annual median migration dates between 27 December and 3 March and an overall median migration date of 12 January (Fig. 5).
The most likely reason for this somewhat unexpected relationship is the younger average age at seaward migration of hatchery ancestry smolts.
In an effort to understand and mitigate increasingly poor adult returns of steelhead, we conducted a four year study using the Seymour River stock to compare the early marine survival and migration of hatchery-reared fish released using various experimental strategies.
Even if barriers were completely effective at preventing upstream migration, the hatchery-produced fish might spawn elsewhere in the basin (Quinn 1993; Dittman et al. 2010).
The differential migration timing of hatchery and wild steelhead permits high harvest rates on the hatchery fish, followed by fishery closures during the later migrations of wild steelhead.
Travel speed differences were observed between wild and hatchery-reared fish during the downstream migration, with wild fish moving faster than hatchery-reared fish on average.
Segregation of salmonid populations is typically achieved by marking hatchery-produced fish and using migration barriers such as dams, weirs, or traps to selectively allow only unmarked wild fish access to spawning grounds (e.g. Mclean et al. 2003; Araki et al. 2007a).
Median annual upstream migration dates for baseline hatchery adults were between 10 and 24 January.
