Steelhead Exhibit a Resident Life History and Repeat Spawning Traits

Expression of a resident life history and repeat spawning traits was documented for steelhead Oncorhynchus mykiss from the Clearwater River, Idaho.  Anadromous adults from a hatchery and wild population were artificially spawned in 1994 and 1995, and their progeny were released as unfed fry in a second order stream above a barrier blocking upstream fish passage.  The number of steelhead remaining in the stream was 840 (+72; 95% CI) in 1998, 418 (+60) in 1999, and 284 (+34) in 2000.  The 1995 year-class composed >93% of the steelhead in the stream in 1998 and 1999; the year-classes could not be accurately differentiated in 2000.  The percentage of steelhead that were flowing milt in August was 44% in 1998 (n flowing milt + not flowing milt = 458), 49% in 1999 (n = 171), and 31% in 2000 (n = 70).  Based on sub-samples, 16% of the fish not flowing milt were males and the rest females in 1998 (n males + females = 32), 42% were males in 1999 (n = 31), and 44% were males in 2000 (n = 25).  Gonadosomatic index (100*gonad weight/whole body weight) for males not flowing milt was never less than 4.5% in any of the three years, suggesting that all males in the stream were mature, even those not flowing milt in 1998.  Survival from 1998 to 1999 was 0.49 (SE = 0.22) for fish flowing milt and 0.36 (SE = 0.12) for fish not flowing milt; thus survival was no lower for mature males than for females which composed the majority of fish not flowing milt in 1998 and 1999.  Females matured and spawned for the first time in 1999 based on recovery of yearling offspring in 2000.  Whether females spawned repeatedly in later years is unknown.  Maintenance of a resident life history alternative may be particularly important for inland steelhead due to risks associated with long distance migrations to and from the ocean.