Adult spring chinook salmon (Oncorhynchus tshawytscha) returning to the Little White Salmon National Fish Hatchery (NFH) by about 1 June are subjected to a reduced photoperiod regime which causes accelerated maturation. These adults are spawned about 4 weeks earlier than untreated fish and by October of the following year the resulting progeny weigh nearly twice as much as juveniles from normally spawned adults. In contrast to normal juveniles, those from the accelerated spawning display behavioral and physiological signs of smolting as subyearlings in the first spring following hatching (Zaugg et al. 1986).

In a study designed to compare survival to adulthood, subyearlings tagged with coded-wire tags were released in May and June 1984 and 1985 while yearling fish were released those same years at their normal release date in April. Recoveries from yearlings were higher than from subyearlings for both years (Table 1), with releases in 1985 contributing a higher percent of adults than those in 1984.

Fish released as yearlings returned to the hatchery in a higher ratio of females to males than did fish released as subyearlings (Table 2). Also, only one of 119 returning males from the subyearling releases returned as a jack (2 years old), whereas 14 of 89 returning males (16%) from the yearling releases were jacks (3 years old).

Sixty-six percent of adults recovered from releases of yearling fish in 1984 and 1985 were 4-year-old fish, 30% were 5-year-olds, and the remaining 4% were 3-year-old jacks (Table 3). This compares with 80% of the adults from subyearling releases recovered as 4-year-olds and 20% as 3-year-olds. Regardless of age at release, the majority of adults matured at age 4 years.

An examination of the lengths of adults returning to the hatchery indicated that 4-year-olds from the subyearling releases were about the same size as 5-year-old adults returning from fish released as yearlings (Table 4). Average length of 4-year-old adults (77 cm) released as yearlings was slightly greater than 3-year-old adults returning from the subyearling releases (72 cm).

The release of subyearling spring chinook salmon has become an integral part of the production program at the Little White Salmon NFH. It has been cost effective and has resulted in an increase of returning adults for both the local fishery and as a source of additional spawners.

Zaugg, W. S., J. E. Bodle, and J. E. Manning. 1986. Smolt transformation and seaward migration in 0-age progeny of adult spring chinook salmon (Oncorhynchus tshawytscha) matured early with photoperiod control. Can. J. Fish. Aquat. Sci. 43:885-888.

Table 1. Adult recoveries from juvenile chinook salmon released from the Little White Salmon Hatchery in 1984 and 1985.

^aRecovered in the fishery and returns to the hatchery, based on 50,000 released.

Table 2. Numbers of tagged female and male adult salmon returning to the Little White Salmon Hatchery.

Table 3. Age distribution of adult salmon taken in the fishery and returning to the hatchery from releases in 1984 and 1985.

Table 4. Mean fork lengths (cm) of adult salmon returning to the hatchery from releases in 1984 and 1985 (number of fish measured in parentheses).

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