The present study examined the effects of chemical antipredator conditioning on antipredator behavior and the relative effects of antipredator conditioning and seminatural rearing environments on postrelease survival of chinook salmon (Onocrhynchus tshawytscha). Hatchery-reared juvenile chinook salmon were exposed to extracts from conspecific tissue or to comparable stimuli from green swordtail (Xiphophorus helleri). These "injured fish" stimuli were paired with water that contained the odour of predatory cutthroat trout (Oncorhynchus clarki). Chinook salmon receiving conspecific stimuli showed higher levels of several antipredator behaviors compared with chinook salmon receiving green swordtail extracts. When the two groups of chinook salmon were tested 2 days later with cutthroat trout stimulus alone, the chinook salmon that had originally received injured conspecific stimuli paired with cutthroat trout odour spent more time motionless than chinook salmon that had received green swordtail stimuli and cutthroat trout odour. In another experiment, complex rearing treatments had a negative effect on instream survival (contrary to previous studies) that was compensated for by the application of the chinook salmon extract and cutthroat trout odour prior to release. Chinook salmon, like rainbow trout (Oncorhynchus mykiss), show antipredator behavior in response to chemical stimuli from injured conspecifics and learn predator recognition when such stimuli are paired with predator odour, improving survival in the wild.

Abstract.-Life history traits in hatchery and wild spring Chinook salmon Oncorhynchus tshawytscha from the upper Yakima River were compared to determine whether locally adapted traits had diverged after one generation of state-of-the-art artificial propagation. Sex composition in wild-and hatchery-origin fish differed in three of four brood years (P 0.01). The proportion of hatchery males, primarily age 3, increased from 38% to 49% over time. Conversely, the sex composition of wild fish did not exhibit a similar linear trend. Most hatchery-and wild-origin fish reached maturity at age 4 (!76%), followed in magnitude by ages 3 and 5. Wild mean age at maturation demonstrated no significant trend over time, while hatchery mean age at maturation declined (P ¼ 0.05). Mean lengths of 3-5-year-old hatchery fish were shorter than those of wild fish of the same age (differences of 2.7 cm for age 3, 1.7 cm for age 4, and 1.9 cm for age 5). Likewise, body weights of hatchery fish were lower than those of wild fish (differences of 0.3 kg for age 3, 0.3 kg for age 4, and 0.6 kg for age 5), representing a change in body size of between 0.5 and 1.0 standard deviation (SD). Median arrival timing of hatchery and wild fish at a broodstock collection site just downstream of ancestral spawning grounds showed no consistent difference. However, the median arrival date of age-3 fish was 19-20 d later than that for fish of ages 4 and 5 (P , 0.01). Mean spawn timing of hatchery fish was significantly earlier (5.1 d) than that of wild fish in a ''common-garden'' experiment (P , 0.05). We estimate that fitness could be reduced by as much as 1-5% for traits diverging from their optima by 0.5-1.0 SD. The degree of genetic determination of the divergence is unknown, but future monitoring will help clarify this. Perhaps the most important conclusion of our study is that even a hatchery program designed to minimize differences between hatchery and wild fish did not produce fish that were identical to wild fish.

In male Chinook Salmon Oncorhynchus tshawytscha, age of maturation is phenotypically plastic, occurring at age 1 (referred to as precocious parr or microjack), age 2 (minijack), age 3 (jack), age 4, or age 5. Microjacks and minijacks are thought to forego migration to the ocean as smolts, instead remaining in headwaters and employing a “sneaking” strategy to fertilize eggs. We compared the prevalence of minijacks (minijack rate) among hatchery‐ and natural‐origin spring Chinook Salmon from the Yakima River, Washington, over seven brood years (2001–2007). We quantified minijack rates and sex ratios in the hatchery population prior to release and during out‐migration at a trap located 230 km downstream. Within this time period, we also monitored minijack rates in a 3‐year (brood years 2002–2004) growth study designed to reduce minijack production at the hatchery. Minijacks made up an average of 41% of the male population in the hatchery, but annual minijack rates varied in response to the growth rate or fish size at release. Average minijack rate was approximately 20% among out‐migrating hatchery fish, about half the rate found prior to release. Among out‐migrants, minijack rates of hatchery fish were approximately 10 times those of natural‐origin fish, but sex ratios were significantly skewed toward females in both hatchery‐ and natural‐origin groups. Data from this study and related studies suggest that the predominant age of early male maturation in the Yakima River and similar rivers is age 2 (minijack) in hatchery fish and age 1 (microjack) in natural‐origin fish. Based on this and other studies, we now recognize three minijack life history types in spring Chinook Salmon: resident, fluvial, and anadromous, depending on the migration pattern exhibited in the spring and summer. Finally, we discuss the broader impacts that high minijack production may have on the establishment of size‐at‐release targets for salmon supplementation programs in the future.

The Cle Elum Supplementation and Research Facility in the Yakima River basin, Washington, is an integrated spring Chinook Salmon Oncorhynchus tshawytscha hatchery program designed to test whether artificial propagation can increase natural production and harvest opportunities while keeping ecological and genetic impacts within acceptable limits. Only natural‐origin (naturally spawned) fish are used for hatchery broodstock. Spawning, incubation, and early rearing occur at a central facility; presmolts are transferred for final rearing, acclimation, and volitional release at sites adjacent to natural spawning areas, where returning adults can spawn with natural‐origin fish. The first wild broodstock were collected in 1997, and age‐4 adults have returned to the Yakima River since 2001. An unsupplemented population in the adjacent Naches River watershed provides a reference for evaluating environmental influences. The program has been comprehensively monitored from its inception. A synthesis of findings, many already published, is as follows: supplementation increased the harvest, redd counts, and spatial distribution of spawners; natural‐origin returns were maintained; straying to nontarget systems was negligible; natural‐origin females had slightly higher breeding success (production of surviving fry) in an artificial spawning channel, while the behavior and breeding success of natural‐ and hatchery‐origin males were similar; hatchery‐origin fish showed differences in morphometric and life history traits; high rates of hatchery age‐2 (minijack) production were reported, but the observed proportions of out‐migrating juvenile and adult (ages 4 and 5) returning males were comparable for hatchery‐ and natural‐origin fish; hatchery smolts did not affect the levels of pathogens in natural smolts; and the ecological interactions attributed to the program were within adopted guidelines. Continued study is required to assess the long‐term impacts on natural production and productivity.

Captive breeding has the potential to rebuild depressed populations. However, associated genetic changes may decrease restoration success and negatively affect the adaptive potential of the entire population. Thus, approaches that minimize genetic risks should be tested in a comparative framework over multiple generations. Genetic diversity in two captive-reared lines of a species of conservation interest, Chinook salmon (Oncorhynchus tshawytscha), was surveyed across three generations using genome-wide approaches. Genetic divergence from the source population was minimal in an integrated line, which implemented managed gene flow by using only naturally-born adults as captive broodstock, but significant in a segregated line, which bred only captive-origin individuals. Estimates of effective number of breeders revealed that the rapid divergence observed in the latter was largely attributable to genetic drift. Three independent tests for signatures of adaptive divergence also identified temporal change within the segregated line, possibly indicating domestication selection. The results empirically demonstrate that using managed gene flow for propagating a captive-reared population reduces genetic divergence over the short term compared to one that relies solely on captive-origin parents. These findings complement existing studies of captive breeding, which typically focus on a single management strategy and examine the fitness of one or two generations.