Comparison of Life History Traits between First‐Generation Hatchery and Wild Upper Yakima River Spring Chinook Salmon

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 = 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.