Juvenile life history, migration, and habitat use of natural- versus hatchery-origin Chinook salmon

Objectives: Our overall objective was to directly compare the early life history and migration patterns of hatchery and natural-origin fish and determine when hatchery fish provide good surrogates for natural-origin winter-run Chinook Salmon (Oncorhynchus tshawytscha) in the Sacramento River system. Specifically, our objectives were to compare the juvenile freshwater habitat use, residence time in freshwater habitats, and size-at-age and relative growth of juveniles.

Methods: Using otoliths collected from adult carcasses, we reconstructed the freshwater life history of hatchery and natural-origin fish through microstructural and chemical analyses. We analyzed the otolith strontium isotope chemistry and daily increment widths to characterize habitat use, migration timing, and growth.

Results: Following their hatchery rearing period, hatchery fish had much shorter residence times (11 days, SD = 16 days) in the river than natural-origin fish (129 days, SD = 37 days). Beyond the reduced duration of river residence, hatchery fish occupied fewer areas of the watershed during ocean outmigration. Despite these differences in outmigration behaviors, adults from both groups reached saltwater during their downstream outmigration at similar sizes (natural-origin: 633 µm, SD = 68 µm; hatchery-origin: 625 µm, SD = 63 µm) and ages (natural-origin: 161 days old, SD = 20 days; hatchery-origin: 165 days old, SD = 22 days) as juvenile smolts.

Conclusions: Considering differences in habitat use and timing between hatchery and natural-origin fish is critical to avoiding bias when applying data collected from hatchery fish to manage and conserve natural populations. Explicitly considering the presence of winter-run Chinook Salmon in habitats beyond the migratory corridor during habitat management is needed to promote and enable the contribution of multiple life histories for productivity, and resilience, and ultimately recovery.

研究目标：本研究的核心目标为直接对比孵化场来源鱼类与野生来源鱼类的早期生活史与洄游模式，并明确在萨克拉门托河水系中，孵化场繁育鱼类可作为冬季洄游型奇努克鲑（Oncorhynchus tshawytscha）野生种群有效替代者的适用场景。具体研究目标包括：对比幼鱼的淡水生境利用情况、淡水生境居留时长，以及幼鱼的年龄体尺与相对生长速率。

研究方法：本研究利用采集自成鱼尸骸的耳石，通过微结构与化学分析手段重建孵化场来源与野生来源鱼类的淡水生活史。具体通过分析耳石的锶同位素化学特征与日生长增量宽度，表征其生境利用、洄游时序与生长情况。

研究结果：在结束孵化场育苗阶段后，孵化场来源鱼类在河流中的居留时长（11天，标准差SD=16天）远短于野生来源鱼类（129天，SD=37天）。除河流居留时长缩短外，孵化场来源鱼类在入海洄游过程中所利用的流域生境面积也更少。尽管二者在洄游行为上存在上述差异，但两组成鱼在作为降海幼鲑进行下游洄游入海时，到达咸水环境的体尺（野生来源：633微米，SD=68微米；孵化场来源：625微米，SD=63微米）与日龄（野生来源：161日龄，SD=20日龄；孵化场来源：165日龄，SD=22日龄）均无显著差异。

研究结论：在利用孵化场来源鱼类的相关数据开展野生种群管理与保护工作时，充分考虑两类鱼类在生境利用与时序上的差异，对避免研究偏差至关重要。在生境管理过程中，需明确关注洄游廊道以外生境中的冬季洄游型奇努克鲑种群，以推动并实现多种生活史策略对种群生产力、恢复力的贡献，最终助力种群复壮。