Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, California, United States of America.
PLoS One. 2012;7(2):e28880. doi: 10.1371/journal.pone.0028880. Epub 2012 Feb 8.
Maintaining viable populations of salmon in the wild is a primary goal for many conservation and recovery programs. The frequency and extent of connectivity among natal sources defines the demographic and genetic boundaries of a population. Yet, the role that immigration of hatchery-produced adults may play in altering population dynamics and fitness of natural populations remains largely unquantified. Quantifying, whether natural populations are self-sustaining, functions as sources (population growth rate in the absence of dispersal, λ>1), or as sinks (λ<1) can be obscured by an inability to identify immigrants. In this study we use a new isotopic approach to demonstrate that a natural spawning population of Chinook salmon, (Oncorhynchus tshawytscha) considered relatively healthy, represents a sink population when the contribution of hatchery immigrants is taken into consideration. We retrieved sulfur isotopes ((34)S/(32)S, referred to as δ(34)S) in adult Chinook salmon otoliths (ear bones) that were deposited during their early life history as juveniles to determine whether individuals were produced in hatcheries or naturally in rivers. Our results show that only 10.3% (CI = 5.5 to 18.1%) of adults spawning in the river had otolith δ(34)S values less than 8.5‰, which is characteristic of naturally produced salmon. When considering the total return to the watershed (total fish in river and hatchery), we estimate that 90.7 to 99.3% (CI) of returning adults were produced in a hatchery (best estimate = 95.9%). When population growth rate of the natural population was modeled to account for the contribution of previously unidentified hatchery immigrants, we found that hatchery-produced fish caused the false appearance of positive population growth. These findings highlight the potential dangers in ignoring source-sink dynamics in recovering natural populations, and question the extent to which declines in natural salmon populations are undetected by monitoring programs.
维持野生鲑鱼种群的生存能力是许多保护和恢复计划的主要目标。亲源来源之间的连接频率和范围决定了种群的人口统计学和遗传边界。然而，养殖成年鱼的移民可能在改变自然种群的动态和适应性方面所起的作用在很大程度上仍未被量化。量化自然种群是否是自我维持的，作为来源（在没有扩散的情况下种群的增长率，λ>1），还是作为汇（λ<1），可能会因为无法识别移民而变得模糊。在这项研究中，我们使用一种新的同位素方法来证明，当考虑到养殖移民的贡献时，一个被认为相对健康的野生奇努克鲑鱼（Oncorhynchus tshawytscha）的自然产卵种群是一个汇种群。我们从成鱼的耳骨（耳石）中提取了硫同位素（(34)S/(32)S，称为δ(34)S），以确定个体是在养殖场还是在河流中自然产生的。我们的研究结果表明，只有 10.3%（置信区间为 5.5 至 18.1%）在河里产卵的成鱼耳石的δ(34)S 值小于 8.5‰，这是自然产生的鲑鱼的特征。当考虑到返回流域的总鱼量（河里的鱼和养殖场的鱼）时，我们估计返回的成年鱼中有 90.7%至 99.3%（置信区间）是在养殖场中产生的（最佳估计值为 95.9%）。当考虑到自然种群的增长率以适应先前未识别的养殖场移民的贡献时，我们发现养殖鱼导致了自然种群正增长率的虚假出现。这些发现强调了在恢复自然种群时忽略源汇动态的潜在危险，并质疑监测计划未能检测到自然鲑鱼种群下降的程度。
