Data from: Validity of inferring size-selective mortality and a critical size limit in Pacific salmon from scale circulus spacing

Size-selective mortality owing to lack of energy reserves during the first marine winter has been suggested to be a result of juvenile salmon failing to reach a critical size or condition by the end of their first marine summer and not surviving the following winter due to this presumed energy deficit. This hypothesis implies a knife-edge mortality function based upon size, and is subject to empirical data support for acceptance. Scale circulus spacing has been interpreted as an index for body size, and we reviewed the effect of size-selective mortality with a knife-edge mortality function on descriptive statistics for a scale circulus spacing index (SCSI). In order to invoke size selection as an important driver of mortality during the first year of ocean rearing, it is necessary to demonstrate not only that size-selective mortality is directed towards the smaller members of the population, but that the selective nature of the mortality can account for a substantial portion of the observed mortality. If the assumption is made that a random sample of a single juvenile population has been obtained, then studies that employ a SCSI to infer size-selective mortality coupled with a critical size limit must demonstrate a shift toward larger values of the SCSI, but also a concomitant reduction in the variance and range of the SCSI and an increase in the skewness and kurtosis of the SCSI values. Through simulation we found that the percentage of adults that displayed a SCSI value greater than the maximum observed in the juvenile sample was highly dependent on the initial juvenile sample size and size-selective mortality rate. Geographical distributions of juvenile Pacific salmon can be stratified by size, with larger individuals migrating earlier from local ocean entry locations than smaller individuals, and thus differential timing migration of juveniles based upon body size prior to the collection of the marine juvenile sample may be a more plausible explanation of published trends in the SCSI, rather than invoking substantial size-selective mortality and a critical size limit.有研究提出，幼鲑在首个海洋越冬期因能量储备不足发生的大小选择性死亡（size-selective mortality），源于其在首个海洋夏季结束时未能达到临界体长或生理状态，并因此因能量亏缺无法度过后续冬季。该假说提出了一种基于体长的刀刃式死亡函数（knife-edge mortality function），其有效性需得到实证数据的支持。鳞片环纹间距（scale circulus spacing）被视为体长的替代指标，我们针对刀刃式死亡函数下的大小选择性死亡对鳞片环纹间距指数（SCSI）描述性统计量的影响展开了综述。为证实大小选择是海洋育幼期第一年死亡的关键驱动因素，不仅需要证明大小选择性死亡倾向于作用于种群中的小型个体，还需证明该选择性死亡能够解释观测到的大部分死亡事件。若假设已获取某一幼鲑种群的随机样本，则采用SCSI结合临界体长限制来推断大小选择性死亡的研究，不仅需要证明样本向SCSI更大的方向偏移，还需同时伴随SCSI方差与极差的缩减，以及SCSI数值偏度与峰度的升高。通过模拟实验我们发现，成体中SCSI值高于幼鲑样本观测最大值的个体占比，高度依赖于初始幼鲑样本量与大小选择性死亡率。太平洋鲑（Pacific salmon）幼体的地理分布可按体长分层，体型更大的个体会比小型个体更早从当地海洋洄游入口处开始洄游。因此，在采集海洋幼体样本前，基于体长的幼体洄游时间差异，或许比引入显著的大小选择性死亡与临界体长限制，更能合理解释已发表的SCSI观测趋势。