Salmonid fish often experience size-selective mortality when descending the river (i.e., seaward migration). However, it is unknown whether size-selective mortality is specific to this life history (i.e., migrants), or is shared by an alternative life history (i.e., residents). In this study, we investigated the size-dependent mortality patterns of masu salmon (Oncorhynchus masou) migrants and residents during the migration period (i.e., April to June) in the Horonai River, northern Japan. By conducting an individual-based study using PIT tags and antennas, we show that larger migrants more likely survived the seaward migration than smaller migrants, but size-dependent survival was not detected in river residents during the same period. These results suggest that size-selective mortality is specific to the river-descending migrants in masu salmon in their seaward migration period. We attribute this finding to the presence of piscivorous fishes (e.g., brown trout Salmo trutta) which occupy the migration corridor and consume masu salmon migrants, whereas such piscivorous fish do not occur in the river section farther upstream inhabited by residents.

鲑鱼在顺流而下（即向海洄游）时，通常会出现大小选择性死亡。然而，尚不清楚大小选择性死亡率是特定于这种生活史（即移民），还是由另一种生活史（即居民）共享。在这项研究中，我们调查了马苏鲑（Oncorhynchus masou) 迁移期间（即 4 月至 6 月）在日本北部 Horonai 川的移民和居民。通过使用 PIT 标签和天线进行基于个体的研究，我们表明与较小的移民相比，较大的移民更有可能在向海迁移中幸存下来，但在同一时期，河流居民中未检测到与大小相关的生存。这些结果表明，大小选择性死亡率是马苏鲑鱼在向海洄游期间顺流迁移的特有的。我们将此发现归因于食鱼鱼类（例如，褐鳟Salmo trutta）的存在，它们占据了洄游走廊并消耗了马苏鲑鱼的迁徙，而这种食鱼鱼类不会出现在居民居住的更上游的河流部分。