Overlap in spawning habitat characteristics between two salmonids in relation to stream size: redd superimposition hypothesis on longitudinal species replacement

Abstract

Redd superimposition, spawning on a previous spawner's redd by a later spawner, reduces reproductive success of the previous spawner. Therefore, in streams where redd superimposition frequently occurs, late-spawning species have a competitive advantage over early-spawning species. We hypothesised that smaller channels of upper reaches would have higher potential of redd superimposition owing to lower availability of spawning habitat, thereby providing a competitive advantage to late-spawning species, to explain a displacement of native masu salmon ( Oncorhynchus masou ishikawae) by non-native white-spotted char ( Salvelinus leucomaenis) (late spawner) specific to small upper reaches in a Japanese river. We examined (1) the availability of spawning habitat and (2) overlap in spawning habitat characteristics between the two species in streams with different channel size. The results showed that (1) the habitat availability decreased upstream as channel size decreased, and (2) characteristics of spawning habitat highly overlapped between the two species in small channels, whereas those differed significantly between the two species in larger channels, supporting our hypothesis. Our results provide a new perspective on longitudinal changes in competitive advantages in salmonids.