Predicting Riverbed Particle Size Reveals Spatial Patterns of Fragmentation in Salmonid Spawning Habitats Across Multiple Basins

Habitat fragmentation in freshwater ecosystems severely hinders the reproduction of migratory aquatic species by obstructing their spawning migrations. Given the widespread presence of artificial barriers, strategically allocating restoration efforts based on the distribution of suitable spawning habitats is essential. As substrate particle size is a critical factor in determining spawning habitat for many freshwater fish species, mapping suitable spawning habitats for target species based on this factor can be an effective approach for prioritizing habitat restoration. In this study, we assessed the impact of habitat fragmentation on spawning habitats for anadromous salmonids and identified habitats that should be prioritized for restoration. We did so by predicting the median particle size of riverbed materials across Hokkaido Island, northern Japan, using a machine learning algorithm. In addition, we verified through field surveys whether the predicted particle size was related to the observed substrate coarseness, and whether salmonids were spawning in the identified spawning habitats. The median particle size was mainly explained by the average elevation of river segments and the mean slope of the upstream catchment area. Field validation confirmed that observed substrate coarseness was related to the median particle size, and salmonids predominantly spawned in the predicted spawning habitats at the river segment scale. Overlaying barrier data onto the spawning habitat map revealed that 68.12% of suitable spawning habitats on Hokkaido Island are isolated. These results demonstrate that predicting particle size is an effective approach to assessing the impacts of fragmentation on spawning habitats of anadromous salmonids at a broad scale, and that the majority of suitable habitats are fragmented. Based on these results, priority should be given to restoring connectivity in rivers where large portions of spawning habitats are isolated from anadromous salmonids.