Combined effects of immigration potential and habitat quality on diadromous fishes

In river systems, considerable debate exists as to why small-scale conservation with a primary focus on habitat quality brings little improvement to biological communities. To reveal potential causes of the enigmatic consequences, further explorations should focus on how species' response to local environments is determined within a watershed. Here, we examined how immigration potential influences habitat associations of four diadromous fishes in the Shubuto River system in Hokkaido, Japan: sculpins (Cottus sp. ME and C. hangiongensis), gobies (Gymnogobius opperiens) and sea lamprey larvae (Lethenteron japonicum). We used closeness to the sea (i.e., distance from the sea) as a proxy for immigration potential, since the source of immigrants of the diadromous fishes is the river mouth. A multilevel modeling approach revealed that immigration potential had a "bottom-up effect" on local habitat-abundance relationships (current velocity and substrate coarseness), but habitat preferences did not vary along the gradient of immigration potential. This likely reflects the limited dispersal capabilities of the study species, whereby shorter travel distances may greatly enhance colonization success. Our study raises the possibility that the effectiveness of small-scale riverine conservation can be improved by selecting areas where successful colonization would be expected. Explicitly incorporating this consideration into conservation decision-making would help achieve greater success in environmental restoration and preservation actions.