Effects of Ocean Currents and Watershed Factors on the Fish Fauna of River Estuaries of a Peninsular Bordering Biogeographic Zone.

This study investigated the fish fauna diversity of the Shimokita and Tsugaru peninsulas in the northernmost part of the main island of Japan. Since ocean currents influence brackish and saltwater fish distribution and dispersion, it is assumed that in the target area where ocean currents are complex, the ichthyofaunal diversity of river estuaries may vary significantly between rivers. In addition, the target area is located on a peninsula with complex topography, and topographical factors are also considered major factors influencing the fish fauna. During the field survey, 6879 individuals of fish from 34 species were collected from 25 river estuarine areas. Furthermore, the cluster analysis results indicated that the Tsugaru Warm Current, which flows from the warm current (Tsushima Current) on the Sea of Japan side to the Pacific Ocean side, strongly influences the presence of fish fauna in the regional river estuarine areas. Moreover, the wave exposure was marginal in rivers flowing into the Tsugaru Strait (7.4 ± 5.8), and saltwater fishes were absent, whereas fishes that depend on the gravel bed were observed. The bed slope of the estuarine river area of this group was large (1.65 ± 0.99%). The riverbed comprised stone and gravel, suggesting that in addition to biogeography, environmental factors strongly influence the presence of fish fauna. Furthermore, compared with previous studies conducted in the Shimokita Peninsula, the distribution area of the southern Gymnogobius species expanded, possibly due to the increase in seawater temperature caused by climate change. In the target area, where currents and topography are complex, it became clear that the area into which the river flows and topographical factors determine the fish fauna of river estuaries, suggesting that elucidating the relationship between complex environmental factors and biota is important for riverine environmental management and conservation ecology. [ABSTRACT FROM AUTHOR]