Nesting habitat use and partitioning of three sympatric ninespine sticklebacks (genus <Emphasis Type="Italic">Pungitius</Emphasis>): implications for reproductive isolation

The nest site characteristics and distribution patterns of nests of three sympatric ninespine sticklebacks, the brackish- and fresh-water types and Pungitius tymensis, were investigated in the Shiomi River, eastern Hokkaido, Japan. In this river, most of the males of the brackish-water type were found to build their nests in the lower reach with higher salinity (mean 13.5‰) and water temperature (mean 13.3°C), whereas the males of the freshwater type and P. tymensis built their nests in the upper reach with lower salinity (mean 0.6 and 0.8‰, respectively) and water temperature (both mean 11.3°C). The ranges of their nesting areas, however, overlapped within the river, because the nests of the brackish-water type were widely distributed in the river. A comparison of the nest site characteristics in the area where nest overlapped indicated that there were no significant differences for all of five characteristics (the distance from the nest to the shore, the distance from the nest to the bottom, the distance to the nearest nest, the nest density and the amount of cover around the nest). Using canonical discriminant analysis, the nests of the three sticklebacks could not be distinguished using the five nest characteristics. These results suggest that habitat isolation through physical factors functions as an important mechanism of reproductive isolation between the brackish-water type and the other two sticklebacks.