Migratory histories of three types of Cottus pollux (small-egg, middle-egg, and large-egg types) as revealed by otolith microchemistry

Abstract

 Migratory histories of three types of Cottus pollux, the small-egg type (SE type), middle-egg type (ME type), and large-egg type (LE type), were studied by examining strontium (Sr) and calcium (Ca) in their otoliths with wavelength dispersive X-ray spectrometry on an electron microprobe. The Sr : Ca ratios in the otoliths changed both with ontogenetic development and with salinity of the habitat. Otolith Sr : Ca ratios of LE-type samples and the ME-type samples from the Honmyo River, Kyushu Island, showed consistently low ratios, averaging 1.8 × 10⁻³ and 2.4 × 10⁻³ from the core to the edge, respectively. In contrast, otolith Sr : Ca ratios of SE-type samples and the other four ME-type samples from Hokkaido and Honshu Islands fluctuated strongly along the life history transects in accordance with migration patterns from freshwater to the sea and vice versa. The otolith Sr : Ca ratios of SE-type samples showed low ratios from the core to a point around 15 μm, averaging 1.5 × 10⁻³, and subsequently increased sharply with a high Sr : Ca ratio phase to a point around 400 μm, averaging 5.5 × 10⁻³, and followed again a low ratio phase to the edge with averages of 3.1 × 10⁻³. Similar fluctuation patterns in otolith Sr : Ca ratios were found for the four ME-type samples. These findings clearly demonstrated that otolith Sr : Ca ratios reflected the sculpin's life histories, as being fluvial for the LE type and the Honmyo River ME type and amphidromous for the SE type and the other four populations of ME type.