Migratory history of the threespine stickleback Gasterosteus aculeatus

The migratory history of two highly divergent forms (the Japan Sea and Pacific Ocean forms) of the threespine stickleback Gasterosteus aculeatus collected from Japanese brackish water (seawater) and freshwater was studied by examining strontium (Sr) and calcium (Ca) concentrations in their otoliths using wavelength dispersive X-ray spectrometry on an electron microprobe. The Sr:Ca ratios in the otoliths changed with salinity of the habitat. The otolith Sr:Ca ratios of the freshwater resident-type samples of the Pacific Ocean form showed consistently low Sr:Ca ratios, averaging 0.85-0.96 x 10(-3) from the core to the edge. In contrast, the otolith Sr:Ca ratios of the anadromous type of both the Japan Sea and Pacific Ocean forms fluctuated strongly along the life history transects in accordance with their migration patterns from seawater to freshwater. The higher ratios in the anadromous type, averaging 5.4 x 10(-3), in the otolith region from the core to 200 mum, corresponded to the seagoing period, suggesting that otolith Sr:Ca ratios are affected by ambient water salinity. These findings clearly indicate that otolith Sr:Ca ratios reflect individual life histories, and that these two highly divergent forms of stickleback have a flexible migration strategy.