Use of otolith microchemistry to estimate the migratory history of the threespine stickleback, Gasterosteus aculeatus

Ontogenic change patterns in otolith Sr:Ca ratios in the threespine stickleback, Gasterosteus aculeatus , collected from Japanese brackish waters and freshwater, were examined by wavelength dispersive X-ray spectrometry on an electron microprobe. Two-dimensional images of the Sr concentration in the otoliths showed a variety of patterns of Sr concentration relative to salinity of habitat in all specimens collected in Hokkaido Island. Otolith Sr:Ca ratios in line analysis of all specimens collected in Hokkaido Island fluctuated strongly during the life history transect in accordance with the migration (habitat) pattern from sea to freshwater, via brackish water. In contrast, the Sr concentration or the Sr:Ca ratios of the Otsuchi River stickleback remained at consistently low levels throughout the otolith (1·5×10 −3 in the Sr:Ca ratios). The higher ratios (3·2–6·3×10 −3 ) in sticklebacks collected in Hokkaido Island probably reflect the ambient salinity or the seawater–freshwater gradient in Sr concentration. The findings clearly indicated that otolith Sr:Ca ratios reflected individual life histories, and that the stickleback had a flexible migration strategy in the ambient water.