Marine, brackish, freshwater habitats and the corresponding otolith Sr:Ca ratios: a review of the literature

Diadromous fishes may inhabit fresh, brackish or sea waters at different life history stages. However, it is difficult to precisely monitor these spatial dynamics due to the weaknesses of most current research methods. Numerous studies, including our own, have demonstrated that fish otolith microchemistry is a powerful tool to reconstruct the ambient environmental scalars of diadromous migration. Amongst otolithic strontium:calcium (Sr:Ca) ratios have been most widely applied to these cases. Because all of the Sr:Ca ratio studies have been conducted separately by different authors even using different analytical approaches, it is difficult to assess the agreement between these varying ratios, and, moreover, if these Sr:Ca ratio data can be summarized to a general reference base for the habitats of diadromous fishes. To fill the gap, we reviewed the Sr:Ca ratios of forty-three species reported in the literature so far. We actually found that the values of Sr:Ca x1000 are significantly different between the certain concentric zones in otolith, which reflect the fresh (2.4±1.5), brackish (5.4±0.6), and sea (8.0±4.4) waters of the fishes studied by different authors even using EPMA or LA-ICPMS. These data can be used as a general reference base for discrimination the habitat zones of diadromous fish otolith.