Potential application of otolith Sr:Ca ratios as indicators of brown-algae feeding histories in herbivorous fishes

Brown algae (Phaeophyceae) are the largest and most ecologically important group of marine algae, serving as habitat-forming organisms shaping coastal ecosystems. Understanding the feeding pressure exerted by herbivorous fishes on brown algae is essential for elucidating the dynamics of both coral reefs and algal forests. This study focused on the unique ability of brown algae to accumulate strontium (Sr) and tested the hypothesis that frequent consumption of brown algae elevates otolith Sr:calcium (Ca) ratios in herbivorous fishes. We analysed stomach contents and otolith Sr:Ca ratios in four sympatric species collected in southwestern Japan. Among three Kyphosus species, higher Sr:Ca ratios were associated with the frequency of brown-algae consumption. Notably, K. bigibbus, a brown-algae specialist, exhibited an exceptionally high otolith Sr:Ca ratio (0.12), far exceeding the common range of marine fishes (0.004–0.02). In K. vaigiensis, Sr:Ca ratios were high in areas where brown algae were frequently consumed, whereas they were moderate in areas where red algae were predominantly consumed. In K. cinerascens, only the latter pattern was observed. Both patterns are consistent with our hypothesis. Furthermore, the results suggest that K. bigibbus begin feeding on brown algae at approximately 1 year of age. By contrast, Siganus fuscescens exhibited Sr:Ca ratios within the common range, despite frequent feeding on brown algae, deviating from this hypothesis. Therefore, otolith Sr:Ca ratios may serve as a species-specific proxy for reconstructing feeding histories on brown algae, providing a novel tool for investigating herbivory over several decades in long-lived species.