Stable isotope records from otoliths as tracers of fish migration in a mangrove system

The ratios of stable isotopes O-18:O-16 and C-13:C-12 were measured in otolith carbon taken from nine species of fishes caught within mangroves and on the reef at Gazi Bay, Kenya. Before analysis, otoliths were divided into 'larval' 'post-larval' and 'adult' sections using a drill. Fishes were putatively classified as 'mangrove residents' 'offshore residents' or 'migrants' on the basis of information from the literature, and depending on where they were caught (mangroves only, offshore only or both mangroves and offshore) in the present study. Eight of the species exhibited an increase in otolith C-13:C-12 with age, but this was significant only in the two migrant species Lethrinus harak and Lutjanus fulviflammus. There were no consistent patterns in O-18:O-16 with age, or between migrants and non-migrants. These results suggest that comparing absolute values of otolith oxygen and carbon isotope signatures between fish species is not a useful way of determining migration patterns at this site, because of species-specific differences in carbon metabolism and insufficiently steep gradients in temperature and salinity. Changes in carbon isotope signatures between life stages within a species, however, do hold promise as migration tracers.