Formation of a stress-induced chemical check in fish otoliths

Abstract

The effect of physiological stress on the Sr/Ca, S/Ca, Na/Ca, and K/Ca ratios measured in the sagittae of Australian salmon Arripis trutta Bloch et Schneider, a marine percoid teleost, was determined for a group of fish trapped in a stream with a lower salinity and higher temperature than the adjacent sea. Abnormally low conditions factors for these fish, when compared with other extensive collections of Australian salmon, provided evidence that they stressed. Life-history transects of Sr/Ca, S/Ca, Na/Ca, and K/Ca ratios made with a wavelength dispersive electron microprobe showed that there were consistent changes in otolith trace chemistry at the otolith edge. Both the Sr/Ca and S/Ca ratios increased at the otolith edge, whereas the Na/Ca and K/Ca ratios decreased. Similar changes in otolith chemistry were not evident in Australian salmon collected in nearby marine habitats. The results indicate that stress, in a general sense, can result in the incorporation of increased levels of Sr in fish otoliths, and that it can also affect other trace element levels in a systematic manner. Stress can produce a recognizable chemical check on fish otoliths and these marks may be useful as natural markers for age validation and stock discrimination studies.