OXYGEN AND CARBON STABLE ISOTOPES IN THE OTOLITHS OF WILD AND LABORATORY-REARED AUSTRALIAN SALMON (ARRIPIS-TRUTTA)

Australian salmon, Arripis trutta, collected from the east coast of Tasmania, Australia, in 1987, were weighed and measured and their otoliths marked by immersing fish in an oxytetracycline hydrochloride/seawater solution before placement in constant-temperature aquaria. Individual somatic and otolith growth rates were determined for input into mass balance models. Mass balance models were used to determine the oxygen and carbon isotopic composition of otolith material produced during captivity. There was a significant relationship between delta-O-18 measured in the otolith aragonite and ambient temperature (r2 = 0.77). The linear relationship between these data, where delta-O-18 = 6.69 - 0.326 (T, degrees-C), was not significantly different from a relationship indicative of equilibrium deposition of oxygen isotopes in aragonite. Otolith carbon was significantly depleted in C-13 relative to equilibrium deposition, with depletions > 6.0 parts per thousand at all temperatures. There was no relationship between delta-C-13 and temperature. It was estimated that > 30% of the otolith carbon was from metabolically derived sources. Significant differences in otolith carbon isotopes among wild juvenile Australian salmon were hypothesised to be attributable to differences in diet. Levels of variability for both oxygen and carbon isotopes in laboratory-maintained and wild fish were similar to that found by other researchers for foraminifera and these results highlight the importance of large sample sizes when estimating environmental temperatures from oxygen isotopes measured in fish otoliths.