Fish otolith chemistry as an indicator of physiological, ecological and environmental events

Fish otoliths are calcium carbonate aggregates in the membranous labyrinth of all teleost fishes. The deposition of these structures is affected by both physiological and environmental factors which can cause changes in both the rate of otolith depositon and in the composition of the material deposited. The rate of otolith deposition, particularly in regard to the alternation of calcium carbonate-rich and protein-rich zones, has been widely investigated in an attempt to understand processes of ageing and growth in fishes. Aspects of the chemistry of these structures, particularly trace elements and stable isotopes, that may vary in response to physiological and environmental change, have not been studied in detail and are the basis of this research. The effects of temperature, somatic growth, otolith growth, condition factor, RNA/DNA ratio, age and season on the incorporation of Sr, Na, K and S into the sagittal otoliths of Australian salmon and blue grenadier were investigated by a combination of laboratory rearing experiments and monthly collections of wild fish. Microchemical analyses of otolith chemistry were carried out with a wavelength dispersive electron microprobe. There were significant differences in otolith Sr/Ca ratios among Australian salmon maintained in the laboratory at different temperatures and a slight positive correlation with temperature, but there was no evidence for a linear relationship between Sr/Ca ratio and temperature. Biologically significant relationships between other factors were not evident in laboratory-maintained fish. Furthermore, the variability of elemental ratios within temperature treatments and within individual otoliths was very significant. There were highly significant correlations between otolith chemistry and fish age in wild blue grenadier and it was hypothesized that the seasonal and age-related variation in otolith Sr content is largely the result of changes in the proportions of free and bound Ca and Sr present in the blood plasma and that this is in turn a function of the quantity and type of proteins present in the plasma. Data on the level of Sr present in the saccular endolymph and the sagittae of 12 fish species showed that there was a very strong relationship between the composition of the endolymph and the otoliths. Seasonal collections of otoliths, blood plasma, saccular endolymph and biological data from Pseudophycis barbatus indicated that physiology was largely responsible for changes in endolymph composition. Measurements of weight, length, gonad weight, and Sr, Ca, Na, K, protein, triglyceride, phosphate and glucose in the plasma and endolymph were used to develop multivariate models to explain endolymph and otolith composition. It was possible to explain up to 98% of the variance in the endolymph Sr content of female Pseudophycis barbatus. The range of otolith Na and K content could be estimated using models in the geochemical literature. These results showed that otolith trace element composition was based on the interaction of physiological, ecological and environmental factors. Life-history transects of otolith microchemistry in a range of species indicated that the factors that ultimately effect otolith composition are generally not under strong environmental control. However, it was found that variations in otolith microchemistry can be useful in studies aimed at determining distributional relationships among contemporaneous individuals, particularly in studies seeking to identify nursery grounds or where there is an interest in fine resolution "stock" discrimination. Recognizable "signatures" in otolith life-history transects may result when co-occurring individuals experience extreme environmental conditions. In most species there appears to be little information that can be gained directly from the interpretation of otolith Sr/Ca, Na/Ca, K/Ca and S/Ca ratios. The complex interaction of factors affecting trace element levels in fish otoliths makes it virtually impossible to determine those factors which result in a particular quantity of Sr, Na, K or S in an otolith or, to use these elements as indicators of physiological or environmental change. Otolith microchemistry of anadromous and nonanadromous salmonids was investigated to determine if there were differences among migratory and nonmigratory individuals and to determine if the habitat where vitellogenesis took place would affect the composition of the otolith primordia of the progeny. There were significant differences in otolith Sr/Ca ratios among adult anadromous and nonanadromous salmonids and the otolith Sr/Ca ratios in the primordia were greater in the progeny of anadromous salmonids than in the otolith primordia of the progeny of nonanadromous individuals. Studies of oxygen and carbon isotopes in the otoliths of laboratory maintained Arripis trutta showed that oxygen isotopes were deposited near to equilibrium with seawater and, contrary to evidence presented in the literature, can be used to predict environmental temperatures. Approximately 30% of the otolith carbon was from metabolically derived sources. Oxygen isotopes in otoliths from a wide range of fish species were found to be deposited in equilibrium with seawater, while there are varying levels of carbon isotope disequilibria. The hypothesis that the magnitude of carbon isotope disequilibria is related to metabolic rate (V02 ) was developed and evidence in support of this hypothesis was presented. Scanning and transmission electron microscope studies of fish otoliths showed the relationship between organic and inorganic material in microincrements and presented evidence for the complete cessation of calcium carbonate deposition during periods of stress. Ultrastructural observations showed that aragonite crystals in fish otoliths are highly variable in morphology and size and, thus, may be an important factor in determining within otolith variations in trace element chemistry