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Seasonal variation in the chemistry of bearded rock cod Pseudophycis barbatus plasma. endolymph and otoliths was investigated to determine the effects of physiology and environment on the trace element composition of otoliths. Cod blood, endolymph and otoliths were collected from freshly caught fish at sea. Quantities of protein, triglycerides, phosphate, glucose, calcium, sodium, potassium and strontium were determined in the blood plasma and the endolymph. These data, in conjunction with temperature data and basic biological information such as length, weight, sex, gonad weight, condition and age, were used to explain the levels of Sr found in cod otoliths. Using multiple regression models, it was possible to explain up to 98.5 % of the total variance in female endolymph Sr levels on the basis of basic biological data and data on blood plasma chemistry. The results indicated that temperature does not directly affect the Sr, Na, K or S content of otoliths but, in adults, that changes in temperature are associated with major physiological changes, specifically the development of the gonads. These seasonal physiological changes are associated with changes in the chemistry of the blood plasma and the endolymph, which can result in differences in the trace element composition of otolith material. A brief investigation of the variability in the relative protein composition of the plasma indicated that there are large intraspecific and seasonal variations In protein complement, notably the albumins and globulins. These changes may ultimately affect the composition of endolymph and otoliths due to differences in metal-binding capacity. This is an area of potential further investigation. Data presented here show that the interpretation of otolith microchemistry data cannot be based on temperature data alone and that variations in otolith chemistry are due to the interaction of numerous factors. In the case of bearded rock cod, the negative correlation between temperature and otolith Sr is a fortuitous one, with changes in physiology being the major force. In other species changes in physiology similar to those observed here are likely to occur at very different times of the year Therefore, before the interpretation of otolith microchemistry data is undertaken, the researcher must carefully consider the various environmental and physiological factors unique to each species.

marine ecology. progress series

Determinants of otolith chemistry seasonal variation in the composition of blood plasma, endolymph and otoliths of bearded rock cod Pseudophycis barbatus