Summary1. Trace element concentrations in fish earstones ('otoliths') are widely used to discriminate spatially discrete populations or individuals of marine fish, based on a commonly held assumption that physiological influences on otolith composition are minor, and thus variations in otolith elemental chemistry primarily reflect changes in ambient water chemistry. 2. We carried out a long-term (1-year) experiment, serially sampling seawater, blood plasma and otoliths of mature and immature European plaice (Pleuronectes platessa L.) to test relationships between otolith chemistry and environmental and physiological variables. 3. Seasonal variations in otolith elemental composition did not track seawater concentrations, but instead reflected physiological controls on metal transport and biokinetics, which are likely moderated by ambient temperature. The influence of physiological factors on otolith composition was particularly evident in Sr/Ca ratios, the most widely used elemental marker in applied otolith microchemistry studies. Reproduction also triggered specific variations in otolith and blood plasma metal chemistry, especially Zn/Ca ratios in female fish, which could potentially serve as retrospective spawning indicators. 4. The influence of physiology on the trace metal composition of otoliths may explain the success of microchemical stock discrimination in relatively homogenous marine environments, but could complicate alternative uses for trace element compositions in biominerals of higher organisms.

Most studies that infer geographic distributions of fish using otolith microchemistry assume that environmental factors (e.g. temperature, salinity) outweigh intrinsic effects (e.g. size, condition); however, this assumption has not been rigorously tested, particularly in marine fish. Here, we report the results of a long-term experimental study of European plaice Pleuronectes platessa L. and explore relationships between blood plasma and ambient water chemistry over a 12 mo reproductive cycle. Overall, blood plasma was found to be highly regulated, with few elements exhibiting strong, if any, correlation with ambient concentrations. This sets a first order limit on the sensitivity of otolith chemistry to fluctuations in ambient seawater chemistry. The observed temporal, ontogenetic and sex-specific variations in blood plasma elemental concentrations indicated significant physiological influences on elemental uptake and processing mechanisms. Physiological variables exerted relatively strong influences on the uptake and regulation of the softer, more thiophilic elements (Mn, Cu, Zn, Se and Pb), as well as Sr and Ca. By contrast, seasonal and sex-related variations were relatively minor among the hard acid metal ions (Li+, Mg2+, K+, Rb+, Ba2+). Overall, plasma elemental concentrations covaried most strongly and consistently with plasma protein concentrations. For this exclusively marine species, seasonal changes in physiology governed intra-annual variations in blood chemistry and, by implication, also regulate ion availability to the otolith. Based on these observations, we recommend that sex and age should be controlled for in future experimental designs using otolith microchemistry to infer stock structure or migration patterns

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The effect of 9 min of net confinement on two marine teleost species, the flounder and the Atlantic salmon, was investigated in order to gauge how different species respond to the same stress stimulus. Net confinement in both species induced significant elevations in plasma cortisol, glucose, lactate, osmolality, and monovalent ion levels; the responses to net confinement in salmon were generally of a greater magnitude. In both species, handling induced significant alterations in PFFA levels though there were marked species differences in the qualitative and quantitative aspects of this response. Plasma protein levels were significantly elevated only in the flounders. In general, the two species responded similarly to the net confinement with differences only in the absolute levels and durations of the responses.