ABSTRACT:The fish otolith (earstone) has long been known as a timekeeper, but interest in its use as a metabolically inert environmental recorder has accelerated in recent years. In part due to technological advances, applications such as stock identification, determination of rnigralon pathways, reconstruction of temperature and salinity history, age validation, detection of anadromy, use as a natural tag and chemical mass marking have been developed, some of which are difficult or impossible to implement using alternative techniques. Microsamphg and the latest advances in beam-based probes allow many elemental assays to be coupled with daily or annual growth increments, thus providing a detailed chronologcal record of the environment. However, few workers have critically assessed the assumptions upon which the environmental reconstructions are based, or considered the possibility that elemental incorporation into the otolith may proceed differently than that into other calcified structures. This paper reviews current applications and their assumptions and suggests future directions. Particular attention is given to the premises that the elemental and isotopic composition of the otolith reflects that of the environment, and the effect of physiological filters on the resulting composition. The roles of temperature, elemental uptake into the fish and the process of otolith crystahzation are also assessed. Drawing upon recent advances i n geochemistry and paleoclirnate research as points of contrast, it appears that not all applications of otolith chemistry are firmly based, although others are destined to become powerful and perhaps routine tools for the mainstream fish biologst.

Many calcified structures produce periodic growth increments useful for age determination at the annual or daily scale. However, age determination is invariably accompanied by various sources of error, some of which can have a serious effect on age-structured calculations. This review highlights the best available methods for insuring ageing accuracy and quantifying ageing precision, whether in support of large-scale production ageing or a small-scale research project. Included in this review is a critical overview of methods used to initiate and pursue an accurate and controlled ageing program, including (but not limited to) validation of an ageing method. The distinction between validation of absolute age and increment periodicity is emphasized, as is the importance of determining the age of first increment formation. Based on an analysis of 372 papers reporting age validation since 1983, considerable progress has been made in age validation efforts in recent years. Nevertheless, several of the age validation methods which have been used routinely are of dubious value, particularly marginal increment analysis. The two major measures of precision, average percent error and coefficient of variation, are shown to be functionally equivalent, and a conversion factor relating the two is presented. Through use of quality control monitoring, ageing errors are readily detected and quantified; reference collections are the key to both quality control and reduction of costs. Although some level of random ageing error is unavoidable, such error can often be corrected after the fact using statistical (' digital sharpening ') methods.

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The chronological properties of otoliths are unparalleled in the animal world, allowing accurate estimates of age and growth at both the daily and the yearly scale. Based on the successes of calcified structures as environmental proxies in other taxa, it was logical that researchers should attempt to link otolith biochronologies with otolith chemistry. With the benefit of hindsight, this anticipation may have been naive. For instance, the concentrations of many elements are lower in the otolith than in corals, bivalves, seal teeth, or the other bony structures of fish, making them less than ideal for elemental analyses. Nevertheless, there is growing interest in the use of otolith chemistry as a natural tag of fish stocks. Such applications are directed at questions concerning fish populations rather than using the fish as a passive recorder of the ambient environment and do not rely upon any explicit relationship between environmental variables and otolith chemistry. The questions that can be addressed with otolith chemistry are not necessarily answerable with genetic studies, suggesting that genetic and otolith studies complement rather than compete with each other. Thus, we believe that otolith applications have the potential to revolutionize our understanding of the integrity of fish populations and the management of fish stocks.

The elemental composition of fish otoliths may represent a permanent record of the environmental conditions an individual has experienced as trace elements, incorporated into the growing surface of the otolith, reflect the physical and chemical characteristics of the ambient water. We tested the utility of trace element signatures in otoliths as natural tags of the river of origin of juvenile American shad (Alosa supidissima) collected from the Connecticut, Hudson and Delaware Rivers in August and October 1994. Four elements (K, Mn, Sr, and Ba) showed significant variability among sites within rivers in August, although only Mg showed a significant site effect by October. Four elements (Mg, Mn, Sr, and Ba) differed significantly among rivers in both months. Linear discriminant functions based on the trace element signatures classified fish to their natal river with -90% accuracy in both August and October collections. The discriminant function generated from the August data was able to classify fish collected in October successfully with better than 80% accuracy. On the basis of our findings, the river of origin of adult fish could be accurately determined by quantifying the trace element composition of the juvenile portion of their otoliths.

Trace element incorporation into fish otoliths varies among samples collected at different sites. If otolith elemental composition (the elemental "fingerprint") somehow reflects the characteristics of the ambient water, the elemental fingerprint of the otolith nucleus could serve as a natural marker of fish hatched at different sites. To test this hypothesis, Atlantic cod (Gadus morhua) otoliths collected from five spawning grounds in the northwest Atlantic were tested for differences in elemental and isotopic composition. Laser ablation – inductively coupled plasma mass spectroscopy (LA-ICPMS) was used to assay the concentration of 14 isotopes (nine elements) in otolith nuclei. The sensitivity of the laser ablation system exceeded that of the electron microprobe by 2–4 orders of magnitude, with an average CV of 21% for any given isotope. Most isotopic concentrations were consistent between left and right otoliths of a given fish, and most differed significantly among sample sites; there were no significant differences by age, sex, or fish length. Multivariate analyses of the elemental fingerprints resulted in significant discrimination among sample sites. While the mechanism underlying trace element incorporation into otoliths is still unclear, otolith elemental fingerprinting has the potential to become an effective and accurate means of stock identification.

Stable carbon isotope ratios (13C) were measured in annual layers of otoliths of Atlantic cod (Gadus morhua) from the northeastern Scotian Shelf, Atlantic Canada. Layers deposited during the first 4-6 years of otolith growth increased in 13C from minimum values between -5‰ and -2.5‰ to a maximum near 0‰. This pattern of increase was independent of the years in which the fish was collected. Layers formed after reaching the maximum 13C value displayed decreasing or nearly constant isotopic ratios. Early rise in 13C may be a combined result of (i) decrease in the fraction of metabolic oxidized carbon in the fishes' blood as they mature, relative to the proportion of seawater-derived carbon, and (ii) dietary shift to higher trophic-level foods with higher 13C values. Age of maximum in 13C may be indicative of age of maturity of cod. The maximum 13C value attained by otoliths decreased steadily between 1983 and 1993, while cod stocks in Atlantic Canada were declining. Drop in age of attainment of maximum 13C between 1984 and 1985 coincides with changes in population dynamics of the 4Vs stock. This decrease, as well as the post maximum decrease in 13C values of the mature cod otoliths may represent movement of the fish to deeper waters of the shelf, where 13C of dissolved inorganic carbon is lower.