Elemental Composition as a Natural Tag

To the extent that populations or stocks of fish inhabit different environments, the otolith elemental composition should serve as a proxy for population identity. Two assumptions underlie the use of the otolith elemental composition as a stock discriminator: 1) material deposited on the otolith is metabolically inert after deposition, and is not susceptible to resorption; and 2) the physical and chemical environment influences the rate of trace element incorporation into the growing otolith surface. Elements under strong physiological regulation (eg- Na, K, S, P, Cl) probably do not meet the second assumption, and thus are of limited value for stock identification studies. However, both assumptions appear to be met with respect to elements such as Sr, Ba, Mn, Fe and Pb (and perhaps Li, Mg, Zn, Cu and Ni), in which both ambient element:Ca concentrations and temperature produce significant effects on otolith composition. Environmental responses such as these, recorded permanently in the otolith, imply that the otolith concentrations of selected elements and isotopes can be used as a biological tag to discriminate among groups of fish which have spent at least part of their lives in different environments. Discriminatory power is increased greatly by treating the selected elements as a group, using multivariate statistics, rather than individually; hence the term "elemental fingerprint". An appealing feature of this application is that the elemental fingerprint need not be linked to potential sources or locations in the environment. Rather, the presence of significant differences in the fingerprints of two or more groups of fish implies that the groups cannot all be random samples from the same population. This deduction holds even if physiological effects have influenced elemental composition, since random samples from the same population would have experienced the same mean physiological effects. Of course, the presence of different fingerprints cannot be used to infer the length of time that the groups of fish remained separate, since even occasional residency in a different environment has the potential to introduce a detectable difference in the elemental composition. By corollary, the absence of differences does not necessarily imply that the groups of fish are of common origin. As a result, it is fair to categorize otolith elemental fingerprints as powerful discriminators when differences exist, but of negligible value when differences cannot be detected.

It is probably inappropriate to refer to the use of elemental fingerprints as stock discriminators, since genetic differences are not implied and spatial heterogeneity in the stock environment can result in different fingerprints for different stock components. Perhaps more importantly, ontogenetic effects and age-related differences in exposure history can result in very different fingerprints for fish of different size classes from the same population. Nevertheless, the presence of different fingerprints among matched groups of fish necessarily implies different environmental histories. Accordingly, the elemental fingerprint would appear to be an excellent biological tracer of groups of fish, and it is this application of elemental fingerprints which has met with success in both freshwater and saltwater.

Two forms of elemental fingerprinting are in general use: one based on whole dissolved otoliths, and the other based on analysis of the otolith core. Advantages of whole-otolith assays include ease of preparation, absence of error associated with sampling or identifying growth increments, and the availability of accurate and precise assay protocols. The major disadvantage is associated with the inability to take advantage of the chronological growth sequence recorded in the otolith. Atomic absorption spectrometry (AAS), inductively-coupled plasma atomic emission spectroscopy (ICP-AES), neutron activation analysis, and inductively-coupled plasma mass spectrometry (ICPMS) are among the techniques which have been used to analyze otoliths. However, it is ICPMS which has emerged as the instrument of choice for such assays, due largely to its capability for rapid and accurate isotopic and elemental assays over a wide range of elements and concentrations. Isotope dilution ICPMS (ID-ICPMS), a variant of ICPMS often used to certify reference materials, is the most accurate of the otolith analytical techniques currently available. Sample sizes required for most of the above assays are on the order of 5-10 mg of otolith material, although ICPMS units outfitted with high efficiency nebulizers are capable of handling otolith weights as low as 0.3 mg. See the Methods page for detailed information on preparing otoliths for elemental analysis.

With the realization that elemental fingerprints can be used very effectively to separate mixtures of fish coming from different sources, there is increasing demand for the maximum-likelihood based software to separate the group mixtures. Discriminant analysis is not a good option here, since the 'priors' parameter is unknown. In the Methods section of this web site, we've released a working copy of the Integrated Stock Mixture Analysis (ISMA) program (written for the S-Plus environment) for use in separating stock mixtures based on elemental fingerprints or other continuous or categorical variables.

Our laboratory has devoted much effort to the study and application of otolith elemental fingerprints, both with respect to stock identification and to factors influencing trace element uptake into the otolith. This work can be reviewed in Thorrold et al. (1998), Farrell and Campana (1996), Campana and Gagné (1995), Campana et al. (1995), and Fowler et al. (1995a). A comprehensive review of the field of otolith composition and chemistry is provided in Campana (1999). An in-depth examination of the value of elemental fingerprints as natural tags of fish is presented in Campana et al. (2000). Recent applications of otolith elemental fingerprints as natural tags are available in Campana et al. (2007) for redfish and Jonsdottir et al. (2006) for cod around Iceland.