Elemental composition of otoliths as a stock delineator in fishes

The elemental composition of otoliths is increasingly being examined as a means of resolving the fine scale stock and population structure of fishes. The technique has been applied to three sets of problems: (1) discriminating between marine and freshwater populations (or life history stages), (2) determining links between natal rivers or nursery areas and adult stocks, and (3) assessing population structure in marine fishes. Thus far, there are few consistent patterns in the data. Diagnostic elements differ between species and systems; experimental and correlative studies provide little support for strong environmental effects on otolith composition (other than effects of extreme differences in salinity/strontium concentrations); and the physiological mechanisms that would allow such an effect for most elements are not well documented. The area of investigation suffers from: a lack of well-established benchmark studies against which ‘reality’ of results can be assessed; small sample sizes and incomplete analysis; and non-critical evaluation of results. Two areas of research look promising: the use of marked differences between marine and freshwater fishes (or the corresponding portions of otoliths of anadromous fish) in strontium concentrations to track migrating coastal fishes, and apparently robust indications of regional structuring of some marine fish populations. The relative youthfulness of the field dictates the need for work in a number of areas, but priorities at this stage should include developing a better understanding of the physiological mechanisms that determine otolith composition and its variability, including distinguishing the effects of handling and contamination from ‘real’ composition, and validating apparent patterns of stock structure both by independent replication of results and comparisons against other sources of information, such as genetic and tagging studies. To assure data quality and form a base for corroboration of results, widespread use of certified reference materials and careful documentation of analytic procedures are essential.