Elemental composition of otoliths as a stock delineator in fishes

The elemental composition of otoliths is increasingly being examined as a m eans of resolving the fine scale stock and population structure of fishes. The technique has been applied to three sets of problems: (1) discriminatin g between marine and freshwater populations (or life history stages), (2) d etermining links between natal rivers or nursery areas and adult stocks, an d (3) assessing population structure in marine fishes. Thus far, there are few consistent patterns in the data. Diagnostic elements differ between spe cies and systems; experimental and correlative studies provide little suppo rt for strong environmental effects on otolith composition (other than effe cts of extreme differences in salinity/strontium concentrations); and the p hysiological mechanisms that would allow such an effect for most elements a re not well documented. The area of investigation suffers from: a lack of w ell-established benchmark studies against which 'reality' of results can be assessed; small sample sizes and incomplete analysis; and non-critical eva luation of results. Two areas of research look promising: the use of marked differences between marine and freshwater fishes (or the corresponding por tions of otoliths of anadromous fish) in strontium concentrations to track migrating coastal fishes, and apparently robust indications of regional str ucturing of some marine fish populations. The relative youthfulness of the field dictates the need for work in a number of areas, but priorities at th is stage should include developing a better understanding of the physiologi cal mechanisms that determine otolith composition and its variability, incl uding distinguishing the effects of handling and contamination from 'real' composition, and validating apparent patterns of stock structure both by in dependent replication of results and comparisons against other sources of i nformation, such as genetic and tagging studies. To assure data quality and form a base for corroboration of results, widespread use of certified refe rence materials and careful documentation of analytic procedures are essent ial. (C) 1999 Elsevier Science B.V. All rights reserved.