Otolith elemental fingerprints as biological tracers of fish stocks

Abstract

Specific trace elements incorporated into the growing surface of the fish otolith reflect the physical and chemical characteristics of the ambient water, although not necessarily in a simplistic manner. Since fish which spend at least part of their lives in different water masses often produce otoliths of different elemental composition, the otolith elemental composition (‘elemental fingerprint’) can serve as an environmentally induced tag of groups of fish. On the basis of isotope dilution ICPMS (ID-ICPMS) assays of nearly 2500 dissolved adult cod ( Gadus morhua) otoliths, it has become clear that cod otolith elemental fingerprints based on the elements Li, Mg, Mn, Sr and Ba are physically stable, reproducible and consistent between left and right otoliths. Highly significant differences existed among the fingerprints of all of the spawning aggregations, resulting in a characteristic marker for each aggregation. Long-term stability (4–13 years) of the fingerprints for a given spawning group was not evident, indicating that the fingerprint was not a proxy for genetic identity. However, the fingerprint was very stable over the short-term (up to 1 year), suggesting that it could serve as a seasonally stable biological tracer, or natural tag, of pre-defined groups of fish, even during situations of extensive stock mixing. As an illustration of the tracer approach, a maximum likelihood-based stock mixture analysis was applied to feeding (summer) and over-wintering stock distributions, using the fingerprints of the spring spawning aggregations as known-stock reference samples. The results of the summer stock mixture analyses suggested that the mixture analysis was accurate within 1%, while the stock mixture analysis of the over-wintering schools produced stock-specific distributions which would have been difficult to obtain using alternative approaches. While the use of elemental fingerprints as natural tags is not suited to all stock mixing situations, suitability can probably be determined beforehand on the basis of existing environmental and biological information.