Research Document 1997/96

The stock identity of 4Vn cod based on an analysis of otolith elemental fingerprints

By S.E. Campana, T. Lambert, G. Chouinard, M. Hanson, A. Fréchet, and J. Brattey

Abstract

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 otoliths grow continuously without resorption throughout the life of the fish, individuals which spend at least part of their lives in different water masses produce otoliths of different elemental composition. Thus the otolith elemental composition ("elemental fingerprint") serves as an environmentally-induced tag of fish aggregations, independent of genetic identity, and remarkably stable from season to season. We used isotope dilution ICPMS (ID-ICPMS) to determine the elemental fingerprint of 1044 adult cod, representing 5 Atlantic cod (Gadus morhua) stocks in and around the approaches to the Gulf of St. Lawrence. There were highly significant differences among the elemental fingerprints of all of the stocks. To confirm that these fingerprints can be used to confidently track cod during periods of migration or stock mixing, we compared the fingerprints of 3Pn4RS, 4T and 4Vn cod samples taken in the fall (prior to out-migration) and in the spring (on the spawning grounds just after in-migration), as well as various locations from within each stock area. The results indicated that the otolith elemental fingerprint was stable across seasons, and that geographic variability in the elemental fingerprints within stocks was small compared to the differences among stocks. An important exception was that the elemental fingerprint of fish collected in spawning condition in 4Vn in May was significantly different from that of the fall 4Vn fish, but essentially identical to that of the 4T spring spawners. These findings indicated that significant numbers of 4T cod were still present in the 4Vn area at a time when fisheries management assumes that they are absent

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