Regime Shift Signatures from Stable Oxygen Isotopic Records of Otoliths of Atlantic Cod (Gadus morhua)

Abstract

The sudden collapse of Atlantic cod (Gadus morhua) may relate to ocean climate, or regime shifts as demonstrated in production of Pacific salmon. This paper reports the results of stable oxygen isotope ratio analyses (18O/16O or δ18OA) from 91 otoliths of cod over a period of about 20 years. Seasonal δ18OA variations of individual otoliths started at an initial value of about −0.5 to 0‰ VPDB, and then reached a stable level in the range of +2.5 to +3.5‰ VPDB after 4–5 years. The initial low values correspond to the natal sources of mature cod, while the higher δ18OA values represent the water conditions before the cod was caught. This pattern of δ18OA variation was observed over the life history of all cod examined. Furthermore, the calculated isotopic temperatures agreed with those obtained from summer bottom trawl survey, indicating that δ18OA of otoliths could be used as a thermometer in determining the ambient seawater temperature where the cod lived. Comparison of long-term δ18OA records and biological and meteorological observations suggested that decadal-scale ecosystem changes did occur in the late 1970s and early 1990s in Atlantic Canada, comparable to regime shifts occurred in the North Pacific.