Stable carbon isotope variations in otoliths of Atlantic cod (Gadus morhua)

Stable carbon isotope ratios (delta(13)C) were measured in annual layers of otoliths of Atlantic cod (Gadus morhua)from the northeastern Scotian Shelf, Atlantic Canada. Layers deposited during the first 4-6 years of otolith growth increased in delta(13)C from minimum values between -5 parts per thousand and -2.5 parts per thousand to a maximum near 0 parts per thousand. This pattern of increase was independent of the years in which the fish was collected. Layers formed after reaching the maximum delta(13)C value displayed decreasing or nearly constant isotopic ratios. Early rise in delta(13)C may be a combined result of (i) decrease in the fraction of metabolic oxidized carbon in the fishes' blood as they mature, relative to the proportion of seawater-derived carbon, and (ii) dietary shift to higher trophic-level foods with higher delta(13)C values. Age of maximum in delta(13)C may be indicative of age of maturity of cod. The maximum delta(13)C value attained by otoliths decreased steadily between 1983 and 1993, while cod stocks in Atlantic Canada were declining. Drop in age of attainment of maximum delta(13)C between 1984 and 1985 coincides with changes in population dynamics of the 4Vs stock. This decrease. as well as the post maximum decrease in delta(13)C values of the mature cod otoliths may represent movement of the fish to deeper waters of the shelf, where delta(13)C of dissolved inorganic carbon is lower.