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

Stable carbon isotope ratios ( 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 13 C from minimum values between -5‰ and -2.5‰ to a maximum near 0‰. This pattern of increase was independent of the years in which the fish was collected. Layers formed after reaching the maximum 13 C value displayed decreasing or nearly constant isotopic ratios. Early rise in 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 13 C values. Age of maximum in 13 C may be indicative of age of maturity of cod. The maximum 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 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 13 C values of the mature cod otoliths may represent movement of the fish to deeper waters of the shelf, where 13 C of dissolved inorganic carbon is lower.