Carbon isotopic records from the otoliths of Atlantic cod (Gadus morhua) from eastern Newfoundland, Canada

We used micro-milling methods to reconstruct individual lifetime stable carbon isotope (δ13C) records from otoliths of Atlantic cod (Gadus morhua) collected off eastern Newfoundland, Canada (NAFO Divisions 3KL and 3O) over a period of several decades. Overall lifetime trends of otolith δ13C were similar among individuals and typically showed an increase of approximately 1.5–2‰ during the first several (3–4) years followed by a more stable “adult” portion with maximum values between −1 and 0‰. These trends are consistent with those reported for cod elsewhere in Atlantic Canada and from the northeast Atlantic and, based on results herein, have persisted for about 50 years among cod off eastern Newfoundland. Despite this general consistency, our study gives evidence of geographic differences which may reflect different stock components within northern (3KL) cod based on characteristic patterns of δ13Coto. Notably, cod living in very deep water off the edge of the shelf appear to have distinctly lower δ13Coto patterns related to the lower δ13CDIC of these deep waters. Our results also suggest that there are differences in the feeding behaviour among juvenile cod from different regions. Seasonal variations in δ13Coto were noticeable among juvenile cod, but not among adults. There appeared to be an abrupt shift towards lower δ13Coto values in cod spawned during the mid-1980s and sampled in Div. 3KL. A downward shift in δ13C of otoliths of mature cod was previously reported among cod on the Scotian Shelf, and may be related to migration of the fish to deeper waters. Further information on the isotopic composition of potential prey as well as DIC at varying water depths in this region will be necessary to better interpret these changes in δ13Coto.