Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Dr. H.P. Schwarcz


The study was concerned with analyzing the stable isotopic compositions (18O/16O, 13C/12C ratios) of otoliths from Atlantic cod (Gadus morhua) of the northern cod stock off the east of Newfoundland and Labrador, Canada. The purpose was to examine whether environmental changes in the northwest Atlantic, particularly cold temperatures, in the early 1990s may have influenced the collapse of this stock. A number of separate studies were also conducted to examine some of the factors which may influence the isotope ratios of otoliths to aid our interpretation.

Cod from both inshore and offshore locations in Newfoundland were collected and 13C/12C and 15N/14N ratios of muscle tissue were analyzed to quantify the effect of ontogenetic diet change on the isotopic composition of metabolic carbon. Parallel isotopic increase of carbon (+2.6%) and nitogen (+3.6%) in muscle tissue confirmed that cod undergo almost continuous trophic level change related to ontogenetic diet change in early life. Otoliths from some of the same fish were then examined to determine if this trophic level change effects otolith 13C/12C ratios. A significant correlation was found between muscle isotope ratios of tissue and otolith. Ontogenetic diet change can explain up to 80% of the otolith's variation in 13C/12C with age. An additional decrease of 20% in the proportion of metabolic carbon between juvenile and adult cod explains the remainder of the variation. We then examined otoliths from nothern cod to look at their lifetime 13C/12C records. These records were remarkably consistent across the study area indicating that similar biological factors are at work in all cod. Stock components could be identified based on their characteristic isotopic signatures and there was some indication that juveniles from different areas were feeding at different trophic levels. Long-term environmental change seemed to be reflected in decreased 13C/12C during the mid-1980s. The exact cause of this change, however, remains uncertain.

To aid in the interpretation of the oxygen records, an extensive survey of 18O/16O in seawater was carried out over the Newfoundland and Labrador Shelves, examining spatial and temporal variation. There is an inshore to offshore gradient between low salinity and 18O/16O near shore and high salinity and isotope ratios offshore and also a strong linear correlation between isotope ratios and salinity. These observations were combined with isotopic records of individual northern cod otoliths to examine migration and environmental change. Most of the individual records displayed increasing ratios with age indicating an important ontogenetic habitat change towards higher salinity waters. Inshore cod form a distinct group with distinctly different lifetime records of 18O/16O. There is also a temporal trend towards lower mean adult and juvenile values which may indicate changes in recruitment and is consistent with a southward shift in distribution of northern cod.

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