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Author

Yongwen Gao

Date of Award

8-1997

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

Henry P. Schwarcz

Co-Supervisor

Uwe Brand

Abstract

Stable isotopic analysis has been widely used in study of proxy materials with concentric layers. such as fossil shells. corals. and tree rings. and has recently begun to blossom in fish otoliths. This study describes some new developments of stable isotopic analyses in otoliths of cod (Gadus morhua) from the northeast Scotian Shelf, Nova Scotia, and demonstrates how powerful those analyses are in study of marine environmental changes.

It is known that the relationship between oxygen isotope (δ¹⁸Ow) and salinity is very useful in study of coastal settings, especially for study of the dynamic processes, water currents and possible sources of mixing, and the estimation of isotopic composition in paleo-oceans. In this study, I found that the best equation for describing this relationship in the 4Vs area is: δ¹⁸Ow = 0.524S -18.38. I also found that this relationship varies vertically with water depth, especially at depths greater than 200 m. The oxygen isotopic records in 4 V s over the last 20 years show that the variation trend for surface water was steadily more negative in δ¹⁸Ow and lower in salinity. However, at depths greater than 200 m, a warmer water mass appeared around 1985, indicating that some distinct environmental events happened during that period.

Sagittal otoliths of cod were analyzed based on seasonal translucent (winter) and opaque (summer) aragonite zones, by a newly-established micromilling technique (DM 2800). The δ¹⁸Ow values range from 0 to +3.0 %0, with minor isotopic variations between seasons. However. there were significant differences in the growth of the first-year, indicating that juvenile cod are more active than adults at the early stage of their life history. Comparing opaque otolith zones of the 4-5 year group from different years of capture, the secular isotopic variation gradually increased by about 1 % over the period of 1984-94, corresponding to a temperature decrease in the surrounding seawater of about 5⁰C. This result is in agreement with the data from summer groundfish trawling survey and with other independent investigations. Based on the δ¹⁸O values in otoliths (greater or less than 0.75 %), two types of cod in 4Vs were recognized. It suggests that about 20 % of the total cod were recruited from the Gulf of St. Lawrence (migratory) where the δ¹⁸O was relatively lower, whereas the other 80 % were indigenous to the 4Vs area. Such a classification may have the potential for using δ¹⁸Ow signals from different water masses to study recruitment and migration of cod. and possibly other fish species.

The life-long δ¹⁸C variation in otoliths of cod shows a consistent increase during the first 4 to 6 years, from the minimum values between -5 and -2.5 % to a maximum near 0%. The early rise in δ¹⁸C is partly due to the fraction of metabolic HCO₃⁻ in the cod's blood, and partly due to a dietary shift or trophic effect. The maximum in δ¹⁸C is attained at an age (4-5 y) close to that of the age of maturity, and used to determine the age of sexual maturity of the 4Vs cod. Furthermore, the maximum δ¹⁸C variation attained by otoliths is in synchrony with the coeval temperature decrease in seawater from 1984 to 1994, during which the cod stocks experienced a drastic decline in the Canadian Atlantic.

In this study, I also analyzed otoliths of known-history Norwegian cod from experiments. These cod were reared in ocean-pens for as long as 6 years, and provide a unique opportunity to examine their isotopic variations and environmental influences. Comparing these to otoliths from the 4 V s field cod, there are distinct differences in the range of isotopic composition, the life-long variations in δ¹⁸O and δ¹⁸C, and the degree of correlation between δ¹⁸O and δ¹⁸C. These differences probably arise from the living environmental constraints and food supply in the two different settings. The isotopic temperature estimation was very close to the records during rearing, suggesting that temperature is a controlling factor in the forming of seasonal otolith zones.

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