Stable Isotope and Elemental Variation in Cod Otoliths in Relation to Environmental and Physiological Signals

Otolith growth is regulated by the condition of an individual fish, and subject to energetic and physiological variation. At the same time, external factors have a direct influence on otolith formation – both on the rate of growth and on the structure and chemical composition. The combination of micro-milling techniques for stable isotope analysis with laser ablation techniques for ICPMS analysis of elemental composition can provide detailed data on the external and internal environment of an individual fish. Whereas δ¹⁸O, Ba, Sr, and other elements provide valuable information about fish movements and environmental conditions, δ¹³C, Sr, Na, and otolith opacity hold relevant clues about an individual’s condition and physiological responses. We present three examples of combined analysis of such otolith clues, from studies of NE Atlantic cod populations. Cod otoliths from medieval middens revealed evidence of contrasting warm and cold periods with resulting effects of fish growth. In a second study, on fecundity and skipped spawning in NE Arctic cod, δ¹³C and Sr profiles from otoliths of female fish were used to study the prevalence of skipped spawning, and the validity of spawning checks in cod otoliths. In the last example, the elemental and stable isotope profiles were combined to present a biochronology series matched with growth variations in cod from the Faroe Islands.