Applying Otolith Chemistry to Population Studies in Fisheries: Is Environment or Biology the Key to Variation in Multi-Element Signals?

Fish otoliths are unique recording structures, since they form before hatching and grow continuously throughout life. During otolith formation and organic matrix is secreted which controls the precipitation of CaCO₃ together with other elements as impurities. The elemental concentration of the otolith is influenced by both physiological and environmental factors, particularly the water chemistry and temperature. The otolith chemical composition is often distinct in different populations, and came be used to discriminate between populations, and measure connectivity between life stages. This utility is usually assumed to be the result of the close connection between the unique otolith signal and the surrounding environment. Any biological or physiological effect is more difficult to explain, and usually ignored during stock identification, although it might lead to some miss classification. This is the first review to evaluate the relative effects of environmental or biological factors on otolith chemical composition. The summary is focused on species that are distributed across different basins and areas in European waters. ANOVA comparisons and multivariate analyses were used for the evaluation. In cod (Gadus morhua), bluemouth (Helicolenus dactylopterus) and sardine (Sardina pilchardus), there are species-specific patterns in otolith chemical composition. However, even within these species, there are still significant spatial differences at either small or large geographic scales. These phylogenetic differences emphasize the need for a more fundamental understanding of elemental incorporation during otolith formation. Yet, based on an increasing amount of empirical data, we are still able to successfully study changes and patterns in fish populations using otolith elemental composition.