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A spatially, temporally and behaviourally resolved comparison of cod (Gadus morhua L.) otolith growth rates by age-class for the ecosystems studied

To compare the behavioural pattern of cod in the different ecosystems and raising this to population level the task of etho-typing includes extracting average habitat characteristics and activity pattern at scales comparable to otolith macrostructure analysis was undertaken. It was clear from our results that, although daily increments are visible in cod up to five years old, the otoliths of adult cod are unlikely to provide a reliable record of short-term (weekly) changes in behaviour because otolith properties could change dramatically at a time when neither temperature nor depth varied in any significant way. The cause of such changes was not clear.

However, longer term changes in optical density and accretion rate do appear to provide a proxy for seasonal differences in behaviour that are associated with a long-term change in somatic growth or temperature experience. For example, the pattern of annual zone formation in cod from the southern and northern North Sea was clearly different and this is marked enough to be used a means of distinguishing between fish from each region. In cod from the Baltic Sea and the Faroe plateau, significant decreases in opacity were observed as cod changed their dominant behaviour from feeding to spawning, suggesting that otoliths could be used to provide reliable indications of spawning events and maturity ogives in otolith collections from these regions.

The otolith accretion model developed in the IBACS project (QLRT-2001-01610) was applied to DST tagged individuals of the Baltic Sea and the North Sea to evaluate output of otolith macrostructure with actual observations of these individuals. Major biological drivers were identified that primarily influence otolith macrostructure between ecosystems. This modelling of otolith macrostructure using the environmental experience data from each ecosystem produced promising results.

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