Community Research and Development Information Service - CORDIS

Evaluation of the influence of environmental features on cod spatial dynamics

The results of the CODYSSEY field programme demonstrate that cod can occupy habitat over the temperature range -1.5°C to 19°C and, while the majority of temperature experience was centred between 6° to 10°C in all ecosystems, individuals can tolerate extended periods of time at the extreme temperatures and endure thermal shocks as great as 10°C in a short period of time as they cross the boundaries between different water masses. Although individual cod are known to adapt to changes in temperatures, they are thought to behave in a way that reduces thermal stress (behavioural thermoregulation, Claireaux et al 1995) and to be unable to tolerate large, abrupt changes in temperature. As a result, it is believed that thermoclines of only a few degrees can constitute fairly impenetrable boundaries and that this aspect of their biology may contribute to the segregation of cod populations in nature (Claireaux et al 1995). Our observations represent a significant development in our understanding of cod biology because they suggest that thermal tolerance of wild cod is considerably greater than that suggested by laboratory studies.

Simulation experiments have explored the extent to which the behaviour of cod can be explained by simple behavioural rules. In the Baltic Sea, cod appear to use a combination of behavioural rules to ensure that they remain within preferred habitat. Simple depth or oxygen preference rules do not accurately recreate their observed behaviours. In the North Sea, simulations of cod response to water temperature suggested that simple hypotheses that relate cod movements to optimal temperature alone create highly unrealistic seasonal distributions. Simulating cod movements within a tolerance range provide more realistic results, and support the results of the electronic tagging experiments that described a tolerance of higher temperatures than expected. Together, these results suggest that the minimisation of physiological stress and the maximisation of growth rate are not be the only factors to consider as the basis of, or inputs to, predictive models.

Overall the assessment of thermal and anoxic tolerance of cod, and the analysis of depths occupied, has demonstrated that cod are tolerant to a wider range of environmental conditions than previously thought. These results help to explain the widespread distribution of cod in the Atlantic, and also provide evidence that, while human-induced environmental change will inevitably shift the distribution of cod, the capacity of stocks and individuals to tolerate thermal stress offers hope for stock recovery if fishing mortality can be controlled, and if recruitment of young cod occurs at historical levels.

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United Kingdom
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