This version available http://nora.nerc.ac.uk/7671/ NERC has developed NORA to enable users to access research outputs wholly or partially funded by NERC. Copyright and other rights for material on this site are retained by the authors and/or other rights owners. Users should read the terms and conditions of use of this material at http://nora.nerc.ac.uk/policies.html#access This document is the author's final manuscript version of the journal article, incorporating any revisions agreed during the peer review process. Some differences between this and the publisher's version remain. You are advised to consult the publisher's version if you wish to cite from this article. www.elsevier.comContact CEH NORA team at nora@ceh.ac.ukThe NERC and CEH trade marks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Wilson et al. 1994;Réale et al. 2007). Notwithstanding a certain lack of consensus in terminology, the evolution of these analogues of human personality has evoked considerable scientific interest. Among the questions raised are by which mechanisms behavioural syndromes are inherited, how the variation is maintained, which circumstances benefit different complex phenotypes, and what are the costs and benefits of limited plasticity imposed by more or less fixed trait associations (DeWitt et al. 1998;Koolhaas et al. 1999;Sih et al. 2004; Dingemanse and Réale 2005;McElreath et al. 2007;Réale et al. 2007;Wolf et al. 2007). Furthermore, in the barn owl (Tyto Alba) melanin-based colouration is associated with several behavioural, morphological and physiological characteristics that have been linked to stress coping ability (Almasi et al. 2008).Based on observations that HR and LR trout differed in melanin pigmentation our hypothesis was that spotted fish, in the closely related specie Atlantic salmon (Salmo Salar), 5 would show a lower cortisol reponse to stress when compared to a non-spotted group of individuals. Furthermore, several documented changes in the behaviour of the two trout lines made us predict that also behaviour would differ between groups of spotted and non-spotted fish. The main focus of this study was therefore to test the hypothesis that there exist an association between melanin-based pigmentation and hypothalamus-pituitary-interrenal (HPIaxis) reactivity in salmonid fish. Furthermore, we tested the prediction that behavioural differences are present in two groups of fish chosen divergently on melanin pigmentation. Materials and methods Quantification of melanin-based colorationIn the studies described below, photographs of experimental fish were captured with a Canon EOS 450D digital camera and transferred to a computer for further analyses of melanin-based coloration. Prior to photography, each fish were sedated in 200 mg/l MS-222 and placed adjacent to a measuring tape with millimetre resolution, allowing for an accurate estimation of the area where spots were counted. The d...