Journal of Fish Biology (2002) 61, 1540–1548 doi:10.1006/jfbi.2002.2173, available online at http://www.idealibrary.com on Fish may fight rather than feed in a novel environment: metabolic rate and feeding motivation in juvenile Atlantic salmon C. J. C*, N. B. M  A. C. T Fish Biology Group, Division of Environmental & Evolutionary Biology, Graham Kerr Building, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow G12 8QQ, U.K. (Received 31 May 2002, Accepted 21 October 2002) This study tested the hypothesis that juvenile Atlantic salmon Salmo salar with a high resting metabolic rate and probability of dominance will also have a correspondingly higher feeding motivation in a novel environment to offset their greater costs of maintenance. The opposite was found to be the case: Atlantic salmon with a high standard metabolic rate had a slightly but significantly lower feeding motivation. It is hypothesized that Atlantic salmon with higher maintenance costs opt to be more aggressive at the expense of the costly activities associated with feeding, since elevated aggression in a new habitat is a more successful strategy for acquiring a feeding territory (and hence sustaining food intake in the long-term) than a high feeding motivation alone.  2002 The Fisheries Society of the British Isles. Published by Elsevier Science Ltd. All rights reserved. Key words: metabolism; competitive ability; energy budgets; appetite; metabolic scope. INTRODUCTION Animals must often compete with conspecifics for important resources such as food, space or mates. While social dominance is determined by several asymmetries between individuals, such as size, age and sex (Arcese & Smith, 1985; Lemel & Wallin, 1993), competitive ability often seems not to conform to such fixed predictors of dominance. Instead, the importance of such predictors may be conditional on the motivational state of contestants (Elwood et al., 1998). Furthermore, individuals may differ in inherent characteristics that allow them to respond quickly to prey (Salvanes & Hart, 2000). Therefore, there may also be physiological and motivational asymmetries that affect the outcome of an interaction. For example, hunger can increase aggression and strengthen social hierarchies in birds (Andersson & Ahlund, 1991) and fishes (Symons, 1968; Dill et al., 1981), with the proviso that higher hunger levels increase competitive ability through higher feeding motivation (Milinski & Parker, 1991). In rainbow trout Oncorhynchus mykiss (Walbaum), energy demand and subsequent feeding motivation were found to influence dominance status, by elevating aggression to promote competitive exclusion (Johnsson & Bjornsson, 1994). Fasting only, however, induced a short-lived increase in dominance status, since the positive *Author to whom correspondence should be addressed at present address: Seafish Marine Farming Unit, Ardtoe, Acharacle, Argyll PH36 4LD, U.K. Tel.: +44 (0) 1397 875304; fax: +44 (0) 1397 875001; email: C – Cutts@seafish.co.uk 1540 0022–1112/02/121540+09 $30.00/0  2002 The Fisheries Society of the British Isles. Published by Elsevier Science Ltd. All rights reserved.