Swimming performance and metabolism of 0+ year Thymallus arcticus L. A. D EEGAN *†, H. E. G OLDEN *, J. H ARRISON ‡ AND K. K RACKO § *The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, U.S.A., ‡Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ 08901, U.S.A. and §Lockeed Martin/REAC, 2890 Woodbridge Avenue, Edison, NJ 08837, U.S.A. (Received 16 April 2004, Accepted 31 March 2005) The prolonged swimming speed and metabolic rate of 0þ year Arctic grayling Thymallus articus were examined with respect to current velocity, water temperature and fish size, and compared to conditions fish occupy in the river. Oxygen consumption (mg O 2 h 1 ) increased with fish mass and temperature (6–23  C), with a steep increase in metabolic rate between 12 and 16  C. Absolute prolonged swimming speed (cm s 1 ) increased rapidly with fish size (total length, L T , and mass), however, fish in the natural stream habitat occupied current velocities between 15 and 25 cm s 1 or 4 L T s 1 , approximately half their potential prolonged swimming speed (10 L T s 1 ). # 2005 The Fisheries Society of the British Isles Key words: fish metabolism; habitat use; swimming performance. INTRODUCTION Riverine fishes are subject to a number of environmental factors, including varying water and current velocities that can influence their metabolism, growth and activity. As temperature and current velocities increase, the energetic demands of fishes increase, sometimes in a non-linear fashion if temperature or current speed thresholds are exceeded (Beamish, 1964; Brett & Groves, 1979; Henry & Houston, 1984; Tang & Boisclair, 1995). As metabolic demands increase, additional consumption of food is required to maintain growth (Jobling, 1994). River discharge and current velocities also affect the foraging ability and growth of stream-dwelling fishes (Deegan et al., 1999; O’Brien et al., 2001). Prolonged swimming speed is considered an indicator of the ability of fishes to swim and hold position in currents and is widely used to assess the effects of environmental conditions on fishes (Plaut, 2001). At high discharge fishes may be forced into sub-optimal lateral habitats or they may be displaced downstream †Author to whom correspondence should be addressed. Tel.: þ1 508 548 3705; fax: þ1 508 457 1548; email: ldeegan@mbl.edu Journal of Fish Biology (2005) 67, 910–918 doi:10.1111/j.1095-8649.2005.00784.x, available online at http://www.blackwell-synergy.com 910 # 2005 The Fisheries Society of the British Isles