325 J. exp. Biol. 175, 325–331 (1993) Printed in Great Britain © The Company of Biologists Limited 1993 *Present address: University of Durham, Department of Biological Sciences, South Road, Durham DH1 3LE, UK. Key words: annular respirometer, optomotor response, swimming, oxygen consumption, schooling, Salmosalar, Scomberscombrus. SHORT COMMUNICATION AN ANNULAR RESPIROMETER FOR MEASURING AEROBIC METABOLIC RATES OF LARGE, SCHOOLING FISHES M. C. LUCAS*, A. D. F. JOHNSTONE and J. TANG Accepted 6 October 1992 Scottish Office Agriculture and Fisheries Department, Marine Laboratory, PO Box 101, Victoria Road, Torry, Aberdeen AB9 8DB, UK Aerobic metabolic rates of fish at controlled levels of activity are usually measured with individual fish isolated in a flume-type respirometer, in which the fish must swim against a known water velocity (Brett, 1964; Beamish, 1978; Gehrke e ta l. 1990). Their use for large and fast-swimming fish is difficult owing to engineering problems and water turbulence and because swimming in the confined test section of the respirometer affects the performance of the fish. Some of these problems have been partially dealt with by applying corrections to compensate for the altered water flow around the body of the fish (Webb, 1971) and by developing highly sophisticated flume respirometers (Gehrke et al. 1990). Rheotactic stimuli have also been used to control activity in annular rotating vessels such as the ‘fish wheel’ of Bainbridge (1958) and respirometers (Wohlschlag, 1957; Fry, 1971). An alternative way of controlling swimming activity is to utilize the optomotor response of fish (Harden-Jones, 1963). The main advantage of such a system is that the fish swim at a controlled speed, set by a moving background pattern, through still water. Dabrowski (1986) used a small annular respirometer where swimming speed of salmonid fry was controlled by a pattern rotating around the chamber. The apparatus described below has been developed to provide a new solution to the problem of measuring oxygen consumption in larger active fish, such as adult salmon, and in fast-swimming obligate- schooling fish, such as mackerel, which are highly susceptible to stress and for which flume respirometers are unsuitable. The respirometer tube (Fig. 1) was submerged in sea water in the 10m diameter annular tank at the Marine Laboratory, Aberdeen (He and Wardle, 1988). Size and shape of the tube were a compromise between minimal restriction of fish swimming ability and minimal volume to give reliable measurements of oxygen consumption rates. The respirometer was constructed from twelve, 5mm thick, clear polyvinylchloride (PVC) (Darvic) tubular (0.78m diameter) modules, each containing an angle of 150˚, in plan