Rainbow trout Oncorhynchus mykiss (Walbaum) are usually cultured at high densities to maximize production, but little is known about the physiological and behavioural consequences of high-density ®sh culture. The purpose of this study was to develop quantitative correlates of activity for ®sh held under conditions of increasing density. Fifteen hatchery-reared rainbow trout (mean fork length = 432.3 6 9.2 mm) were implanted with activity (electromyogram; EMGi) transmitters and randomly assigned to each of three replicate tanks. Original tank densities (15 kg m ±3 ) were then increased to 30 and ®nally to 60 kg m ±3 at weekly intervals by adding additional ®sh. Remote telemetry signals indicated that activity increased with increasing stocking density. Fish were relatively inactive during the middle of the day, with diel activity patterns not differing among treatments. Fish were more active during periods of darkness, with activity increasing with increasing stocking density. Relationships between swimming speed, EMGi activity and oxygen consumption were developed using a respirometer and used to estimate oxygen consumption of the ®sh in the density treatments. Average oxygen consumption estimates increased with increasing density treatments as follows: low density = 75.6 mg kg ±1 h ±1 ; medium density = 90.0 mg kg ±1 h ±1 ; and high density = 102.6 mg kg ±1 h ±1 . Telemetry permits quanti®-cation of the effects of increasing density on ®sh activity. Physiological telemetry devices may provide a useful tool for remotely monitoring animal welfare correlates under controlled conditions for ®sh exposed to different husbandry conditions and may prove a valuable tool for the aquaculture industry.

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Red and white axial muscle activity of adult Atlantic salmon Salmo salar was examined using conventional electromyography (EMGx) and activity radio-transmitters (EMGi) at 0·5 and 0.7 body lengths (L) along the body of the fish. Critical swimming trials were conducted and maximum sustainable speeds (U crit ) were unaffected by the presence of electrodes, being 1·51 0·21 m s 1 (3.33 0.34 L s 1 ) (n=44). Regardless of longitudinal position of the electrodes within the musculature, both EMGxs and EMGis indicated increasing red muscle activity with increasing swimming speed, whereas white muscle fibres were recruited only at speeds >86 5% U crit . Telemetered EMGi signals indicated that muscle activity varied significantly for electrodes implanted at different longitudinal positions along the fish (P<0·001). These results suggest that electrode placement is an important influence affecting the signals obtained from radio transmitters that estimate activity and location should be standardized within biotelemetry studies to allow accurate and consistent comparisons of activity between individuals and species. Optimal location for electrode placement was determined to be in the red muscle, towards the tail of the fish (0·7 L). 1999 The Fisheries Society of the British Isles

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