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Caudal differential pressure as a predictor of swimming speed of cod (Gadus morhua)

D. M. Webber^1,2,*, R. G. Boutilier², S. R. Kerr¹ and M. J. Smale³

¹ Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1,
² Department of Zoology, Cambridge University, Downing Street, Cambridge CB2 3EJ, UK and
³ Port Elizabeth Museum, PO Box 13147, Humewood 6013, South Africa

View larger version (23K): [in a new window]   Fig. 1. Diagram of a fish tail illustrating the placement of a differential pressure sensor (1 cmx1 cmx1.25 cm) to measure swimming speed through the measurement of tailbeat differential pressure. The sensor was mounted on one side of the caudal peduncle at the position of the second last vertebral centrum, approximately 2 cm anterior to the base of the caudal fin rays and approximately 1 cm dorsal to the vertebra.

View larger version (35K): [in a new window]   Fig. 2. Tailbeat differential pressure versus swimming speed and time for a typical cod (Gadus morhua) swimming in a ‘Brett-style’ respirometer at 5°C. At each swimming speed, a continuous 9 s segment of pressure data sampled at 63 Hz is shown. Note the increase in positive and negative pressure (amplitude) and in the frequency of the tailbeat pressure signal as swimming speed increases. The section of the recording labelled ‘Burst’ represents a cod repeatedly burst swimming in the swimming chamber at high speeds prior to exhaustion.

View larger version (41K): [in a new window]   Fig. 3. Tailbeat differential pressure and water speed versus time for a typical cod (Gadus morhua) swimming in a ‘Brett-style’ respirometer at 5°C. (A) A single increase in the water velocity of 0.3 m s–1. (B) Repeated smaller increase in water velocity of 0.1 m s–1. The pressure data were sampled at 63 Hz. The lower traces in A and B represent the water speed setting of recirculating sea water. The upper traces are the tailbeat pressure data. Note the increase in both tailbeat pressure amplitude and tailbeat frequency with increases in water velocity.

View larger version (22K): [in a new window]   Fig. 4. (A) The classic figure-of-eight path (Pettigrew, 1873) followed by the tail through one pressure cycle or tail beat of a cod (Gadus morhua) swimming 0.3 m s–1 in a ‘Brett-style’ respirometer at 5°C. The larger figure of eight (dotted line, ) is the path of the tip of the caudal fin, and the smaller figure of eight (solid line, ) is the path of the tail at the position where the pressure sensor was sutured to the caudal peduncle. The x and y coordinates of the tail position were measured from frame-by-frame (30 Hz) playback of 8 mm film. A video frame number is indicated for each x,y data point. (B) Tailbeat differential pressure with corresponding video frame numbers versus time of the tailbeat cycle shown in A.

View larger version (15K): [in a new window]   Fig. 5. Relationship between average tailbeat amplitude and swimming speed for two cod (cod1 and cod2) (Gadus morhua) swimming in a ‘Brett-style’ respirometer at 5°C. Amplitude was measured at the tip of the caudal fin (Camp) and at the position of the pressure sensor (Samp). Filled squares, cod1 (0.58 m), Samp=10.84+8.56log10U, r2=0.95, N=201, P<0.0001; open squares, cod1 (0.58 m), Camp=12.73+4.51log10U, r2=0.58, N=158, P<0.0001; filled circles, cod2 (0.68 m), Samp=11.84+6.32log10U, r2=0.86, N=235, P<0.0001; open circles, cod2 (0.68 m), Camp=14.84+5.75log10U, r2=0.68, N=189, P<0.0001; where U is swimming speed in m s–1. Values are means ± S.E.M.

View larger version (16K): [in a new window]   Fig. 6. Tailbeat frequency versus swimming speed for seven cod (Gadus morhua) fitted with pressure sensors and two cod without instrumentation (filled circles) swimming in a ‘Brett-style’ respirometer at 5°C. fTB=22.73+97.08U, r2=0.85, P<0.0001, N=144; where fTB is tailbeat frequency in beats min–1 and U is swimming speed in m s–1.

View larger version (21K): [in a new window]   Fig. 7. (A) Integrated tailbeat pressure versus swimming speed for seven cod (Gadus morhua) swimming in a ‘Brett-style’ respirometer at 5°C. IP=19.934x10.611U, N=84, r2=84.9, P<0.0001; where IP is ‘integrated pressure’ (Pa) and U is swimming speed (m s–1). (B) Maximum minus minimum pressure or ‘pressure difference’ versus swimming speed for seven cod. PD=44.63x48.22U, r2=0.87, P<0.0001, N=147, where PD is ‘pressure difference’ (Pa). The double circles represent pressure data calculated from Dubois and Ogilvy (Dubois and Ogilvy, 1978) for swimming bluefish (Pomatomus saltatrix).

View larger version (18K): [in a new window]   Fig. 8. Rate of oxygen consumption (O2; µmol min–1 kg–1)versus swimming speed (U; m s–1); for six cod (Gadus morhua) swimming in a ‘Brett-style’ respirometer at 5°C. Different symbols represent individual fish. O2=22.33x5.75U, r2=0.77, P<0.0001, N=60, where 22.33 is O2 at zero velocity (the standard metabolic rate).

View larger version (19K): [in a new window]   Fig. 9. Rate of oxygen consumption (O2; µmol min–1 kg–1) versus integrated (IP) and difference (DP) pressure (Pa) for six cod (Gadus morhua) at 5°C. Different symbols represent individual fish. (A) O2=23.4+0.469IP, r2=0.68, P<0.0001, N=52, where 23.4 µmol min–1 kg–1 is O2 at zero ‘integrated pressure’. (B) O2 versus maximum minus minimum pressure or ‘pressure difference’ (PD). O2=34.7+0.051PD, r2=0.43, P<0.0001, N=48, where 34.7 µmol min–1 kg–1 is O2 at zero pressure difference.