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Kinematics of plaice, Pleuronectes platessa, and cod, Gadus morhua, swimming near the bottom
School of Natural Resources and Environment and Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109-1119, USA
* e-mail: pwebb{at}umich.edu
Accepted 24 April 2002
The kinematics of plaice (Pleuronectes platessa, L=22.1 cm) and
cod (Gadus morhua, L=25.0 cm, where L is total fish length)
swimming at various speeds at the bottom and lifted to heights, h, of
10, 50 and 100 mm by a thin-wire grid were measured. For cod, tailbeat
frequency, amplitude, body and fin span and propulsive wavelength were
unaffected by h and varied with speed as described for fusiform
pelagic species. In contrast, the kinematics of plaice was affected by
h. Body and fin spans and propulsive wavelength were independent of
swimming speed and h. Tailbeat amplitude was independent of swimming
speed, but averaged 1.5 cm at h=0 and 2.5 cm at h
10 mm.
Plaice tailbeat frequency increased with swimming speed for fish at the bottom
but was independent of swimming speed at h=10, 50 and 100 mm,
averaging 4.6, 6.0 and 5.8 Hz respectively. Total mechanical power,
P, produced by propulsive movements calculated from the bulk-momentum
form of elongated slender-body theory was similar for cod and plaice swimming
at the bottom but, at h10 mm, P for plaice was larger
than that for cod. Plaice support their weight in water by swimming at a small
tilt angle. The small changes in swimming kinematics with swimming speed are
attributed to decreasing induced power costs to support the weight as speed
increases. The contribution of the tail to power output increased
monotonically with the tail gap/span ratio, z/B, for
z/B=0.23 (h=0 mm) to z/B=1.1 (h=50 mm).
The smaller tailbeat amplitude of the tail decreased both z/B and the
power output for plaice swimming at the bottom. For the maximum body and fin
span of plaice, the contribution to power output increased for local
z/B values of 0.044 (h-0 mm) to 0.1 (h=10 mm) and
declined somewhat at larger values of z/B. The smaller effect of the
bottom on power output of the largespan anterior body sections may result from
the resorption of much of the upstream wake at the re-entrant downstream
tail.
Key words: kinematics, plaice, Pleuronectes platessa, cod, Gadus morhua, swimming, power, tailbeat amplitude
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