Fish 511/Fall 2014
Physiological Responses of Fish to Exercise
I. Effect of exercise on the metabolic demand for oxygen:
A. Several measures of oxygen demand are in use:
•Standard (= basal) metabolic rate. Animal is at complete rest and in post-absorptive state.
Fish confined in a respirometer are usually not totally inactive, so the standard rate may
be estimated by extrapolation of a VO2 vs. activity regression to zero activity.
•Routine metabolic rate. Includes effect of "normal" activity; hence, not well-defined.
•Active metabolic rate. Usually standardized as the max. VO2 sustainable for 30 min (or 1 h).
B. The difference between the standard and active rates is the scope for activity or
metabolic scope. (The metabolic scope is affected by temperature–the maximum
metabolic scope has been shown to coincide with the preferred temperature of a number
of species.)
C. Oxygen delivery is adequate to meet the O2 demand of the tissues at exercise levels up to
the maximum sustainable swimming speed. Higher speeds are possible, but the extra cost
is met by anaerobic metabolism, with accumulation of lactic acid. Anaerobic metabolism
is inefficient: as swimming speed increases, the time to exhaustion decreases rapidly
(Figure).
Swimming speed
4 lengths/s
5 lengths/s
8 lengths/s
10 lengths/s
Time to exhaustion
Rainbow trout
Jack mackerel
1- 6 h
indef.
1 m (note change!)
-< 20 s ("sprint")
6h
< 3 s ("burst")
3m
"Burst" or "sprint" swimming activity is followed by minutes or even hours of recovery with
elevated gill ventilation (hyperventilation) and disturbed acid-base balance. Some fishes (e.g.
upstream-migrating adult salmon) may perform at 75% or more of the active metabolic rate
for weeks.
II. Comparison of metabolic rates of fish with other vertebrates:
A. Aerobic metabolism: Fish and other poikilotherms have much lower standard, routine
and active metabolic rates than homiotherms. Active rates for fish are 10% of rates for
medium to large mammals and only 1% of rates for small mammals and birds. These
differences are apparently related to differences in the pumping capability of the hearts of
warm- and cold-blooded animals (see conclusion re cardiac output of fish heart as
limiting factor, page 2).
B. Anaerobic metabolism. The instantaneous metabolic energy demand during burst
swimming of fish (largely anaerobic) may exceed the maximum sustainable rate
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Fish 511/Fall 2014
(aerobic) by 10-fold. Differences between burst metabolic rates for poikilotherms and
homiotherms are much less than differences between active metabolic rates.
III. Immediate physiological responses to exercise include changes in:
1) Gill ventilation rate
2) Cardiac output
3) Venous oxygen saturation (“venous reserve”).
Gill ventilation rate (Vg): one study with rainbow trout (Table below) showed that at
maximum sustained swimming speed VO2 was increased 7.8x over the resting rate
and Vg was increased 9x (from 200 to 1800 mL water/kg/minute) over the resting
rate. The increase in gill ventilation rate was, however, considerably less than seen
in trout in response to hypoxia (18X over resting; up to 3600 ml/kg/minute).
Many larger species capable of fast, sustained swimming switch over to ram
ventilation at higher speeds (ram ventilation is not important for small fishes because
they can not cruise at an adequately high speed).
.
Cardiac output (Vb): Both heart rate (fh) and stroke volume (SV) increase during
exercise. In the previously mentioned study with trout, fh increased 1.4x and SV
increased 2.2x, for a total increase in cardiac output of about 3 x.
The increase in SV is primarily a consequence of an increase in circulating
catecholamines, mediating increased contractility of cardiac muscle (inotropic
effect) and increased venous return (increasing cardiac filling and contractility
according to Starling's Law). The increase in fh (of lesser importance than
increased SV) is mediated by decreased vagal (cholinergic) tone.
O2 saturation of blood: Arterial PO2 (PaO2) remains high and O2 content (Ca O2) remains
near saturation at all swimming speeds, but venous O2 saturation may decrease; e.g., in
the previously referenced study on trout, venous O2 saturation declined from 70% at rest
down to 15% at critical swimming speed. Hence, the delivery of O2 per unit volume of
blood to the working muscle may increase several-fold as the "venous O2 reserve" is
reduced (Figure).
Important conclusion: during exercise, the gill ventilation rate does not increase to
the extent possible (i.e. to the extent seen in hypoxia); nevertheless, arterial O2
remains near saturation, even at the maximum sustainable swimming speed.
Hence, O2 transfer across the gill surface does not seem to be limiting maximum O2
uptake: the limiting factor for aerobic oxygen transport must be cardiac output.
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Fish 511/Fall 2014
Changes in the oxygen transport system of rainbow trout during
maximum sustained exercise (from Kiceniuk and Jones 1977)
Resting
Oxygen uptake
VO2
Active
=1
7.8x
Vb
SATA,O2
SATV,O2
O2 Utilization
=1
100%
70%
30%
3x
5%
15%
80%
Vg
CI,O2
CE,O2
O2 Extraction
1
10 mg/l
5 mg/l
50%
9x
10 mg/l
6.5 mg/l
35%
Blood
Water
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