Page 431 - The Toxicology of Fishes

The Osmoregulatory System 411

350

300
Cortisol (ng/mL pasma) 200
250

150

100
50
2 min
0
1 3 5 7 9 11 13 15
Fish number
FIGURE 8.5 Effect of capture stress on plasma cortisol levels of Mozambique tilapia (Oreochromis mossambicus). Open
circles are fish receiving food contaminated with PCB 126 (50 µg per kg fish per day for 5 days). The fish were captured
subsequently at intervals of 2 minutes. Fish number 1 shows a resting cortisol level; the following fish already show a rise
to a plateau level 5 minutes after. The plateau level of the PCB-treated fish is significantly lower than that of control fish
(black squares), indicating an impaired cortisol response to capture by PCB 126. (From Quabius, E. S. et al., Gen. Comp.
Endocrinol., 108, 478–482, 1997. With permission.)

osmoregulation (McDonald and Milligan, 1997); however, the actual impact of the disruptive effects
of some toxic chemicals on this process remains to be established.
The fact that toxic agents can act as stressors also implies that their effects are partially additive with
those of other stressors. This is indicated by observations on Mozambique tilapia exposed to cadmium. In
male fish with a low hierarchical position, the reduction of plasma osmolarity and the rise in plasma cortisol
levels were more pronounced than in the dominant males (Wendelaar Bonga, 1997). Similarly, handling
of the fish or crowding, known to cause a stress response in rainbow trout (Balm and Pottinger, 1995),
makes these fish more sensitive to toxic agents. This is an aspect that requires more study not only in
aquaculture, where, for example, the toxic effects of ammonia can be aggravated by poor holding conditions
or rapid temperature changes, but also in toxicological risk analysis. For proper determination of no-effect
levels or of lethal concentrations, any potentially stressful factor other than the toxic chemical should be
absent. In many cases, the experimental conditions used to estimate toxicity did not meet this criterion.
On the other hand, no effect and other toxicity levels determined under such rigorous conditions will easily
lead to underestimation of the toxicity of a chemical in situations where the toxicant is not the only stressor.

References

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Baldisserotto, B., Kamunde, C., Matsuo, A., and Wood, C. M. (2004b). Acute waterborne cadmium uptake
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