Toxicants and Osmoregulation in Fish

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Abstract
Fish are extremely sensitive to many water-borne toxicants, because these affect the gills by increasing the permeability to water and ions of the gill epithelium and by inhibition of the ion exchange activity of the chloride cells. The compensatory responses of the fish will significantly increase the energy required for maintenance of water and ion homeo- stasis, and this will result in reduced growth and reproduction. The effects of toxicants are to a great extent comparable to those of stressors such as confinement, transport, and handling, not only where the endocrine and metabolic responses are concerned, but also with respect to the osmoregulatory disturbances produced. Stressors may affect osmoregulation indirectly through the action of catecholamines on the gills. Further- more, stressors induce immunosuppression and this may result in gill damage by infectious agents. Many toxicants evoke a stress response, and thus it is difficult to determine the mechanism of action of toxicants on the gills, because the specific effects of the toxicants are hard to distinguish from the effects of non-specific stress responses on the gills. This further implies that the negative effects of many toxicants and non- toxicant stressors on gill structure and hydromineral balance are additive. This aspect needs more attention in aquaculture.

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