The copper-induced reduction of critical swimming speed in rainbow trout (Oncorhynchus mykiss) is not caused by changes in gill structure.

In the present experiments, we investigated, if copper exposure causes a decrease in the critical swimming speed, as has been observed earlier, and if such a decrease can be explained by a disturbance in gill structure. The results show that this is not the case. While copper exposure tended to increase the thickness of gill diffusion barrier in resting fish, a similar difference was not seen in swimming animals. However, copper exposure caused a decrease in the critical swimming speed. This decrease must thus be caused by factors other than structural changes in the gills, associated with copper exposure.