Mortality rate was higher in the contaminated environments (figure 8.17B), possibly as a result of the increased metal levels in the body (figure 8.17C). Note, however, that a certain amount of copper is needed physiologically, so that in a completely deficient environment growth rate must be reduced and mortality rate increased (figure 8.17B). In this study, growth rate was reduced at higher levels of pollution, presumably because the animal moulted more frequently, but metals were not completely eliminated from the body, so mortality rates were still increased.