The monitoring of biological effects: the separation of natural changes from those induced by pollution.

An added mortality rate of eggs, larvae and juveniles of fish populations, or impact, is assumed to be density independent. The total mortality from hatching to recruitment is represented by the fecundity, and any increment in density independent mortality implies a decrement in density dependent mortality. At high stock the condequence is an increase in stock towards a position of less resilience: at low stock less resilience is found with a decrease in stock. In general impact generates a shift of K-strategy, the self-stabilizing strategy, to r-strategy, and opportunistic one. In a fish population very little impact should be tolerated at low stock because it would prevent recovery to a management objective such as maximum sustainable yield. At high stock, impact may generate more stock at an unknown risk.