The basic characteristic of world fisheries is a persistent tendency toward depletion of fish stocks accompanied by high levels of capital and labour devoted to fishing, comparatively low catches and poor economic returns. At times the trend to depletion in the fishery is slow. As grounds close to port are depleted and the catch declines fishermen move further afield working new areas; but in time the productivity of the new grounds is also reduced and fishermen move to still more isolated areas. Such a process cannot continue indefinitely and eventually total catch and overall catch per unit effort (effort being the inputs of factors of production) begin to fall. At other times the fall in catch is sudden as in the collapse of the Peruvian anchovy fishery; 1 the fishery may be exploited to its limit and perhaps a small environmental change has a large effect on population and total catch.