Changes in the distribution of fishing effort over the season can influence trends in annual catch per unit effort (CPUE) because the vulnerability of the target species varies seasonally. We use annual and monthly time series analysis of CPUE data for a lobster fishery to show that time trends in CPUE are dependent upon the time interval selected. Changes in the spatial distribution of effort will almost certainly have similar consequences. Modellers and managers of fisheries need to quantify the effects of economic and regulatory changes on the temporal and spatial allocation of fishing effort. (C) 1997 Elsevier Science Ltd.