In the common models of stock assessment, fish populations are stable and minor disturbing forces (e.g. small increments in fishing effort) cause only minor changes in the equilibrium position. Clark's (1974) conclusion, based on a “salmon” type of fish, that if there is a depensatory part of the stock/recruit relation, there will be an unstable situation, is extended to all types of population. A similar situation, in which a small increment in fishing effort can result in a collapse of the stock can also occur if (as seems to happen in some pelagic fisheries) the catchability coefficient, q, can increase with decrease in fish stock. This potential for collapse can be serious in fishery management unless there is adequate provision for monitoring the stocks, and for rapid implementation of control measures as they become necessary.