1. In fisheries models, the factor (q) relates abundance to stock size (x = qN) and fishing mortality to fishing effort (F = qf). Following the latter, (q) is a measure of fishing mortality generated on a stock by one unit of effort; 2. The fraction of a fish stock which is caught by a defined unit of the fishing effort. When the unit is small enough that it catches only a small part of the stock--0. 01 or less-it can be used as an instantaneous rate in computing population change.