In this paper, I show how data that are routinely collected by survey vessels can be used to estimate the number of concentrations of krill (Butterworth, 1988; Mangel, 1988) in a given region of the Southern Ocean. Sample computations are performed, using data collected by Soviet research/survey vessels in the early 1980s. These examples highlight the need for a navigational log as well as a fishing log in order to make accurate inferences. In the appendix, a method for correcting for the bias in the detection of concentrations is described.