A system and method for determining the concentration of zooplankton in a continuous zooplankton culture system. The system may include a transmissiometer, a sample tube, a first sample containing water and microalgae, and a second sample containing water, microalgae, and zooplankton. The concentration of zooplankton in the system may be calculated by comparing the transmissiometer output from the first sample and the second sample, wherein each of the outputs are obtained by moving each sample into the sample tube such that the transmissionmeter is positioned to take a reading across the tube.