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Nuclei, or microbubble, populations control the inception and dynamics of cavitation. It is therefore important to quantify distributions in cavitation test facilities to rigorously model nucleation dynamics. Measurements of natural nuclei population dynamics were made in two test facilities in Australia and Japan via mechanical activation using a Cavitation Susceptibility Meter (CSM). A range of tunnel operating parameters, including pressure, velocity and dissolved oxygen (DO) content, were investigated. The DO saturation condition upstream of the test section is found to provide a threshold as to whether the population is affected by DO in the Australian test facility. Historical trends in the population are quantified, indicating that regular monitoring is required. Variation of the population around the Australian cavitation tunnel circuit was studied by varying the water sampling location. Provided the water remains undersaturated, as defined above, the natural nuclei population in the test-section can be measured by sampling from the lower-limb resorber. Comparisons are made between test facilities in Australia, Japan and other countries, as well as environmental waters, using different measurement techniques. Optical and acoustic methods show microbubbles in the size range of 10 to 100 µm typical of those used to