(a) Integrated flux versus redshift. The dotted lines show lines of constant H I mass. (b) Measured H I mass as a function of redshift. The coloured round data points in both panels indicate the 219 H I detections; the colours correspond to the spectral over-densities highlighted in the histogram in panel b, and the black data points correspond to the grey histograms that include sources not associated with any over-density. The light grey open histogram represents the optical redshift catalogue from Healy et al. (2021). The dashed grey lines indicate the 5σ H I mass limits at the three velocity resolutions used in the source finding: dashed for 45 km s⁻¹, dot-dashed for 135 km s⁻¹, and dotted for 225 km s⁻¹.