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The morphology of the atomic hydrogen (H  i ) disc of a spiral galaxy is the first component to be disturbed by a gravitational interaction such as a merger between two galaxies. We use a simple parametrization of the morphology of H  i  column density maps of the Westerbork observations of neutral Hydrogen in Irregular and SPiral galaxies (WHISP) project to select those galaxies that are likely undergoing a significant interaction. Merging galaxies occupy a particular part of parameter space defined by Asymmetry ( A ), the relative contribution of the 20 per cent brightest pixels to the second‐order moment of the column density map ( M  20 ) and the distribution of the second‐order moment over all the pixels ( G  M ).  Based on their H  i  morphology, we find that 13 per cent of the WHISP galaxies are in an interaction (Concentration– M  20 ) and only 7 per cent are based on close companions in the data cube. This apparent discrepancy can be attributed to the difference in visibility time‐scales: mergers are identifiable as close pairs for 0.5 Gyr but are identifiable for ∼1 Gyr by their disturbed H  i  morphology. Expressed as volume merger rates, the two estimates agree very well: 7 and 6.8 × 10 −3  mergers Gyr −1  Mpc −3  for paired and morphologically disturbed H  i  discs, respectively.  The consistency of our merger fractions with those published for bigger surveys such as the Sloan Digital Sky Survey shows that H  i  morphology can be a very viable way to identify mergers in large H  i  surveys. The relatively high value for the volume merger rate may be a bias in the selection or WHISP volume. The expected abundance in high‐resolution H  i  data by the planned South African Karoo Array Telescope (MeerKAT), Australian SKA Pathfinder (ASKAP) and Westerbork Synthesis Radio Telescope/APERture Tile In Focus instrument (WSRT/APERTIF) radio observatories will reveal the importance of mergers in the local Universe and, with the advent of the Square Kilometer Array (SKA), over cosmic times.

Quantified H i morphology – IV. The merger fraction and rate in WHISP