We assessed the impact of an additional 100 mm square mesh cylinder (SMC) on the selective property of the demersal whitefish trawl in the Celtic Sea. Sea trials were conducted on board a French trawler using twin trawl rigging. We tested the effects of the position of the SMC (in front of and behind the mandatory 100 mm square mesh panel) and of the insertion of a dispersive float. Selectivity analysis revealed the 50% retention length () to be greater than the Minimum Conservation Reference Sizes regardless of species (haddock, whiting or megrim) or rigging configuration. Results did not reveal a clear effect of the SMC position. However, the insertion of the float led to a decrease in selection ranges suggesting enhanced contact probabilities with the square meshes. With the largest values, the SMC placed in the front position (and without the float) proved to be the most selective rigging configuration and was then tested under commercial conditions. Catch comparisons revealed that the test gear retained less fish across all size classes than the commercial gear. For haddock, the test gear retained less fish below 50 cm in length. Catch comparisons also indicated significant discards of fish above the MCRS, i.e. “high-grading” practices. Under the requirements of the landing obligation (LO), unwanted catches must be stored on-board and landed regardless of economic value. The SMC is thus a valuable tool to mitigate the impact of the LO on the demersal whitefish fishing fleets operating in the Celtic Sea.