Canopy removal by storms is a primary cause of mortality for the small kelp Ecklonia radiata in temperate Australasia. We simulated hydrodynamic drag from storms with in situ pull-tests to determine whether thallus size, life stage, aggregation, wave exposure and substratum affect the canopy removal process. A total of 466 individuals were pulled off 20 rocky reefs at 8-10 m depth in southwestern Australia. The majority (71%) of thalli dislodged at the rock implying that the canopy removal process in southwestern Australia is a substratum-controlled process. Dislodgment bared clean substratum where re-invasion by propagules or encroachment would be necessary to fill up the gaps. Maximum break forces (150-250 N) were found for large late stage kelps and kelp aggregates from wave exposed sandstone and granite reefs, and minimum values (25-100 N) for small juveniles and solitary kelps from protected limestone reefs. By applying size and break force data to the drag equation, water velocities required to break or dislodge E radiata were calculated to 2-5 ms(-1) for large kelps. These velocities are frequently encountered in wave-exposed shallow subtidal habitats, suggesting that thallus size is limited by the hydrodynamic environment.