This paper discusses the first nonlinear numerical simulation results of fishbone excitation by a magnetically trapped supra-thermal electron population with pressure profile peaked on axis due to, e.g. electron cyclotron resonance heating. The precession resonance underlying the linear instability is clearly identified. Meanwhile, mode saturation is shown to occur because of the secular resonant particle motion, pumping particles out of the radial region where the wave–particle power exchange is initially localized. The mode nonlinear saturation is accompanied by downward frequency chirping, due to phase-locking of the mode with resonant particles, consistent with the 'mode particle pumping' mechanism.