Complex recruitment dynamics with Doppler-like effects caused by shifts and cycles in age-at-recruitment

Even with constant production (N-0) and constant time-specific mortality (Z, daily), recruitment can vary greatly. A new variable-lag phenomenon modelled here generates temporal variation in recruitment from temporal shifts in age-at-recruitment (AAR; here synonymous with stage duration). The phenomenon is due to two components, one being analogous to the Doppler effect and the other an interaction of dAAR/dt with Z, and is distinct from those identified in previous contributions not focusing on temporal structure. Recruitment can be predicted from No, Z, and AAR, using either numerical simulation or, in many cases, the modified Doppler equation given here. Seasonal recruitment cycles predicted for a sinusoidal AAR function fitted to natural data, with constant larval production and constant Z (varied among, not within, model runs), were strongly peaked with a difference of 20% between maximum and minimum recruitments at Z = 0, to a difference of 2000% at Z approximate to 0.2, The model gives indirect access to parameters difficult to measure in the field: if temporal variation is adequately described in two of the three input variables (N-0, Z or AAR), differences (residuals) between predicted and observed recruitment can indicate an unaccounted pattern in the unknown.