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

Abstract

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 N 0 , 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 approx 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.