An age-based cohort model for estimating the spawning mortality of semelparous cephalopods with an application to per-recruit calculations for the northern shortfin squid, Illex illecebrosus

Semelparity is a common life history characteristic of squids and other cephalopod species, yet most cephalopod stock assessment models assume a constant natural mortality rate that does not account for post-spawning mortality. Since older individuals are more likely to spawn, natural mortality is likely to increase with age. We provide a method of estimating the spawning mortality rate of adult semelparous cephalopods, using Illex illecebrosus as an example. The model estimates the rates of maturation and spawning mortality as a continuous function of age for an unfished cohort of females. Model estimates of natural mortality were much higher than those typically assumed for semelparous cephalopod stocks under a constant natural mortality rate (0.02–0.06 week−1). Maturation and spawning mortality rates from the model were incorporated into per-recruit models designed to account for the semelparous life history of cephalopods to illustrate the effects of these parameters on biological reference point estimates. Biological reference point estimates were fairly insensitive to the assumed level of non-spawning natural mortality. However, neglecting to account for ageing error in natural mortality rate estimates resulted in a substantial underestimation of reference points. The model produces biologically plausible results and should be applicable to a wide range of semelparous cephalopod stocks.