Estimated development times for stage-structured marine organisms are biased if based only on survivors

Stage duration is a key parameter in modeling population dynamics of stage-structured populations, however stage duration can be difficult to estimate accurately because calculations are based on survivors and fail to account for mortality. Here we show a method by which development time and stage duration can be adjusted after accounting for within-stage mortality. As an example, we use a previously published data set on stage-specific abundances of the euphausiid, Euphausia pacifica from four laboratory-incubated cohorts, raised from egg to juvenile stage. We used a stage-structured population model along with an inverse algorithm to estimate stage durations, both with and without considering mortality rates. Results suggest that the overall stage duration from egg to furcilia was similar to 48 days when mortality was considered, and similar to 60 days when mortality was not included. Stage-specific mortality rates were generally lower than 10% per day for those stages with relatively high mortality rates (egg and naupliar stages and furcilia VI-VII) and < 4% for other developmental stages. However, the difference between the estimates with and without considering mortality rates did not show a clear relationship with stage-specific mortality rates. More robust models that can incorporate the variability in developmental pathways and physiological changes that occur during the molting process may be necessary to further investigate how age distribution within stage and mortality influence the estimation of stage duration.