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

Abstract

Stage duration is a key parameter in modeling population dynamics of stage-struc-tured populations, however stage duration can be difficult to estimate accuratelybecause calculations are based on survivors and fail to account for mortality. Herewe show a method by which development time and stage duration can be adjustedafter accounting for within-stage mortality. As an example, we use a previouslypublished data set on stage-specific abundances of the euphausiid, Euphausia pacificafrom four laboratory-incubated cohorts, raised from egg to juvenile stage. We useda stage-structured population model along with an inverse algorithm to estimatestage durations, both with and without considering mortality rates. Results suggestthat the overall stage duration from egg to furcilia was 48 days when mortalitywas considered, and 60 days when mortality was not included. Stage-specificmortality rates were generally lower than 10% per day for those stages with rela-tively high mortality rates (egg and naupliar stages and furcilia VI–VII) and ,4%for other developmental stages. However, the difference between the estimateswith and without considering mortality rates did not show a clear relationshipwith stage-specific mortality rates. More robust models that can incorporate thevariability in developmental pathways and physiological changes that occur duringthe molting process may be necessary to further investigate how age distributionwithin stage and mortality influence the estimation of stage duration.KEYWORDS: Euphausia pacifica; stage duration; stage-specific mortalitystage-structured population model; inverse technique