Natural mortality (M) is crucially important for stock assessment since it strongly influences estimates of spawning stock biomass, MSY and fishing mortality. Variation in M can occur between sexes, maturity stages and temporally. Estimates of M are confounded with those of catchability, recruitment, and growth. Previous studies, which focused on age-structured population dynamics models, suggest that it would be better to estimate M rather than pre-specify it. Size-structured population dynamics models, used for species that are hard to age such as crabs, have unique characteristics such as a size-transition matrix and the possibility of a terminal molt, which could impact the ability to estimate M. Therefore, this study explores the ability of size-structured stock assessment models to estimate M for several scenarios. These scenarios include whether the operating model and estimation method have a terminal molt, whether M is constant, time-varying, or sex- and maturity stage-specific, the quality/sample size of the biomass index and the size-composition data and whether growth is estimated simultaneously with M. The results show that size-structured models can estimate time-varying, sex- and maturity stage-specific M when the estimation method mirrors the operating model. Surprisingly, terminal molt does not affect the ability to estimate M. However, estimating growth simultaneously with M has a negative impact on the ability to estimate M but a positive effect on the quality of the estimates of spawning stock biomass.

自然死亡率 ( M ) 对于种群评估至关重要，因为它强烈影响产卵种群生物量、MSY 和捕鱼死亡率的估计。M的变化可以发生在性别、成熟阶段和时间上。M的估计与可捕获性、招募和增长的估计相混淆。以前的研究侧重于年龄结构的人口动态模型，表明最好估计M而不是预先指定它。用于难以老化的物种（例如螃蟹）的规模结构种群动态模型具有独特的特征，例如尺寸转换矩阵和终端蜕皮的可能性，这可能会影响估计M的能力. 因此，本研究探讨了规模结构库存评估模型在几种情况下估计M的能力。这些情景包括操作模型和估计方法是否具有终端蜕皮、M是否为常数、时变或性别和成熟阶段特定、生物量指数和大小组成数据的质量/样本大小以及是否增长与M同时估计。结果表明，当估计方法反映运营模型时，规模结构模型可以估计时变、性别和成熟阶段特定的M。令人惊讶的是，终端蜕皮不影响估计M的能力。然而，同时估计增长M对估计M的能力有负面影响，但对产卵种群生物量估计的质量有积极影响。