Modeling growth of multiple recruitment cohorts of Dungeness crab co-occurring within the central Salish Sea

The carapace dimensions of Dungeness crab (Metacarcinus magister) at a given molt stage vary greatly throughout their range. Although growth models have been developed for various coastal Dungeness crab populations, the patterns of growth for juvenile Dungeness crab in the central Salish Sea remain largely unknown. Larval Dungeness crab within this area are believed to be sourced from at least two different cohorts, distinguished by timing of arrival to the region and differences in carapace dimensions. The co-occurrence of multiple cohorts complicates efforts to understand the population dynamics of this critically important species. Thus, in 2019 we began rearing larvae from two phenotypically-distinct cohorts under common conditions in the laboratory to evaluate differences in carapace dimension, intermolt period, and molt increment. Mixed-effect model analyses of growth metrics collected at the individual level revealed that cohort-of-origin had a significant effect on growth at all levels, despite the presence of individual varying growth effects. Overall, late cohort individuals were found to molt more frequently albeit at smaller increments. These individuals are projected to grow slower initially, yet they may have the ability to effectively catch up in size through additional molt events. Through this experiment, we demonstrated that cohort-specific growth schedules were maintained regardless of the influence of temperature and food availability. Our results suggest that timing of settlement and size at settlement could have an outsized effect on overall growth, potentially resulting in variable time to reach significant milestones, such as sexual maturity or harvest size.