Understanding the life cycle of North Sea brown shrimp Crangon crangon: a simulation model approach

A simulation model of the life cycle of Crangon crangon tracking daily cohorts from the egg stage to the adult stage was programmed in R taking into account size-specific, seasonally varying natural mortality rates of all life stages, temperature-dependent development rates of eggs and larvae, juvenile and adult growth rates as a function of temperature and size, sex, maturation, egg production, and seasonally varying size-specific fishing mortalities. The observed seasonal patterns of commercial catch and egg production could only be reproduced with fast growth rates, which linked winter egg production with the subsequent juvenile recruitment in May-June, the commercial catch peak in September and the peak in egg production over the following winter. Likewise, substantially lowered natural mortalities in winter are essential to generate seasonal patterns similar to the observed ones. The seasonal pattern of egg production can only be reproduced with a minimum spawning age. Cohorts from summer eggs rapidly follow the earlier winter egg cohorts, forming an annual joint wave of growing shrimp. The summer egg cohort originates from summer eggs of the previous year and, depending on winter and spring mortality levels, also from large individuals of the preceding winter egg cohort. The model represents a new tool to investigate maximum yield per recruit in this species as a function of seasonal effort levels, mesh sizes and seasonal closures under different predation and temperature scenarios.