Assessing the potential biases of ignoring sexual dimorphism and mating mechanism in using a single-sex demographic model: the shortfin mako shark as a case study

Abstract

Most demographic models explicitly track the dynamics of a single sex. However, single-sex models may lead to rather poor results when vital rates and other demographic parameters differ between the sexes. Consequently, the population may not be optimally managed if management measures are based on a single-sex model, which suggests the need for two-sex models. The consequences of ignoring sexual dimorphism and mating mechanisms when proposed management strategies are based on single-sex demographic models are illustrated for shortfin mako shark, Isurus oxyrinchus, in the Northwest Pacific Ocean. Analyses based on single-sex models would underestimate the probability of decline risk. The findings imply that management decisions should be based on sex-specific models to better achieve management goals. The mating mechanism principally affects the proportion of breeding females, which has a large impact on the perceived population growth rate. Which mating function best describes the dynamics of shortfin mako populations is still unknown. More research is needed to avoid making inaccurate management decisions, such as genetic paternity studies to clarify the mating mechanism for this species.