Evaluation of fishery-induced sperm limitation in Chesapeake Bay blue crab using an individual-based model

Sperm limitation, where the reproductive output of a population is restricted by its sperm production, is a concern for several crustacean species around the world. We used a simulation study to evaluate the effects of different fishing pressures and regulations on the male to female sex ratio and the average number of sperm received per female for the blue crab Callinectes sapidus population in Chesapeake Bay. We created an individual-based model that tracked a cohort of immature females on a daily step over a 2 yr period as the cohort grew, matured, and mated with mature males. The model included sex-specific growth, maturity, and mating of a closed population. We ran the model under multiple scenarios of mate choice behavior to quantify the patterns in sperm counts and sex ratios across a broad range of simulated fishing pressures and regulations. Average sperm counts in mated females and male to female sex ratios of the modeled population varied among scenarios, but were not related to each other. Fishing pressure had a significant negative effect on average sperm per female only in scenarios with unfished females and the exploitation rate for males at 5 times that currently estimated in Chesapeake Bay. Our results suggested that sperm per female is not linearly related to mature male to female sex ratios and that sperm limitation does not appear to be a concern for the blue crab population of Chesapeake Bay under current regulations.