Unnatural selection: Effects of a male-focused size-selective fishery on reproductive potential of a blue crab population

Abstract

A male-focused size-selective fishery, like the one targeting the blue crab ( Callinectes sapidus Rathbun) along the Atlantic and Gulf coasts of the U.S., has the potential to reduce the average size of the males in the population, reduce the density of males in the population, and/or raise the ratio of females to males. All of these may affect the mating dynamics of the population by reducing the amount of sperm that males provide to females and decreasing the number of males available for copulation. We investigated the effect of the fishery on a blue crab population in upper Chesapeake Bay by collecting crabs in areas of markedly different fishing pressure. Crabs were taken as individuals and as mating pairs, which permitted assessing the size of males currently mating in nature, and the seminal resources they possess and transfer to females. Average size of males in subpopulations that have been subjected to heavier fishing pressure is indeed smaller, and the smaller males pass less sperm and accessory fluid to females than would larger males if they were still present. Some males in pre-copulatory pairs are as sperm depleted as males that had just completed copulation, indicating that they are mating more frequently than they can replace their seminal resources. The most sperm-depleted males in the population are not even pairing or attempting to mate.