Spatial differences in female lobster mating success and population characteristics in southern Massachusetts

Abstract

The Southern New England stock of American lobster Homarus americanus is currently severely depleted and experiencing recruitment failure, potentially resulting from increased larval or juvenile mortality, decreased larval production, or both. We examined one aspect of larval production, female mating success, in 2 regions of Buzzards Bay, Massachusetts. The inner Bay areas are typically male-dominated, and the deeper regions at the mouth of the Bay have higher proportions of females. These locations were specifically selected to test our primary hypothesis that female-skewed sex ratios would decrease female mating success. Using a non-invasive method to detect ejaculates stored within females’ seminal receptacles, mating success was determined for 3851 females. Contrary to our initial hypothesis, mating success was higher in the outer areas where the sex ratio was female-skewed. We then explored 2 alternative explanations for this spatial pattern in mating success. First, using the ratio of abdominal width to carapace length (ABD:CL) to approximate maturity stage, a large proportion of females from the inner Bay appeared to be immature or primiparous, while females from the outer regions (with a larger ABD:CL ratio) were likely multiparous. Thus, lower mating activity inside Buzzards Bay corresponded with a higher proportion of immature or newly matured females for any given size. Second, low densities of lobsters inside Buzzards Bay may have resulted in a mate-finding Allee effect, leaving some mature females unmated. These 2 alternative explanations for the spatial pattern in mating success are not mutually exclusive; both may be impacting female mating success in the region.