Implications of a Decrease in the Mature Size of Female Blue Crab, Callinectes sapidus

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Title: Implications of a Decrease in the Mature Size of Female Blue Crab, Callinectes sapidus
Author: Davis, John Howell
Advisors: David B. Eggleston, Committee Member
Donna L. Wolcott, Committee Chair
Thomas G. Wolcott, Committee Member
Abstract: The size-at-maturity of female blue crabs is declining and the proportion of very small mature females (<100mm carapace width (CW)) is increasing North Carolina. Decreasing mature size of the female may have an effect on the mating success and mortality of size-disparate mating pairs. To test the effect of body size on mating behavior, I used controlled mating experiments comparing size-disparate mating pairs with similarly-sized mating pairs. Small males handled large females longer than any other mating pair combination; however, no increase in injury or mortality was found. Neither large nor small males transferred significantly different numbers of sperm to large and small females. Size-disparity appears to have no effect on mating success in blue crabs, suggesting that diversity of size-related genotypic traits can be preserved in the population. I also designed a mathematical model to estimate the lifetime spawning potential of different size-cohorts of female blue crabs subject to different mortality pressures. Cohorts of very small females (<100mm CW) that are not subject to fishery mortality produce 5.1 — 58.1% more eggs over two spawning seasons than larger, harvestable females (127-159mm CW), suggesting an evolutionary advantage to maturing at smaller sizes. I also used the model to estimate the efficacy of a proposed restriction on the commercial harvest of large females (>172mm CW), intended to increase the spawning potential of large females in order to increase population-wide recruitment, as well as the number of recruits that may possess a larger-size genotype. The proposed seasonal (Sept. — Apr.) commercial harvest of large females produced an estimated 23.2 — 35.1% increase in egg production over two spawning seasons. However, since the large females compose less than 2% of the population, on average, the estimated increase in spawning potential for the entire population was negligible (0.46%). Additionally, the 5% tolerance of the regulation allows for the harvest of the entire population of large females, suggesting that the proposed restriction will be ineffective.
Date: 2006-01-26
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences

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