An age-structured population model for horseshoe crabs in the Delaware Bay area to assess harvest and egg availability for shorebirds |
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| Authors: | John A Sweka David R Smith Michael J Millard |
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| Institution: | 1.Northeast Fishery Center,U.S. Fish and Wildlife Service,Lamar;2.Leetown Science Center,U.S. Geological Survey,Kearneysville |
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| Abstract: | The objective of this simulation study was to create an age-structured population model for horseshoe crabs (Limulus polyphemus) in the Delaware Bay region using best available estimates of age-specific mortality and recent harvest levels. Density dependence
was incorporated using a spatial model relating egg mortality with abundance of spawning females. Combinations of annual female
harvest (0, 50, 100, and 200 thousand), timing of female harvest (before or after spawning), and three levels of density-dependent
egg mortality were simulated. The probability of the population increasing was high (>80%) with low and medium egg mortality
and harvest less than 200 thousand females per year. Under the high egg mortality case, the probability of the population
increasing was <50% regardless of harvest. Harvest occurring after spawning increased the probability of population growth.
The number of eggs available to shorebirds was highest when egg mortality was lowest and female abundance was at its highest
levels. Although harvest and egg mortality influenced population growth and food availability to shorebirds, sensitivity and
elasticity analyses showed that early-life stage mortality, age 0 mortality in particular, was the most important parameter
for population growth. Our modeling results indicate areas where further research is needed and suggest effective management
will involve a combination of harvest management and actions to increase early juvenile survival. |
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