Abstract
A Delaware Bay, USA, standardized survey of spawning horseshoe crabs, Limulus polyphemus, was carried out in 1999 − 2013 through a citizen science network. Previous trend analyses of the data were at the state (DE or NJ) or bay-wide levels. Here, an alternative mixed-model regression analysis was used to estimate trends in female and male spawning densities at the beach level (n = 26) with the objective of inferring their causes. For females, there was no overall trend and no single explanation applies to the temporal and spatial patterns in their densities. Individual beaches that initially had higher densities tended to experience a decrease, while beaches that initially had lower densities tended to experience an increase. As a result, densities of spawning females at the end of the study period were relatively similar among beaches, suggesting a redistribution of females among the beaches over the study period. For males, there was a positive overall trend in spawning abundance from 1999 to 2013, and this increase occurred broadly among beaches. Moreover, the beaches with below-average initial male density tended to have the greatest increases. Possible explanations for these patterns include harvest reduction, sampling artifact, habitat change, density-dependent habitat selection, or mate selection. The broad and significant increase in male spawning density, which occurred after enactment of harvest controls, is consistent with the harvest reduction explanation, but there is no single explanation for the temporal or spatial pattern in female densities. These results highlight the continued value of a citizen-science-based spawning survey in understanding horseshoe crab ecology and conservation.
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Acknowledgments
We thank the many individuals and organizations involved in the Delaware Bay spawning survey, including Benjie Swan, Bill Hall, Sherry Bennett, Stewart Michels, Jordan Zimmerman, Glenn Gauvry, Mike Oates, Delaware Coastal Program and the Delaware National Estuarine Research Reserve (DNERR), Audubon, The Nature Conservancy, the Slaughter Beach crew, Delaware Department of Natural Resources and Environmental Control, New Jersey Department of Environmental Protection, and US Fish and Wildlife Service. We thank Ken Gerow for suggesting the mixed-model approach. We appreciate comments from Jane Brockmann, Benjie Swan, Conor McGowan, and Penelope Pooler on early drafts of the paper. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government.
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Communicated by Wayne S Gardner
Appendices
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Appendix 2
Comparison of residuals from regressions with untransformed and square-root-transformed female density as dependent variable. Similar patterns were seen for modeling of male density
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Smith, D.R., Robinson, T.J. Horseshoe Crab Spawning Activity in Delaware Bay, USA, After Harvest Reduction: A Mixed-Model Analysis. Estuaries and Coasts 38, 2345–2354 (2015). https://doi.org/10.1007/s12237-015-9961-3
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DOI: https://doi.org/10.1007/s12237-015-9961-3