The impact of sea level rise on fiddler crab (Uca pugnax ) populations in southern Delaware salt marshes

Hanson, Cynthia
Journal Title
Journal ISSN
Volume Title
University of Delaware
While the fiddler crab, Uca pugnax, is a dominant macrofaunal species in coastal salt marshes along the Atlantic coast of the United States, little research has been done to characterize their population distributions in the Mid-Atlantic. With this region facing increased rates of sea level rise (SLR) due to geological land subsidence and other hydrological factors, understanding local crab populations and how they will be affected by SLR is important to further understanding the ability of marshes in the region to survive and recover. In this thesis, I use data from field surveys to characterize Uca pugnax distribution for two representative southern Delaware salt marshes (Broadkill and Canary Creek) and generate models of fiddler crab presence and abundance. I compare these models with SLR model predictions for the same study marshes to predict how crab distribution may change with SLR, and determine what this means for marsh habitats in the future. The Uca pugnax surveys revealed that local populations are present throughout the entirety of the marshes, except for areas of vegetation dieback and pond formation where the substrates are flooded and inhospitable for burrows. Abundances of Uca pugnax are consistently highest in areas conducive to burrowing and near water banks, including the edges of some of the dead ponds. Sea level affecting marshes model (SLAMM) simulations showed no loss--and even growth in Canary Creek--in Uca pugnax habitat until sea levels surpassed a threshold of 0.5 m of rise; after which both marshes saw sharp declines of 60-100% of suitable habitat and predicted losses of millions of crabs. To examine the effects of population crowding caused by the loss of habitat on Uca pugnax, I conducted an enclosure field experiment in Canary Creek marsh. However, due to a breach in the enclosures all that could be confirmed was that when crowded, fiddler crabs will distribute themselves to a preferred density of ∼50 crabs/m2 with a male-biased sex ratio of 1.4:1. Overall, this indicates that Uca pugnax may spread landward with marsh migration, and adapt to different habitat types in an attempt to distribute themselves as preferred habitats disappear. My research suggests that high levels of SLR in the future will eliminate the majority of marshland and suitable Uca pugnax habitat in southern Delaware. Unless steps are taken toward wetland conservation and climate management, this will have a strong negative effect on crab population size and distribution.