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Spatial distribution and risk analysis data for diamond-backed terrapins relative to crab trapping, Savannah Coastal Refuge Complex, USA

September 14, 2018

There were five objects for compiling these data: 1) to conduct a systematic inventory of diamond-backed terrapins (Malaclemys terrapin) using headcount surveys on four National Wildlife Refuges (Harris Neck, Blackbeard Island, Wassaw, and Wolf Island) that are a part of the greater Savannah Coastal Refuges Complex in southeast Georgia, USA; 2) to use occupancy modeling to determine detection probability and distribution of diamondback terrapins; 3) to perform risk assessments of these terrapin populations in relation to crabbing pressure; 4) to generate maps with associated GIS files of terrapin relative densities related to crabbing pressure; and 5) to provide scientific information that will allow for effective management of diamond-backed terrapins on these National Wildlife Refuges. The data collected during the course of the systematic inventory of diamond-backed terrapins includes information on terrapin detection in tidal creeks on refuges, crab pot numbers and locations, and a variety of environmental-, location-, and observer-related variables to examine the effects of both environmental conditions and observer bias on terrapin detection. Data were collected on crabbing pressure, terrapin detections, and other variables related to date, time, location, environment, observer, and transect length and IDs. Data was collected simultaneously. For example, while performing headcounts, we were also noting crab trap locations, environmental, and observer variables. Data resulting from occupancy modeling is contained in the data.csv file and was used to test hypotheses about the scale at which terrapin head counts respond to crab trapping. Three covariates were considered for generalized linear mixed effects models, including the average observed crab pot density at either the creek scale (creek_cpot) or the refuge/island scale (island_cpot), and the cloud metric (environmental variable as cloud cover can have an influence on terrapin detectability). The risk assessment used data from terrapin head counts and crab pot locations to assess the risk of terrapin mortality due to the presence, density, and distribution of crab trapping activities. Risk was defined as spatial overlap in observations of crab trapping activities and observations of terrapins along head count survey transects in creeks over the entire length of the study. No adjustments for detection probabilities were made, so the risk is based solely on the survey results. Each of the four National Wildlife Refuges sampled were analyzed separately. Maps were generated to show crabbing pressure in relation to terrapin hotspots, thus highlighting areas in need of protection to assure the persistence of terrapin populations and helping to meet the goal of providing information for effective management of diamond-backed terrapins on these refuges.