Appendix B. Data used in the invasion probability model.
The data on physical and hydrological characteristics of bays and estuaries for this analysis came primarily from NOAA’s Coastal Geospatial Data project. Each observation in this analysis corresponds to a unique estuary, defined as an Estuarine Drainage Area (EDA). The physical and hydrological data are not associated with a specific year; rather, they were described as long-term averages.
Eutrophication data from the National Estuarine Eutrophication Survey (Bricker et al. 1999) was used as an indicator of coastal ecosystem health. Because most of the nutrient pollution problems in the estuaries are related the influx of excess nutrients associated with human activities in upland watersheds, we used the overall human influence variable (Human) as an indicator of the eutrophic condition of the waterbody. The overall human influence (Human) variable was calculated as the aggregated qualitative value of nutrient input from upland watersheds (low, moderate, and moderate-high) and susceptibility for an entire estuary to eutrophication (low, moderate-low, and moderate).
The Environmental Monitoring and Assessment Program (EMAP) National Coastal Database provided information on water temperature, salinity, and dissolved oxygen content. All three variables are a representative long-term average of bottom and surface temperature measurements gathered during the period 1997–2001. There was no direct measure of the depth of the estuary (Depth). Thus, the average bottom collection depth at which the data on temperature, salinity, and dissolved oxygen were measured was used as a proxy for estuary depth. Pollutant flow data that measure the average quantity of pollutant flows into each estuary per year (Pol_Flow) were derived from NOAA’s National Coastal Pollutant Discharge Inventory (NCPDI). This variable approximates pollutant discharge conditions during the period from about 1982 to 1991. Major categories of pollutants included are oxygen-demanding materials, particulate matter, nutrients, heavy metals, petroleum hydrocarbons, pathogens, and wastewater.
The data on shellfish abundance were obtained from NOAA’s Estuarine Living Marine Resources (ELMR) Database, which contains information from 1985 and 1994. The Shell_present variable represents the number of shellfish species with an average annual abundance rating of at least “rare” present at each estuary. The ELMR database provides abundance rating for the following prey species: in the database include b, bay mussel (Mytilus spp.), California jacknife clam (Tagelus californianus), Manila clam (Venerupis philippinarum), Pacific littleneck clam (Prototheca stamina), Pacific oyster (Crassostrea gigas), soft-shell clam (Mya arenaria), bay scallop (Argopectin irradians), Eastern oyster (Crassostrea virginica), quahog (Mercenaria mercenaria), and sea scallop (Placopecten magellanicus). The average annual shellfish abundance rating is interpreted as: blank = no data available, 0 = not present, 2 = rare, 3 = common, 4 = abundant, 5 = highly abundant. To measure the presence of other crabs that prey on or compete for food and shelter with green crabs (Oth_Crab). We used the average abundance rating for three crab species: blue crab (Callinectes sapidus), Dungeness crab (Cancer magister), and red rock crab (Cancer productus).
We note that some data (e.g., pollutant discharges and shellfish abundance) represent long term averages from the 1980s and 1990s. This is likely to introduce a greater than desirable uncertainty in the analysis, if there were significant changes in the relevant factors in the last decade. Finally, we used a geographic information system (GIS) to estimate the distance to the nearest estuary with green crabs (DisNear) and the number of adjoining estuaries with green crab presence (Con_Num).
Bricker, S. B., C. G. Clement, D. E. Pirhalla, S. P. Orlando, and D. R. G. Farrow. 1999. National Estuarine Eutrophication Assessment: Effects of Nutrient Enrichment in the Nation’s Estuaries. NOAA, National Ocean Service, Special Projects Office and the National Centers for Coastal Ocean Science. Silver Spring, Maryland, USA.