The methodology used in the study can be grouped into four areas:
A low-level overflight was conducted during the early morning hours of February 5, 2002. Time-stamped oblique video shots and on-board GPS readings taken during the flight were used as the starting point for field reconnaissance of propeller scarring.
The distribution and extension of individual propeller marks was determined using a swimmer-operated Differential Global Positioning System (DGPS) consisting of a Magellan GPS 315® handheld unit coupled with a Magellan DBR-IV® beacon receiver. At intervals selected by the swimmer, a DGPS position was recorded. Notes were also taken regarding landmarks are the position order in the GPS memory. In densely scarred areas, it was necessary to mark individual scars with small buoyant markers prior to recording the scar location. Up to 20 such markers were positioned at a time while surveying the sites. DGPS positions for polygons were taken whenever the collection of individual scars proved impossible due to the concnetration of scars in the same location. Position data were downloaded into a computer and placed in a Microsoft Excel® file where positions in decimal degrees were calculated. Position downloading was conducted immediately after return to the laboratory from the field. Files were created to note the date of sampling, landmark reference, latitude and longitude data, scar or polygon number, and occasional observations of bottom features.
Scar data were overlain on aerial photographs recently published by the National Oceanic and Atmospheric Administration (NOAA) National Ocean Service (NOS) as part of the benthic habitat mapping efforts for Puerto Rico and U.S. Virgin Islands. Two separate shapefiles were created for lines and polygons for each sampling trip. Line shapefiles include data for individual scars while polygon shapefiles include data for areas intensively impacted and where marking individual scars was not feasible. Sampling event-based shapefiles were merged into a single coverage for both lines and polygons upon completion of field sampling for the project.
Shapefiles produced from field data were used to calculate areas of impact. Areas of impact were designated considering water overflight observations, water depth, and knowledge of boat traffic and recreational activities in the area. The classification scheme used was modified from Sargent et al. (1995) as follows:
| Sites | Scar | PrIA | PoIA | Scar/PoIA | PrIA/PoIA |
|---|---|---|---|---|---|
| Sqmeters | Percent | ||||
| Monsio Jose: | 272 | 1984 | 7864 | 3.5 | 25.2 |
| Collao: | 4579 | 13182 | 21457 | 21.3 | 61.4 |
| Enrique: | 33 | 4282 | 9837 | 0.3 | 43.5 |
| Magueyes: | 5394 | 54786 | 73718 | 7.3 | 74.3 |
| Caracoles: | 7566 | 36577 | 59972 | 12.6 | 61.0 |
| Guilligan's: | 57 | 1541 | 30598 | 0.2 | 5.0 |
| San Jacinto: | 47 | 421 | 2543 | 1.8 | 16.6 |
| Ballenas: | 35 | 8608 | 33299 | 0.1 | 25.9 |
Overflight images or photographs can be used only as a preliminary approach to assess the total areas impacted by propeller scars. Many of the patterns observed by high and low level imagery showed only bottom patterns which origins are at best uncertain. Even photography taken during low-level flights (video and still photos) show only the most obvious/severe damages to seagrass beds. Only conducting close inspection of shallow areas, guided by a combination of prior knowledge of general navigational patterns, bathymetry, destination of boaters and aerial photographs allow a better quantification of the actual damages. This approach is the only means to effectively develop conservation strategies to prevent further deterioration of this critical habitat since it uncovers areas subjected to less severe scarring thus indicating areas of priority for management and close monitoring to minimize the retreat of seagrasses from coastal and reef areas.
The use of GIS enabled us to create detailed maps of linear scars and polygons in areas where scarring is intense. GIS greatly facilitates the compilation and manipulation of spatial data and the overlaying of multiple layers on aerial photographs or maps. This also enables the calculation of areas of impacts as well as areas of probable and potential impacts. The distribution patterns of propeller scars support the idea that a major forcing factor for the deterioration of seagrass beds in certain shallow areas of southwestern Puerto Rico is motorized transportation. These data serve as tools for the resource managers within each reserve to determine areas where boat traffic has the greatest impact on shallow seagrass beds and develop management strategies to prevent or lessen these impacts.
Sargent, F.J., T.J. Leary, D.W. Crewz and C.R. Kruer. 1995. Scarring of Florida's seagrasses: Assessment and management options. Florida Department of Environmental Protection, St. Petersburg, FL. FMRI Technical Report TR-1. 46 pp.
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