National Park Service U.S. Department of the Interior South Florida Natural Resources Center Everglades National Park RESOURCE EVALUATION REPORT SFNRC Technical Series 2008:1 PATTERNS OF PROPELLER SCARRING OF SEAGRASS IN FLORIDA BAY Associations with Physical and Visitor Use Factors and Implications for Natural Resource Management PATTERNS OF PROPELLER SCARRING OF SEAGRASS IN FLORIDA BAY Associations with Physical and Visitor Use Factors and Implications for Natural Resource Management RESOURCE EVALUATION REPORT SFNRC Technical Series 2008:1 South Florida Natural Resources Center Everglades National Park Homestead, Florida National Park Service U.S. Department of the Interior Cover photograph of north end of Lower Arsnicker Key by Lori Oberhofer, ENP ii South Florida Natural Resources Center Technical Series (2008:1) Patterns of Propeller Scarring of Seagrass in Florida Bay iii Patterns of Propeller Scarring of Seagrass in Florida Bay: Associations with Physical and Visitor Use Factors and Implications for Natural Resource Management RESOURCE EVALUATION REPORT SFNRC Technical Series 2008:1 EXECUTive SummarY of approximately 10, i.e., there may be as many as 3250 miles of scars in Florida Bay. Everglades National Park (ENP) encompasses over 200,000 Substantially more scarring was identified in this study hectares of marine environments, including most of Florida than in a previous study conducted in 1995. Bay. The ENP portion of Florida Bay was federally designated as submerged wilderness in 1978. Much of Florida Bay sup- ports submerged aquatic vegetation comprised of seagrass Patterns and Associations that provides vast areas of habitat for recreationally and com- mercially important fish and invertebrates. Florida Bay is a The majority of scarring was identified in depths below 3.0 premier shallow-water recreational fishing destination and it ft and scarring density tends to increase with decreasing is heavily used by recreational boaters for access to produc- depth. tive fishing areas. While the primary stressors in Florida Bay are related to watershed management, recreational boat use Dense scarring is more likely in close proximity to marked has also resulted in damage to benthic resources. Identifica- and unmarked channels and shorelines. tion of propeller scarred seagrass beds has been a critical data The density of scarring around marked and unmarked need by park managers and the public in the development of channels is similar. the park’s General Management Plan (GMP) and for natural Scarring density is higher in areas that are most heavily resource management projects. used by recreational boats. To integrate information on propeller scarred seagrass in the development of the GMP and to assist with developing Scarring density was not related to proximity to boat ramps a park-wide seagrass restoration strategy, we analyzed aerial in the Florida Keys or Flamingo. imagery of Florida Bay. We used resulting data to provide Scarring is increasing in specific sites in Florida Bay. information on the pattern and relative density of seagrass A propeller dredged channel, identified in 1995, has been scarring and to determine if scarring damage is getting better steadily increasing in area. or worse. We also used high resolution imagery, available for only a small portion of the bay, to estimate our level of scar de- tection. Finally, we used geospatial analyses to help determine Summary and Recommendations if scarring density was related to water depth and proximity to shorelines, boat ramps, marked and unmarked navigational Seagrass scarring in Florida Bay is widespread with dense channels, and boating activity. scarring found in shallow depths, near all navigational channels, and around areas most heavily used by boats. Key Findings Although seagrass damage assessments using aerial imag- ery often underestimate scarring, they provide useful in- formation on pattern and relative density. General Results Because scarring is not improving with time, new manage- ment strategies are warranted to protect submerged wil- We detected approximately 12,000 seagrass scars. derness resources and reduce stressors as part of an over- Scar lengths ranged from approximately 2 to 1600 meters. all approach to ecosystem management in Florida Bay. The total length of scars was approximately 525,000 me- An adaptive approach, initially focusing on the most heav- ters (325 miles). ily scarred areas, should consider monitoring data from specific indicator sites with a variety of management op- Scars are present throughout the shallow areas of Florida tions including education, improved signage, new en- Bay. forcement efforts, and boating restrictions, such as pole High resolution imagery suggests that our primary imag- and troll zones. ery may underestimate total scarring distance by a factor iv South Florida Natural Resources Center Technical Series (2008:1) Patterns of Propeller Scarring of Seagrass in Florida Bay v TabLE OF CONTenTS EXECUTIVE SUMMARY. .iii CONTRIBUTING AUTHORS. vi ACKNOWLEDGEMENTS. vi FOREWORD . vii INTRODUCTION . 1 METHODS. 5 Scar Mapping . 5 Prop Scar Change Analysis . 5 Geospatial Analysis . 6 RESULTS . 11 Scar Mapping . 11 Change Analysis . 11 Geospatial Analysis . 12 DISCUSSION . 20 Secondary Impacts. 22 Management Implications. 23 LITERATURE CITED . 26 vi South Florida Natural Resources Center Technical Series (2008:1) CONTribuTinG AUTHOrs David E. Hallac1, Jimi Sadle1, Leonard Pearlstine1, and Fred Herling2 1South Florida Natural Resources Center, Everglades National Park, 950 N. Krome Avenue, Homestead, FL 33030-4443 2Planning and Compliance Division, Everglades National Park, 40001 State Road 9336, Homestead, FL 33034-6733 Comments and Questions: [email protected] ACKNOWLEDGMENTS We would like to thank Judson Kenworthy and Mark Fonseca for their review and thoughtful suggestions, as well as Carol Mitch- ell for coordinating the review. We are grateful for the editing and document production perfomed by Alice Clarke, Joy Brunk, and Caryl Alarcon. This project would not have been possible without data collected and maintained by Florida Fish and Wildlife Conservation Commission - Fish and Wildlife Research Institute (FWC-FWRI). The NPS South Florida and Caribbean Network greatly contributed to this project by supplying imagery. Discussions and resources provided by Curtis Kruer and Coastal Re- sources Group greatly enhanced this study. Our discussions with Bob Showler, Bonnie Foist, Cara Cooper, Dave Fowler, Craig Smith, Dave King, Douglas Morrison, William Perry, and Tony Terry helped in the development of this report. We would also like to thank Tad Burke, Sandy Moret, and Mike Ehlers of the Florida Keys Fishing Guides Association, Lain Goodwin of the Key Largo Fishing Guides Association, Bob Stearns, and Lloyd Wruble for their insights and recommendations. We thank the National Parks Conservation Association and their Coalition of Park Users group, based out of the Florida Keys, and the South Florida National Parks Trust for their recognition of these important issues and support of this project. Please reference this report as follows: SFNRC. 2008. Patterns of Propeller Scarred Seagrass in Florida Bay: Associations with Physical and Visitor Use Factors and Implications for Natural Resource Management. South Florida Natural Resources Center, Everglades National Park, Homestead, FL. Resource Evaluation Report. SFNRC Technical Series 2008:1. 27 pp. Patterns of Propeller Scarring of Seagrass in Florida Bay vii FOreWORD This report, “Patterns of Propeller Scarring of Seagrass in Florida Bay,” represents our continuing advancement in Everglades National Park’s ability to perform science-based assessments of the natural resources in Florida Bay. Florida Bay’s submerged aquatic vegetation and bottom communities were defined as federally designated wilderness in 1978, to protect this internationally significant resource. The health of these benthic communities is directly tied to commercially and recreationally important fisheries and the health of the adjacent Florida Keys reef tract. For decades, propeller scarring has been identified as a stressor affecting the park’s vast seagrass resources, but we have been unable to quantify the impacts. While the primary stressors in Florida Bay are related to the impacts of upstream water management, increasing recreational boat use in the shallow waters of the bay has resulted in damage to important seagrass communities. For the first time, this report provides an assessment of the spatial and temporal patterns of propeller scarring of the seagrass communities of Florida Bay. In addition, the report specifically incorporates geospatial statistics to augment our understanding of the factors affecting propeller scarring, ultimately providing applied results that will assist in the development and evaluation of natural resource management plans. As our efforts to update the park’s General Management Plan progress, more in-depth monitoring will be required to establish a baseline for a number of indicator locations. Monitoring will be used to determine if management strategies are effective in reducing the propeller scarring problem and to implement new strategies, if warranted. While the analytical approach reported here works well within Florida Bay, new methods need to be developed in the future for quantifying seagrass cover and damage along the park’s western coastline, where turbidity often masks scarring. Everglades National Park managers and staff will