Patterns of Propeller Scarring

 of Seagrass in Florida Bay

Resource 

Evaluation 

Report

SFNRC Technical Series

2008:1

Associations with Physical and Visitor Use Factors and 

Implications for Natural Resource Management

National Park Service

U.S. Department of the Interior

South Florida Natural Resources Center

Everglades National Park 

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

Everglades National Park (ENP) encompasses over 200,000 

hectares of marine environments, including most of Florida 

Bay. The ENP portion of Florida Bay was federally designated 

as submerged wilderness in 1978. Much of Florida Bay supports submerged aquatic vegetation comprised of seagrass 

that provides vast areas of habitat for recreationally and commercially important fish and invertebrates. Florida Bay is a 

premier shallow-water recreational fishing destination and it 

is heavily used by recreational boaters for access to productive fishing areas. While the primary stressors in Florida Bay 

are related to watershed management, recreational boat use 

has also resulted in damage to benthic resources. Identification of propeller scarred seagrass beds has been a critical data 

need by park managers and the public in the development of 

the park’s General Management Plan (GMP) and for natural 

resource management projects. 

To integrate information on propeller scarred seagrass in 

the development of the GMP and to assist with developing 

a park-wide seagrass restoration strategy, we analyzed aerial 

imagery of Florida Bay. We used resulting data to provide 

information on the pattern and relative density of seagrass 

scarring and to determine if scarring damage is getting better 

or worse. We also used high resolution imagery, available for 

only a small portion of the bay, to estimate our level of scar detection. Finally, we used geospatial analyses to help determine 

if scarring density was related to water depth and proximity 

to shorelines, boat ramps, marked and unmarked navigational 

channels, and boating activity. 

Key Findings

General Results 

We detected approximately 12,000 seagrass scars.

Scar lengths ranged from approximately 2 to 1600 meters. 

The total length of scars was approximately 525,000 meters (325 miles).

Scars are present throughout the shallow areas of Florida 

Bay.

High resolution imagery suggests that our primary imagery may underestimate total scarring distance by a factor 











of approximately 10, i.e., there may be as many as 3250 

miles of scars in Florida Bay. 

Substantially more scarring was identified in this study 

than in a previous study conducted in 1995.

Patterns and Associations 

The majority of scarring was identified in depths below 3.0 

ft and scarring density tends to increase with decreasing 

depth. 

Dense scarring is more likely in close proximity to marked 

and unmarked channels and shorelines. 

The density of scarring around marked and unmarked 

channels is similar. 

Scarring density is higher in areas that are most heavily 

used by recreational boats. 

Scarring density was not related to proximity to boat ramps 

in the Florida Keys or Flamingo. 

Scarring is increasing in specific sites in Florida Bay. 

A propeller dredged channel, identified in 1995, has been 

steadily increasing in area.

Summary and Recommendations 

Seagrass scarring in Florida Bay is widespread with dense 

scarring found in shallow depths, near all navigational 

channels, and around areas most heavily used by boats. 

Although seagrass damage assessments using aerial imagery often underestimate scarring, they provide useful information on pattern and relative density.

Because scarring is not improving with time, new management strategies are warranted to protect submerged wilderness resources and reduce stressors as part of an overall approach to ecosystem management in Florida Bay. 

An adaptive approach, initially focusing on the most heavily scarred areas, should consider monitoring data from 

specific indicator sites with a variety of management options including education, improved signage, new enforcement efforts, and boating restrictions, such as pole 

and troll zones.
























iv South Florida Natural Resources Center Technical Series (2008:1)
Patterns of Propeller Scarring of Seagrass in Florida Bay 

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)

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. 

Contributing Authors

David E. Hallac1, Jimi Sadle1, Leonard Pearlstine1, and Fred Herling2

1

South Florida Natural Resources Center, Everglades National Park, 950 N. Krome Avenue, Homestead, FL 33030-4443

2

Planning and Compliance Division, Everglades National Park, 40001 State Road 9336, Homestead, FL 33034-6733

Comments and Questions: David_Hallac@nps.gov

Acknowledgments

We would like to thank Judson Kenworthy and Mark Fonseca for their review and thoughtful suggestions, as well as Carol Mitchell 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 Resources 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. 
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 use the results of this study to develop a new, bay-wide 

restoration strategy that identifies areas most in need of improvement, and determines optimal assessment 

methods and management actions, while making recreational users full partners in our protection and 

restoration efforts. Everglades National Park looks forward to our continuing efforts to work with our 

partner agencies and the public in the development of sustainable management actions to protect these 

important resources for future generations.

Robert Johnson

Director

South Florida Natural Resources Center

Everglades National Park

November, 2008
viii South Florida Natural Resources Center Technical Series (2008:1)
Patterns of Propeller Scarring of Seagrass in Florida Bay 

Introduction

Everglades National Park (ENP) encompasses approximately 

607,000 hectares (ha) at the southern tip of peninsular Florida. The park comprises the largest subtropical wilderness in 

North America, and is the only area in the United States designated as an International Biosphere Reserve, a World Heritage Site, and a Wetland of International Importance. Legislation establishing ENP in 1934 emphasized that “...areas shall 

be permanently reserved as a wilderness, and no development of the project or plan for the entertainment of visitors 

shall be undertaken which will interfere with the preservation 

intact of the unique flora and fauna and the essential primitive natural conditions now prevailing in this area.” Included 

within the park boundary are over 200,000 ha (500,000 acres) 

of marine environments, including portions of Florida Bay 

(ca. 162,000 ha) (Fig. 1).

Florida Bay is characterized by extensive areas of shallow water, punctuated by deeper natural basins separated by 

banks with natural and man-made channels connecting them. 

Much of the bay bottom supports submerged aquatic vegetation made up of seagrasses (e.g., Thallasia testudinum and

Halodule wrightii) and marine algae of various species. These 

vegetated areas serve as nursery habitat for commercially important fisheries, such as spiny lobster (Panulirus argus), stone 

crab (Menippe mercenaria), pink shrimp (Farfantepenaeus 

duorarum), and a variety of reef fish species. The bay and 

its submerged vegetation also provide habitat and feeding 

grounds for state and federally listed species such as manatees 

(Trichechus manatus), the smalltooth sawfish (Pristis pectinata), and sea turtles including loggerhead (Caretta caretta), 

green (Chelonia mydas), Kemp’s ridley (Lepidochelys kempi), 

and hawksbill (Eretmochelys imbricata). Recreationally important fish species including bonefish (Albula vulpes), spotted seatrout (Cynoscion nebulosus), gray snapper (Lutjanus 

griseus), snook (Centropomus undecimalis), redfish (Sciaenops 

ocellatus), and tarpon (Magalops atlanticus) also utilize Florida 

Bay.

Figure 1. The Florida Bay portion of Everglades National Park (ENP), Florida, and designated wilderness areas. 
 South Florida Natural Resources Center Technical Series (2008:1)

In addition, with the exception of an area in ENP that is 

1/8th of a mile in width paralleling and immediately adjacent 

to the Intercoastal Waterway, the entire ENP portion of the bay 

bottom was designated by Congress as part of the Everglades 

Wilderness in 1978 (Public Law 95-625) and thus is subject to 

administration under the provisions of the Wilderness Act of 

1964. The name of this area was later changed to the Marjory 

Stoneman Douglas Wilderness (Public Law 105-82).

Human caused seagrass damage has been identified in shallow coastal areas throughout Florida (e.g., Kuss 1991, Kruer 

1994, Sargent et al. 1995). Nutrient inputs, altered hydrology, and anthropogenically-induced algae blooms represent 

major stressors to seagrass meadows (Dawes et al. 2004) and 

are substantial stressors in Florida Bay. Vessel groundings and 

boating effects are also among the multiple stressors contributing to seagrass decline (Orth et al. 2006). Physical damage 

by boats to submerged aquatic vegetation has occurred for 

many years and contributes significantly to the disturbance of 

seagrass meadows (Zieman 1976, Sargent et al. 1995). As early 

as 1953, substantial boat damage to seagrass beds in Garfield 

Bight and Snake Bight was visible in aerial photography (Fig. 

2). This damage was thought to be the result of commercial 

fishing vessels netting mullet in Florida Bay.

Damage to submerged aquatic vegetation has also long 

been recognized as a recreational management issue in 

Everglades National Park (Zieman 1976). Currently, Florida 

Bay represents one of the premier shallow-water boating 

and fishing destinations in the world. As a result, recreational 

angling as well as pleasure boating are becoming increasingly 

popular within ENP. Ault et al. (2008) estimate that boating 

use within the boundaries of ENP has increased from 2 to 

2.5 times in the past 30 years. In addition, the number of boat 

registrations in the three southern Florida counties covering 

Everglades National Park has increased substantially since 

1995 (FWC 2007).

Increased boating activity, often by boaters with no previous or only limited experience in navigating the numerous 

shallow flats and complex of narrow channels of Florida Bay, 

make parts of the bay very susceptible to visitor impacts resulting from the operation of motorized watercraft. Damage 

from boats generally occurs when a boat propeller (prop) 

contacts either the submerged vegetation or the vegetation 

and the bay bottom. Prop scarring can be caused by many factors including: 

1. lack of understanding of the relationship of the 

draft of the boat to depth of the water where the 

boat is operating, 

2. poor marking of navigational channels, 

3. use of short cuts around channels and over flats 

where there is insufficient water depth, and 

4. efforts to get a boat up on plane in shallow areas 

where there is insufficient water depth. 

When vessels run aground, prop scars are often coupled 

with large holes (“blow holes”) in the vegetation and substrate 

created by the vessel operator attempting to use the motor’s 

power to free the vessel (Whitfield et al. 2002). In addition, 

prop scarring occurs when boaters use the prop to dredge 

new channels or maintain existing, unmarked man-made 

channels, also referred to as “wheel ditches.” Sediment excavated by boat props from blow holes and wheel ditches can 

form berms adjacent to the holes. Berms may bury seagrasses, 

causing mortality (Duarte et al. 1997, Whitfield et al. 2002).

Regardless of the cause, the resulting prop scars lead to 

direct loss of wilderness resources, seagrass biomass, and in 

more severe incidents, long term damage to seagrass beds. 

Prop scars create structural changes in seagrass communities 

including physical destruction of the seagrasses, increased 

sediment resuspension, and a potential increase in the susceptibility of seagrass beds to damage from hurricanes. The 

effects of prop scars on animals associated with seagrass beds 

are less clear. At a fine scale, scars have been shown to result 

in changes in abundance of organisms including shrimp, crab, 

Figure 2. Aerial photograph of northern Florida Bay (1953) 

showing extensive prop scar damage to seagrass beds, possibly 

caused by commercial fishing boats. A ban on commercial fishing 

in Everglades National Park was implemented in 1986. 

Porpoise Point

Garfield Bight

Hogfish in seagrass. Photo by William Perry, ENP.
Patterns of Propeller Scarring of Seagrass in Florida Bay 

and molluscs (Uhrin and Holmquist 2003). However, on larger scales, no relationship between scarring density and abundance of similar organisms has been detected (e.g., Bell et al. 

2002, Burfeind and Stunz 2006). Burfeind and Stunz (2007) 

did detect reductions in the growth rates of both pinfish and 

white shrimp in heavily scarred areas, indicating a potential 

impact on habitat quality at some scarring levels. Fonseca 

and Bell (1998) observed rapid loss in structural complexity 

of seagrass habitat when fragmentation exceeded 50% coverage. Further studies are needed to clarify the relationship 

between small and large scale effects of prop scarring and the 

overall impacts of prop scars on organisms that depend on 

submerged aquatic vegetation. 

Estimates of recovery time for prop scars vary depending on the severity of the scar and the seagrass species that is 

damaged. Estimates range from as little as 0.9 years (Sargent 

et al. 1995) to 7.6 years (Andorfer and Dawes 2002). Recent 

model-derived estimates of scarring recovery in T. testudinum

beds suggest that some areas in the Florida Keys may require 

60 years for recovery (Fonseca et al. 2004). Recovery rates are 

much slower when scarring is deep. Excavations between 10 

and 20 cm may destroy the connection of seagrass blades to 

belowground seagrass rhizome biomass (Hammerstrom et al. 

2007). The rhizome architecture of T. testudinum, for example, is not flexible enough to grow down into the remaining 

sediment (Marba et al. 1994), and deep excavations are more 

susceptible to secondary continued erosion and expansion of 

scars from currents, winds, waves, and storms (Zieman 1976, 

Kuss 1991, Durako et al. 1992, Rodriguez et al. 1994, Hastings 

et al. 1995, Dawes et al. 1997, Kenworthy et al. 2002, Whitfield 

et al. 2002). In addition, drift algae filling scars may also slow 

recruitment or recovery of seagrass species (Hammerstrom et 

al. 2007). Halodule wrightii and Syringodium filiforme recover 

5 to 7 times faster than T. testudinum in experimental excavations and propeller scars observed by Kenworthy et al. (2002) 

in the Florida Keys. As a result, boat propeller scarring may 

alter the community composition and abundance of different 

seagrass species (Kenworthy et al. 2002). Heavily used areas 

that are continually scarred will probably never recover under 

current boating pressure. Active restoration of damaged seagrass communities is technically possible, but expensive and 

time consuming. 

Prop scarring has been observed throughout Florida Bay 

in shallow flats, bights, bays, and banks and in other high use 

areas (Fig. 3). A large number of historical photographs show 

evidence of prop scarring over the years; however, to date, no 

detailed study has quantified the extent of seagrass damage in 

Florida Bay. Previous systematic efforts to map prop scars in 

ENP have been limited. Sargent et al. (1995) included Florida 

Bay in their statewide assessment of Florida’s seagrass beds 

and identified 814 ha (2013 acres) of scarred seagrass beds 

within ENP. However, lack of sufficiently detailed aerial photography made it unlikely that their assessment accurately 

reflected the state of seagrass beds in Florida Bay at the time 

of the study. The park’s General Management Plan (GMP) 

process that began in 2003 identified the need to better understand seagrass conditions and trends (ENP 2008). 

In order to implement effective marine management strategies that conserve and recover damaged seagrass, managers must understand existing prop scarring conditions and 

potential associations with physical factors, such as water 

depth and presence/absence of aids to navigation, along with 

public use factors that explain how, where, and to what extent 

Florida Bay is visited. Little information on the locations, patterns, and relative density of prop scarring has been available 

for Florida Bay. The objectives of this study are to:

1. characterize seagrass scarring throughout Florida 

Bay, 

2. determine if prop scarring is getting better or 

worse at specific locations, 

3. assess potential relationships between density and 

location of seagrass scarring when analyzed against 

water depth, proximity to shorelines, proximity to 

marine facilities, and patterns of fishing and boat 

use in Florida Bay, 

4. assess potential relationships between density 

and location of seagrass scarring and proximity 

to navigational channels (marked and unmarked), 

and 

5. develop a statistical model to aid in prevention of 

scarring in undamaged areas. 

In this report, we discuss our findings, including implications for management. 

Propeller scar from boat. Photo by Brett Seymour, NPS.
 South Florida Natural Resources Center Technical Series (2008:1)

Figure 3. Examples of propeller scarring in Everglades National Park photographed in 2006 and 2007: 1) Upper Cross Bank adjacent to 

the Intracoastal Waterway; 2) Channel entrance at Twin Key Bank; 3) Shallow passage at the Boggies; and 4) Mouth of Alligator Creek 

at Garfield Bight.

1 2

3 4
Patterns of Propeller Scarring of Seagrass in Florida Bay 

Methods

Scar Mapping 

Georeferenced digital imagery at 0.5 m resolution of Florida 

Bay from April 2004 was used to digitize propeller scarring 

throughout the Florida Bay portion of ENP. Images were created from 1:24,000 scale true color raw scans (FWC 2004), 

georeferenced and reviewed for spatial accuracy according 

to National Map Accuracy Standards (USGS 2007). This imagery was obtained to allow immediate and future evaluation 

of submerged aquatic vegetation in Florida Bay (FWC 2004). 

Previous assessments of propeller scarring (e.g., Sargent et 

al. 1995) indicate that imagery at this scale will not show all 

propeller damage to seagrass beds. However, review of the 

images indicated that enough scarring was evident to identify 

and map heavily scarred areas as well as some individual scars 

in areas with less damage. Therefore, the images were used to 

develop a conservative estimate of scarring, determine relative scarring densities, and visualize spatial scarring patterns. 

While blow holes are often associated with prop scars, there 

was no consistent and reliable way to distinguish man-made 

holes from natural bottom features. As a result, this type of 

damage was not included in this mapping effort. 

Images of the ENP portion of Florida Bay were reviewed 

in 10 – 25 ha (ca. 25 – 62 acres) increments for visible scars 

in ArcMap version 9.2 (ESRI 2006). Each visible scar was 

digitized by tracing as an individual line (Fig. 4). Generally, 

images were viewed at the greatest magnification possible 

that allowed a clear view of the bay bottom. At magnifications 

greater than 1:1000 - 1:2000, image quality prevented discerning and mapping prop scars. Helicopter overflights of scarred 

areas were carried out to observe current scarring levels in 

areas identified as heavily damaged in the digital imagery. No 

attempt was made to ground truth individual scars that were 

identified in the imagery used in this study; however, observations conducted during multiple helicopter flights confirmed 

that scarred areas, as indicated by the 2004 imagery, were 

indeed scarred. Scar line data were overlaid with 100 m2

 grid 

cells to calculate scarring density (m/m2

). The areas with the 

greatest density of scarring (top 10%) were selected and 

mapped to examine possible priority areas to focus on in the 

development of GMP management alternatives and marine 

management strategies. In order to determine if the analysis 

under- or over-estimated scarring when compared to more 

recent and higher resolution imagery, we utilized a partial set 

of high resolution imagery for north central Florida Bay to 

quantify scarring. The high resolution imagery was collected 

in 2006 at 0.30 m resolution. The imagery was only available 

for a small portion of Florida Bay, so it could not be used for 

the entire study area.

Prop Scar Change Analysis 

In addition to the 2004 imagery, partial sets of 1:24,000 digital 

imagery of Florida Bay from 1995, 1999, and 2006 were used 

to conduct a change analysis between these time periods. We 

analyzed three areas of Florida Bay where scarring was visible 

in the same location on aerial photographs taken over multiple 

years and separated by some period of time. The aerial extent 

of these images varied greatly, limiting the amount of potential 

area that could be mapped for comparative purposes. Imagery 

from 1995 and 1999 consisted of digital orthographic quarterquads. Images from 1995 were in .jpg format and 1999 images 

were in MrSID format. Imagery from 2006 was prepared for 

the 2006 Monroe County Florida Orthophoto Project (Woolpert, Inc. 2007). These images were in MrSid format. All three 

Figure 4. Scar mapping is illustrated with this pair of images of Upper Cross Bank. The scars visible in the lefthand image are shown 

mapped in blue in the righthand image (2004 FWC imagery).
South Florida Natural Resources Center Technical Series (2008:1)

digital image sets were at 0.5 m pixel resolution. Three areas 

within Florida Bay were identified as suitable for comparative 

mapping: Twisty Mile, a channel located south of Madeira 

Point; Shell Key Bank, north of Shell Key; and Cross Bank, 

east of Tavernier (Fig. 5). Twisty Mile and Shell Key Bank were 

mapped using 1999 and 2004 imagery and the Cross Bank 

area was mapped using 1999, 2004, and 2006 images. In all 

cases, scar maps at each site were compared between years 

and identical scars were identified by their shape and location. The number of identical scars at each site was divided by 

the total number of scars mapped in the prior year’s photo to 

determine the percent of scars remaining in the subsequent 

photo. The total length of scars was calculated and recorded 

for each mapping year at all sites.

In addition to mapping individual scars, we sought to 

measure the change in area of a new channel (wheel ditch), 

established through repeated prop scarring and boat travel, 

on Shell Key Bank. The wheel ditch was mapped using 1995, 

1999 and 2004 imagery. A polygon was drawn around the area 

denuded of vegetation in each of the three photos to estimate 

area devoid of seagrass. Once mapped, the size of the area was 

then compared among the three years of imagery. 

Geospatial Analysis 

We explored the relationship between scarring and water 

depth and proximity to several types of features (e.g., shorelines, marinas). In addition, regression analysis was used to 

examine relationships between scar density and water depth, 

as well as between scar density and the proximity variables. 

Proximity was generated in 100 m increments, for all factors 

except water depth. The following variables and associated 

data layers were used in these analyses:

Water Depth 

Bathymetry data (NAVD88) for Florida Bay were obtained 

from a 1990 fathometer survey (Hansen and DeWitt 1999). 

Transects ran mostly north-south, typically about 500 to 600 

m apart with depth measurements collected in feet approximately every 3 m along the transect. Ordinary kriging with 

anisotropy was used to create a continuous gridded surface 

Figure 5. Site map of prop scar change analysis locations: Twisty Mile (northern site), Cross Bank (eastern boundary site), and Shell Key 

Bank (southern site) which includes a propeller-dredged channel (wheel ditch). 

Twisty Mile

Shell Key Bank and Wheel Ditch

Cross Bank
Patterns of Propeller Scarring of Seagrass in Florida Bay 

of interpolated bathymetry for the bay (RMS = 0.22 ft, average 

SE = 0.55 ft) at 100 m resolution. 

NAVD88 vertical datum is approximately 1.345 ft (0.41 

m) above mean low water (MLW) in the Florida Bay area 

(Hansen and DeWitt 1999). Water depths were calculated as 

bathymetry at each grid cell converted to feet plus 1.345 and 

multiplied by -1 to represent depth as a positive value. 

Channels 

Centerlines of all channels in Florida Bay were traced using the Florida Bay Map and Guide (NPS 2006a). Unmarked 

channels were identified through digital imagery acquired for 

previous and ongoing seagrass restoration projects being carried out in ENP. The characterization of marked versus unmarked channels was obtained through close communication 

with ENP law enforcement rangers and resource managers. 

Distance of scarring to marked channels and distance of scarring to all channels was mapped in 100 m increments.

Marine Facilities

Data for marine facilities included docks, boat ramps, 

marinas, and other areas at which boats may congregate or 

launch (FWC 2006). Attributes of the point data include location and characteristics of the facility as follows: A = boat 

ramp, B = bank or bridge, F = fish camp, J = jetty, M = marina, 

O = other (e.g., hotel, resort), P = park, and S = pier. Because 

the marine facilities were well distributed along the Florida 

Keys, they generally represent the distance of scarring from 

the Keys and Flamingo in ENP. 

Boat Use 

Boat use data were collected for all marine waters in ENP 

between fall of 2006 and fall of 2007 using aerial methods (Ault 

et al. 2008). We used motorized watercraft data based on the 

following categories: fishing versus transit. Because ENP fishing reports over many years demonstrate that more than 90% 

of boating activity in the park is associated with recreational 

fishing (NPS 2006b), it is presumed that most transit activity is 

associated with a boat going to or from a fishing-related activity, or as a recreational or commercial (guided) trip (Fig. 6). 

Shorelines 

Shoreline data were obtained from the National Park 

Service official map of ENP. These data include shorelines 

for all keys in Florida Bay and the entire northern coastline 

of ENP, from Cape Sable to Long, Little Blackwater, and 

Blackwater Sounds.

Analyses

Areas in which seagrasses cannot be damaged because 

submerged rooted vascular (SRV) cover was not present were 

excluded from the analysis by screening the imagery with 

benthic cover type data. Benthic cover type data included 47 

descriptive classes for sea bottom and an upland class. These 

classes were aggregated into two classes relevant to this study: 

areas of continuous or discontinuous SRV plants and areas 

without SRV cover including: hard bottom, turbid plume, 

unconsolidated sediments, attached macroalgae, upland, and 

others (Fig. 7, FWC 2005). Areas with water depth greater 

than 6.5 ft MLW were also excluded because 99% percent 

of prop scarring occurs in water depths shallower than 6.5 ft 

MLW (Fig. 8). 

Nine explanatory variables were evaluated using regression analysis: 

1. Water depth (Depth) 

2. Proximity to all channels (ChanPx) 

3. Proximity to marked channels (MChanPx) 

4. Proximity to marine facilities (DockPx) 

5. Proximity to shorelines (ShorePx) 

6. Proximity to boats engaged in fishing (FishPx) 

7. Proximity to boats cruising transiting between 

locations (TransitPx) 

8. Proximity to recreational boats (RecPx) 

9. Proximity to commercial boats (ComPx).

Due to the widespread nature of prop scarring in Florida 

Bay and the likelihood that park management strategies 

would focus on the most heavily scarred areas, we estimated 

the maximum scarring that may occur at a location. A scatter 

plot of scarring density versus proximity to boats engaged in 

fishing demonstrates that scarring tends to increase as dis- Flamingo marina. ENP photo.
South Florida Natural Resources Center Technical Series (2008:1)

tance to boats engaged in fishing decreases (Fig. 9). At any 

given distance, however, all scarring densities from zero to the 

maximum at that distance are also present; thus, causing poor 

correlation in regression models. This difficulty was resolved 

by refocusing the analysis on most heavily scarred areas; a reasonable approach given that management effort should also 

concentration on most intensely damaged areas. Regression 

models tracking maximum scarring with distance often appear to have a strong trend as illustrated in the example (Fig. 

9). Areas with heavy scarring may also be more likely to have 

a maximum density of deep scarring. Deep scarring is of 

particular management concern because recovery rates may 

be much slower at these sites. Maximum scarring data were 

selected as the top 10% of scar density values within each 100 

m interval over the range of proximity data (see example in 

Fig. 9). For depth data, the top 10% of scar density values was 

selected within each 0.03 m (0.1 ft) interval over the range of 

depths. 

To develop a statistical model of maximum scarring, mean 

maximum scarring was calculated from the variables with 

significant regressions. Pearson’s product-moment correlations were performed for each unique pair of the nine variables to eliminate redundant variables. Where variables were 

highly correlated, only one of the variables was retained in the 

model. 

Multiple regression techniques were not used because data 

values selected as a running maximum will, in the majority of 

cases, be selected from different locations for one explanatory 

variable than they will for another variable. The difficultly created is that any attempt at multiple regression analysis will 

have values at a location for one variable and most likely not 

have values at that location for the other variables. Rather 

than attempt complex “missing value” data manipulations, 

an overall estimate of maximum likely scar densities was created by calculating the mean of the most influential mapped 

data layers into a composite index model. That is, we used 

data layers that explained the greatest amount of variability 

as measured by their r2

 values. Linear regressions were run in 

the R statistical software package (R Development Core Team 

2007) for maximum scarring versus each of the variables. A 

linear model was used as the best description of each variable. 

Maximum estimated scar density was mapped in ESRI ArcGIS 

for each spatial data layer based on the regression coefficients 

for that variable. 

Figure 6. Distribution of boat use activities and boat use type in Florida Bay from 2006 and 2007 (Ault et al. 2008).
Patterns of Propeller Scarring of Seagrass in Florida Bay 

Figure 7. Mapped occurrence of submerged rooted vascular (SRV) plants in Florida Bay (FWC 2005).

Sediment plume emanating from a prop scar made during a 2004 redfish tournament around Murray Key. Photo by Lori Oberhofer, 

ENP.

Benthic Cover Type

SRV

No SRV
10 South Florida Natural Resources Center Technical Series (2008:1)

Mapped Scarring by Depth

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 to <2.5

2.5 to <2.75

2.75 to <33 to <3.25

3.25 to <3.53.5 to <3.75

3.75 to <44 to <4.25

4.25 to <4.54.5 to <4.75

4.75 to <55 to <5.25

5.25 to <5.55.5 to <5.75

5.75 to <66 to <6.25

6.25 to <6.56.5 to <6.75

6.75 to <77 to <7.25

7.25 to <7.57.5 to <7.75

7.75 to <88 to <8.25

8.25 to <8.58.5 to <8.75

8.75 to <99 to <9.25

9.25 to <9.5

Depth (ft MLW)

Percent of Total Scarring

0

0.2

0.4

0.6

0.8

1.0

1.2

Cumulative Percent Scarring

Figure 8. Frequency and cumulative frequency of scars by water depth (ft MLW) in Florida Bay.

Figure 9. Scarring density (m/m2) in relationship to distance from 

boats engaged in fishing. Blue circles are all observations. Red 

triangles represent the most densely scarred cells (top 10% of 

observations at each distance in 100 m increments). The r2 for all 

observations is 0.005 and the r2 for the top 10% of oberservations is 0.82.

Propeller scars off Garfield Bight. Photo by Lori Oberhofer, ENP.
Patterns of Propeller Scarring of Seagrass in Florida Bay 11

Results

Scar Mapping 

A total of 11,751 line segments representing 527,498 m (326.2 

miles) of propeller scars were mapped throughout Florida 

Bay (Fig. 10). This does not represent the actual number of 

individual scarring events. In some cases, scars cut across 

patchy grass flats or traversed deeper areas or were otherwise 

not continuous. These discontinuous areas were not mapped 

and the resulting “map” of an individual scarring event may 

be composed of one or more individual lines. Mapped scars 

ranged in length from 2.1 m to 1680 m with a mean length of 

44.5 (S.D. ± 52.7) m. Calculated scar densities ranged from 0 

to 0.025 m/m2

 with the majority (> 75%) of the 100X100 m 

grid cells mapped with scarring less than 0.0125 m/m2

. Scar 

density mapping suggests that scars cover a large proportion 

of shallow water areas and patterns generally match those of 

the shallow flats and mud banks that separate the more than 

40 basins that comprise Florida Bay. Comparison of the 2004 

imagery with a partial set of 2006 higher resolution imagery 

resulted in detection of 340 scars totaling 23,443 m using the 

2004 imagery versus 3975 scars totaling 155,550 m using the 

2006 imagery (Fig. 11). 

Change Analysis 

The number and total length of prop scars increased between 

1999 and 2004 at all three sites (Table 1). All sites had 4 to 5 

times as many scars in 2004 than in 1999. Between 9.8 and 

15.6% of scars that were present in 1999 were still visible 

5 years later. At Cross Bank, where imagery from 2006 was 

available, the number and total length of scars decreased between 2004 and 2006 by 20% and 17% respectively, yet in this 

same time 36.7% of the scars that were visible in 2004 were 

still visible in 2006 (Table 1). Impacted vegetation at the wheel 

ditch on Shell Key Bank (Fig. 12) increased in area by 3.5 times 

between 1995 and 2004 (Table 2).

Figure 10. Overview of propeller scarring in Florida Bay, Everglades National Park. 
12 South Florida Natural Resources Center Technical Series (2008:1)

Figure 11. Comparison of propeller scars mapped using 2004 digital imagery (yellow) with 2006 higher resolution imagery (red).

Location Year Number 

of Scars 

Length 

(m)

Percent 

of scars 

remaining

Cross Bank

Cross Bank

Cross Bank

1999

2004

2006

83

387

311

4342

14395

11959

n/a

15.6

36.7

Shell Key Bank

Shell Key Bank

1999

2004

52

225

3475

9986

n/a

15.4

Twisty Mile

Twisty Mile

1999

2004

61

300

2936

12360

n/a

9.8

Table 1. Number of scars, total length of scars, and percent of 

scars remaining from one mapping period to the next at three 

locations in Florida Bay, Everglades National Park.

Geospatial Analysis 

A wide range of scar densities occur at all distances for the 

proximity variables and at all depths for the water depth variable (Fig. 13). A larger number of cells have prop scars when 

they are in shallow water depths and in close proximity to: 

channels; shorelines; and locations where boating activities 

include fishing or transit for both recreational and guided 

trips. For all variables, regressions were significant (P≤0.001) 

toward higher densities of scarring as distance decreased or 

with decreasing water depth (Table 3 and Figs. 14 and 15). The 

exception was marine facilities (DockPx), which has a very 

low r2

 and, although significant, the relationship with scarScars mapped from 2004 imagery

Scars mapped from 2006 imagery 

Shoreline
Patterns of Propeller Scarring of Seagrass in Florida Bay 13

Figure 12. Time sequence of digital imagery (1995-2004) showing continual expansion of propeller dredged wheel ditch on Shell Key 

Bank.

Table 2. Area of damaged submerged aquatic vegetation 

associated with a propeller dredged channel through Shell Key 

Bank.

Location Year Area of wheel ditch 

(ha)

Shell Key Wheel Ditch

Shell Key Wheel Ditch

Shell Key Wheel Ditch

1995

1999

2004

0.15

0.33

0.52

Propeller scar and associated turbidity in Florida Bay, freshly created by a fishing boat in January 2008 (left and above). Photos by 

William Perry, ENP.

ring density suggests a very slight increase in scarring density 

as distance from marine facilities increases. Proximity to all 

channels (ChanPx) and marked channels (MChan Px) produced similar r2

 values and the regression analysis suggests 

that adding unmarked channels into the regression analysis does not result in an increase in scarring near channels. 

Regressions on boating use showed the highest r2

 value for 

proximity to fishing boats (FishPx), followed by proximity to 

1995 

Wheel Ditch

1999 

Wheel Ditch

2004 

Wheel Ditch
14 South Florida Natural Resources Center Technical Series (2008:1)

transiting boats (TransitPx), recreational boats (RecPx), and 

commercial boats (ComPx). 

Similar trends were observed with increased presence, 

regardless of density, of scarring in the grid cells as the feature proximity or water depth decreased (Fig. 8). The lack 

of observations in areas shallower than 2.0 ft in the depth 

variable is partly an artifact of the methods used for data collection. Access to shallow areas was limited because bathymetric data were collected with a fathometer mounted to a 

boat. Consequently, depths of 2.0 ft represent depths ≤ 2.0 ft. 

Improved bathymetric data are required to better understand 

conditions in these most shallow areas of the bay. 

In most cases, maximum scar densities are strongly related 

to the explanatory variable. Most notable are the relationships 

between proximity to most boating activities and maximum 

likely scar density. Proximity to channels and water depth 

both display a moderate relationship, while there is weak, yet 

significant inverse relationship with close proximity to marine 

facilities. Proximity to commercial boat activity has the weakest relationship. 

All Channel Proximity

0

0.02

0.04

0.06

0.08

0.1

0.12

500 2500 4500 6500 8500

Distance, m

% Cells with Scarring

Commercial Boat Proximity

0

0.01

0.02

0.03

0.04

0.05

0.06

1000 3000 5000 7000 9000 >10000

Distance, m

% Cells with Scarring

Transit Boat Proximity

0

0.01

0.02

0.03

0.04

0.05

0.06

1000 3000 5000 7000 9000 >10000

Distance, m

% Cells with Scarring

Water Depth

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

< 2.5 2.5 - 3 3 - 3.5 3.5 - 4

Water Depth, ft

% Cells with Scarring

Dock Proximity

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1000 7000 13000 19000

Distance, m

% Cells with Scarring

Fishing Boat Proximity

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

1000 3000 5000 7000 9000 >

10000

Distance, m

% Cells with Scarring

Marked Channel Proximity

0

0.02

0.04

0.06

0.08

0.1

0.12

500 2500 4500 6500 8500

Distance, m

% Cells with Scarring

Recreational Boat Proximity

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

200 600 1000 1400 1800 > 2000

Distance, m

% Cells with Scarring

Shore Proximity

0

0.01

0.02

0.03

0.04

0.05

0.06

1000 3000 5000 7000 9000 11000 13000

Distance, m

% Cells with Scarring

Figure 13. Proportion of scar presence for all nine explanatory variables. Scar presence is the count of 100x100 m cells which overlap 

with mapped scars. 
Patterns of Propeller Scarring of Seagrass in Florida Bay 15

Proximity to boat use for recreation, fishing, and transit, 

along with proximity to shore, had the strongest influence 

on estimates of maximum likely scarring (Table 3). Most 

recreational boat use is related to fishing activity, so only one 

variable, fishing proximity (with the higher r2

 of the two) was 

selected for the composite index model. Distance from channels and distance from commercial boats have less influence 

on the composite model. Dock proximity turns out to be an 

important influence on the model near shore. This variable 

separates out shoreline proximity so that distances from the 

small keys within the ENP portion of Florida Bay, where scarring is often present, is distinct from the distances associated 

N Estimate r2

ChanPx 147 -9.702e-06 0.49***

MChanPx 147 -1.096e-05 0.49***

DockPx 3297 7.292e-07 0.09***

ShorePx 137 -1.358e-05 0.73***

FishPx 37 -5.454e-05 0.82***

TransitPx 58 -3.667e-05 0.75***

RecPx 37 -5.015e-05 0.70***

ComPx 297 -3.515e-06 0.43***

Depth 81 -0.022526 0.61***

Table 3. Linear regression results for the nine explanatory variables versus maximum likely scar density (*** P≤0.001).

Figure 14. Scar density and water depths in Florida Bay (zoom below).

High 

Low 

High : 9.8

Low : 1.8
16 South Florida Natural Resources Center Technical Series (2008:1)

0 2000 4000 6000

0.00 0.10

ChanPx (m)

Scar Density, m/m2

0 2000 4000 6000

0.00 0.10

MChanPx (m)

Scar Density, m/m2

0 5000 15000

0.00 0.10

DockPx (m)

Scar Density, m/m2

0 2000 4000 6000

0.00 0.10

ShorePx (m)

Scar Density, m/m2

500 1000 2000

0.00 0.10

FishPx (m)

Scar Density, m/m2

0 1000 2000 3000

0.00 0.10

TransitPx (m)

Scar Density, m/m2

500 1000 2000

0.00 0.10

RecPx (m)

Scar Density, m/m2

0 5000 10000

0.00 0.10

ComPx (m)

Scar Density, m/m2

2 3 4 5 6

0.00 0.10

Depth (m)

Scar Density, m/m2

Figure 15. Top 10% of scar densities in relationship to proximity to all channels (ChanPX), proximity to marked channels (MChanPx), 

proximity to marine facilities (DockPx), proximity to shorelines (ShorePx), proximity to fishing boats (FishPx), proximity to transiting 

boats (TransitPx), proximity to recreational boats (RecPx), proximity to commercial boats (ComPx), and water depth (Depth). Regressions of data points are shown as a solid line. Dashed lines indicate 95% confidence limits. 

with the main keys (Key Largo to Long Key) that are outside 

of ENP (often 1 to 3 miles from the park boundary) where 

most of the marine facilities are located and where scarring 

was limited. Pearson’s product-moment correlation scores 

for each unique pair of variables are presented in Table 4. 

FishPx and RecPx are highly correlated, as are RecPx and 

TransitPx. FishPx and TransitPx have moderately high correlations. ChanPx and MChanPx are also moderately highly 

correlated. 

The variables used in spatially-explicit data layers for 

statistical modeling of maximum expected scarring densities 

were selected based on r2

 values (Table 3) and correlation 

scores (Table 4). FishPx has the strongest linear relationship 

to scarring density. RecPx and TransitPx also had a significant 

relationship to scarring, however their high correlation with 

FishPx allowed these two variables to be removed from the 

composite index model. ShorePx and Depth also have good 

relationships with scar density and were included. Channel 

proximity was only modestly associated with scar density, but 

was retained because it appears to influence the final score of 

some shallow areas and because of the potential to explore 

channel management options in order to reduce prop scarring. 

ChanPx and MChanPx are highly correlated, so only ChanPx 

was selected. ComPx and DockPx were not retained because 
Patterns of Propeller Scarring of Seagrass in Florida Bay 17

Figure 16. Modeled maximum scar densities based on 1) proximity to channel, 2) proximity to fishing boats, 3) proximity to shoreline, 

and 4) water depth.

of their low r2 values (Table 3). The spatially-explicit regressions of the four selected variables (FishPx, ShorePx, ChanPx, 

and Depth) are shown in Fig. 16. The composite index model 

suggests that large parts of Florida Bay are susceptible to 

scarring (Fig. 17). In addition, scarring data generally overlap 

areas that were predicted by the model to be scarred, with no 

substantial omission or commission. Assigning a threshold of 

0.085 m/m2

 or greater to the composite index model captures 

most of the areas and patterns of observed prop scarring in 

Florida Bay (Fig. 18).

ChanPX MChanPX DockPx ShorePx FishPx TransitPx RecPx ComPx

MChanPx 0.76

DockPx -0.08 0.34

ShorePx 0.52 0.59 0.34

FishPx 0.51 0.35 -0.04 0.19

TransitPx 0.47 0.35 0.05 0.14 0.76

RecPx 0.6 0.41 -0.05 0.22 0.92 0.88

ComPx -0.32 -0.07 -0.44 -0.31 0.03 0.17 0.02

Depth 0.39 0.29 0.06 0.46 0.14 -0.01 0.15 -0.58

Table 4. Pearson’s product-moment correlations for all pairs of variables (df= 197626, P ≤ 0.001 for all correlations).

m/m2

m/m2

m/m2

m/m2
18 South Florida Natural Resources Center Technical Series (2008:1)

Figure 17. Modeled maximum-likely boat-prop scarring using shoreline proximity, fishing proximity, channel proximity, and water 

depth. Observed scars are overlain in orange.

Channel between the western-most of the Bob Allen Keys and the small unnamed key to the southwest of west Bob Allen Key. Photo 

by Lori Oberhofer, ENP.
Patterns of Propeller Scarring of Seagrass in Florida Bay 19

Panhandle Key looking south toward Gopher keys. Photo by Lori Oberhofer, ENP.

Figure 18. Modeled maximum boat scar densities are shown in green and observed scars are shown in orange. 

modelled scarring density ≥ 0.85 m/m2

scars mapped from 2004 aerial imagery
20 South Florida Natural Resources Center Technical Series (2008:1)

Discussion

Results of the prop scarring analyses document that scarred 

areas are extensive and ubiquitous throughout the shallow 

waters of Florida Bay. Scarring was limited to shallow areas 

(≤ 6.5 ft), but did not appear to be restricted to any particular 

bottom feature, e.g., only on banks, near channels, or near the 

Florida Keys. Scarring presence is substantially greater than 

that reported by Sargent et al. (1995) (Fig. 19). Our approach 

to the quantification of scarring density was different that 

employed by Sargent et al. (1995); we used a 100 m2

 grid cell 

layer to calculate density, as opposed to using a density scale 

and hand-drawn polygons around scarred areas. We recommend the grid-based approach for future studies because it 

eliminates any ambiguity as to where scarred polygons begin 

or end. Given the limitations of the 1995 map, as expressed by 

Sargent et al. (1995), it is unclear what portion of the current 

mapping effort represents an increase in scarring and what 

portion reflects the availability of better mapping resources. 

Regardless, the current mapping effort provides an improved 

baseline of scarred areas inside the Florida Bay portion of 

ENP. Our comparison of the 2004 imagery to the 2006 higher 

resolution imagery in north-central Florida Bay suggests that 

we may have underestimated the number of scars by a factor of ~ 6.5 and the total length of scars by a factor of ~ 11.5. 

Consequently, our results represent a conservative estimate of 

scarring presence, mean length, total length, and density. We 

emphasize that the results of this study substantially underestimate total scarring (length of scars and quantity of scars) 

(Fig. 11) and scarring density based on the following factors:

1. In heavily damaged areas, it was not possible to 

map every individual scar because scars tend to be 

indiscernible from each other. Polygon data would 

be necessary in these areas of scarring and better 

represent scarring impacts adjacent to heavily used 

areas, such as channels.

2. In addition to intrinsic photo quality issues, surface 

glare, wind related surface disturbance, and sun 

angle in some areas of the imagery prevented 

differentiation of the bay bottom and hampered 

our ability to map scars; therefore, causing us to 

miss scars.

Figure 19. Comparison of prop scarring polygons identified by Sargent et al. (1995) with individual scars identified in this study.
Patterns of Propeller Scarring of Seagrass in Florida Bay 21

3. Water transparency, limited by turbidity and 

possibly by algal blooms at the time of acquisition, 

also inhibited our ability to map scars in some 

areas. This factor made it infeasible to map prop 

scars in ENP’s western coastal seagrass beds, i.e., 

from the Ten Thousand Islands to Cape Sable; 

hence, alternative methods are being pursued. 

4. Field observations suggest that, in some cases, 

scars fill in with drift algae; rendering scars in 

these areas indiscernible in aerial photographic 

interpretation. 

Given these limiting factors, we stress the fact that our results neither represent a full accounting of scarring by quantity or by length, nor accurate estimates of scarring density. 

Rather, the results are useful to describe relative scar density, 

patterns of scarring density as they relate to measurable visitor use factors, and areas where management strategies can 

be focused to achieve improved resource and visitor use conditions, including restoration of prop scarred seagrass beds. 

Further, these analyses do not assess the depth that individual 

prop scars extend into the bay bottom, which can influence 

the ability of seagrass beds to recover. 

Our scarring change analysis documents that some prop 

scarred areas are, at best, maintaining the same number and 

length of scars (i.e., there is no net recovery); while the data also 

suggest that in some locations the quantity of prop scars and 

their length are increasing over time. Many factors, including 

the four factors listed above, contribute to uncertainty related 

to interpretation of the imagery used for the change analysis. 

These sources of error make it difficult to develop definitive 

conclusions related to trends in prop scarring at specific locations, but it is reasonable to conclude that scarring levels are 

not improving and are probably increasing. The substantial increases in mapped scars between 1995 and 2004 at individual 

sites, the persistence of scars at all change analysis sites, the 

increase in the south Florida population, and the increase in 

motor boat registrations, suggest that prop scarring is likely to 

remain a prominent resource concern in ENP. In addition, recent upward trends in gasoline prices may make operation of 

large powerboats, those that typically fish and recreate farther 

offshore, less financially practical and therefore an increase 

in the number of boaters using smaller flats and bay boats is 

likely to occur. Moreover, prop scarring represents a problem 

unlikely to improve without new strategies employed through 

natural resource management initiatives.

Prop scarring is directly related to water depth. While it is 

not surprising that prop scarring is greatest in shallow areas, 

our analysis suggests that the bulk of scarring occurs at depths 

of 3.5 ft or less. This finding is important because management 

plans may consider the use of depth thresholds to manage 

access of motorized watercraft to areas that are most susceptible to prop scarring. The lack of high resolution bathymetry 

makes it difficult to estimate scarring density at the shallowest 

levels (i.e., 0.5, 1.0, and 1.5 ft depths); however, the density of 

scarred areas in depths of 2.5 ft and < 2.0 ft clearly suggests 

that shallower depths are likely to experience even more scarring. Our analysis also suggests that scarring is denser near 

shorelines within ENP portions of Florida Bay. 

The strong relationship between scarring and proximity to navigational channels suggests that resource managers 

should, among other factors, focus on strategies that manage 

navigation in and around Florida Bay. The channels are a conduit from which boaters venture out into the bay, so it is not 

surprising that areas near channels will suffer greater scarring. 

Channels may also be impacting adjacent areas. For example, 

shallow areas relatively near channels may be more likely to 

have heavier scarring than shallow areas farther away from 

channels. The composite index (Fig. 18) reveals this indirect 

effect. 

Interestingly, there was no clear difference in this relationship when examining marked and unmarked channels 

although we initially hypothesized that marked channels may 

have a lower density of scarring around them than unmarked 

channels. The lack of difference when comparing scarring 

between marked versus unmarked channels may be a result 

of more intensive use of marked channels versus unmarked 

channels. Florida Bay is difficult to navigate; therefore, new 

and inexperienced users are likely to choose to use those 

channels that are clearly marked. In addition, inexperienced 

anglers may subsequently focus exploratory trips, i.e., trips 

away from channels to explore new areas for fishing, in and 

around marked channels. In contrast, unmarked channels 

are likely to be used less intensely and most often by boaters 

that have substantial experience. While it is desirable to mark 

more clearly navigational channels that assist with navigation 

throughout Florida Bay, improving these aids to navigation will 

probably only help reduce scarring in areas that are very close 

to the channel, a clear area where navigational mishaps occur. 

Therefore, it is not likely that these markings will result in a 

large scale decrease in scarring across Florida Bay. Moreover, 

in some areas it is possible that scarring could become more 

intense adjacent to newly marked channels because they may 

attract more users and increase the frequency of exploratory 

trips emanating from the channels. In other areas, such as 

southeastern Florida Bay, there is extensive scarring of mud 

banks, presumably because motorboat operators are trying to 

Boat driver causing benthic damages and associated turbidity 

at the east end of Crocodile Dragover. Photo by Lori Oberhofer, 

ENP.
22 South Florida Natural Resources Center Technical Series (2008:1)

traverse these banks. Navigational markings in these areas, if 

used appropriately, should reduce scarring.

Scar density is also related to the presence of boats that 

are in the park on fishing trips. Because scarring is caused by 

motorboats, these results suggest that managing the intensity 

of motorboat activity in specific, high-impact areas should 

result in a decrease in scar density. We did not explore relationships between scarring and scar density by boat type or by 

boating practices. Additional studies on the size of boats using ENP may lead to more specific management recommendations based on a range of ideas that have been suggested 

by the public during public involvement phases of the GMP. 

Managers should also consider successful strategies that have 

been implemented by other federal, state, and local marine resource management agencies, including lessons learned from 

the many other marine natural areas in Florida. These strategies have taken into consideration factors such as boat size, 

potential draft, hull shape, horsepower, and use of adjustable 

outboard motor mounts, e.g., jack plates. We are not aware of 

analyses that support the hypothesis that a particular horsepower rating or boat size would result in more or less prop 

scarring. In Lignumvitae Key Submerged Lands Management 

Area, boat size was not related to an increase in the area 

of benthic damage, but it was related to damage severity 

(Engeman et al. 2008). Generally, managers should consider 

the notion that larger horsepower motors, larger propellers, 

longer outboard motor shaft lengths, and deeper running 

draft depths could result in more scarring and greater damage in shallow waters. For example, managers could consider 

restricting access of large, twin outboard boats from the shallowest waters in Florida Bay. It is worth noting that the large 

majority of mapped scars appear to be caused by single engine 

boats. Twin engine prop scars in ENP were concentrated near 

the Intercoastal Waterway along the park boundary near the 

Florida Keys. More detailed analyses are necessary to understand further the relative contribution of boat and outboard 

motor type to scarring density and pattern. 

Secondary Impacts

The literature on secondary impacts of prop scarring on 

higher trophic level organisms is mixed. Even pristine seagrass 

beds comprise heterogeneous landscapes that include areas 

without seagrass. These zones and edges arguably provide 

transitional areas where important ecological interactions 

occur, e.g., ambush points affecting predator-prey dynamics. 

However, negative impacts of prop scars have been observed. 

For example, Uhrin and Holmquist (2003) observed lower 

abundances of crabs and molluscs in and around scars, and 

Burfeind and Stunz (2007) observed a decrease in shrimp 

growth in scarred areas. In contrast, several studies indicate 

no impact to fish in scarred areas (Bell et al. 2002, Uhrin and 

Holmquist 2003, Burnfield and Stunz 2007) and no impact to 

overall nekton communities (Burnfield and Stunz 2006). 

Concern is warranted related to impacts of sediment suspension and wave activity that is caused by motorboats and 

the wakes they create. These secondary impacts could impact 

intact seagrass beds by reducing overall light transparency and 

reduce the potential for seagrass recovery in scarred areas. 

However, boat wakes have been found to have small impacts 

on sediment suspension and water quality relative to natural 

wave action (Koch 2002). It is not clear if the chronic effects 

of multiple boats traveling through concentrated transit areas, 

such as boat channels, may negatively impact seagrass recovery in adjacent areas. 

Prop scarring may also impact visitor use experiences in 

ENP. As a premier sportfishing destination, Florida Bay is extremely important to the recreational fishing community, including professional fishing guides in the Miami, Homestead, 

and Florida Keys area. These stakeholders have a vested interest in providing visitors from all over the world with a quality 

experience in the park. Visitor use satisfaction during recreational fishing trips is likely to be affected by aesthetics and 

fishing success. Park aesthetic values are clearly affected by 

prop scarring. Florida Bay tends to have relatively high water 

transparency and flats fishermen typically employ sight-fishing techniques. As such, visitor experiences may be negatively 

influenced by the aesthetic impacts caused by prop scarring. 

Further study would be needed to evaluate the effect of prop 

scarring on visitor experience.

At the same time, since ENP is also the largest designated wilderness area east of the Rocky Mountains, it is a 

premier destination for visitors seeking backcountry experiences. Some of these experiences are sought in Florida Bay. 

Wilderness values that are provided include opportunities for 

solitude, wildlife viewing, paddling, fishing, interpretive and 

education programs, and camping. Because some of these opportunities can be adversely impacted by prop scarring, and 

the activities and behaviors that cause prop scarring, the public has indicated that future management of Florida Bay and 

other marine areas should enhance wilderness experiences. 

Visitors to Everglades National Park kayaking in Florida Bay. 

Photo by Rodney Cammauf, ENP.
Patterns of Propeller Scarring of Seagrass in Florida Bay 23

Management Implications

The issues and challenges presented in this report will be addressed as ENP continues development of its GMP in 2008 

and 2009. The GMP will identify objectives and desired conditions to be sought by park managers in cooperation with the 

public related to natural and cultural resource conditions and 

visitor use experiences that should be provided. The GMP 

will be accompanied by an Environmental Impact Statement, 

which requires public involvement at several key steps in its 

development. Specific to managing Florida Bay, a variety of 

preventative management options including mandatory education programs and boat permits for all boaters in ENP, improved aids to navigation, pole and troll zones, idle and speed 

zones, limiting motorized watercraft access by watercraft 

characteristics, and area-specific seasonal access limits or closures, should be considered to minimize damages caused by 

prop scarring.

Education is an important first step in the prevention of 

prop scar damage. Implementing a comprehensive and widereaching education program will be challenging considering 

the number of entry points to Florida Bay, the geographically 

diverse group of users, and the fact that education is not a 

substitute for on-the-water experience and local knowledge. 

Given that Duarte et al. (2008) report seagrass ecosystems 

receive the lowest level of coverage in the media when compared to mangroves and coral reefs, it is not surprising that 

public awareness is limited. They recommend improved 

communication of scientific information through formal and 

informal efforts. Regulatory requirements, paired with education, such as those that require users to have mandatory safe 

boating courses may be more effective. However, Florida Bay 

is complex; perhaps the largest and most complex shallow 

water body to be examined for marine management options. 

All potential management options will be challenging and require substantial cooperation with the public. Application of 

our findings to the GMP process and review of management 

efforts in other coastal areas will help to ensure that the best 

approaches are being considered. 

Slow speed and idle zones may be used to slow motorized watercraft and reduce the potential for seagrass damage. 

These zones are widely used in Florida in an attempt to reduce collisions between boaters and wildlife. However, boats 

generally draft more water when idling. Therefore, idling or 

slow speed transit may also result in some level of scarring 

and associated turbidity. 

Implementation of pole and troll zones appear to be 

on the rise in coastal areas of Florida recently. These zones 

greatly reduce or eliminate the possibility of prop scarring by 

only allowing human-powered transit via push poles and very 

low horsepower electric motors for boat locomotion. Impacts 

from poling activities and trolling motors are possible, but 

likely to be much less intense compared to prop scarring. Pole 

and troll zones are likely to present solutions that have a high 

probability of reducing prop scar impacts while still allowing 

access. However, managing motorboat access does not guarantee that scarred areas will recover or that the area of damage will decline. For example, Engeman et al. (2008) report a 

continual increase in benthic damages despite the establishment of a no-motor-zone area in Lignumvitae Key, Florida. 

A full review of the existing pole and troll zones is warranted 

prior to implementation in ENP. Many challenges are associated with implementing pole and troll zones. Maintaining a 

reasonable level of access to traditional use areas that become 

pole and troll zones can be difficult if the areas are very large 

and require miles of human or electric-motor transit to desirable fishing grounds. In addition, marking and communicating the boundaries of pole and troll zones and enforcement of 

such zones would require creative solutions given the size of 

Florida Bay and the large extent of scar cover. 

Limiting access to an area completely, or to all motorized 

watercraft, is likely to be the most effective option to reduce 

seagrass scarring and would be relatively easy to enforce, 

but these regulations would cause the most impact to existSouth end of largest Whipray key looking south toward small Whipray. Photo by Lori Oberhofer. ENP.
24 South Florida Natural Resources Center Technical Series (2008:1)

ing users of Florida Bay. Complete closures should only be 

implemented if less restrictive strategies prove unsuccessful 

or are limited to areas that have extremely high scar density. 

Closing areas to motorized watercraft may have other values, 

such as maintaining pristine reference sites for scientific study, 

improving submerged wilderness character, and allowing seagrass restoration programs to be successful without the threat 

of further impacts. Signage should be used to mark closed or 

limited access areas, however it should be noted that large 

sign implementation projects have largely failed to eliminate 

scarring in Pinellas County (Stowers et al. 2002); while the 

rate of scarring increase was reduced, an increase in scarring 

still occurred. 

Management options that are preventative in nature 

should be coupled with options that directly mitigate boat 

related injuries to seagrass beds. Methods of assessment and 

restoration of damaged seagrass beds should be formalized in 

ENP. Kirsch et al. (2005) outline the Mini-312 program currently being used to quantify damage efficiently in Florida Keys 

National Marine Sanctuary. This program enables managers 

to prepare defensible resource damage claims and provides 

the baseline information needed for restoration efforts. The 

Mini-312 program, developed by National Oceanographic 

and Atmospheric Administration, has three components: a 

rapid, standardized assessment of seagrass boat-grounding 

damages with GIS products, a model of damage recovery 

rates, and a court-accepted calculation of compensation (the 

party responsible for the damage is responsible for compensation of loss of habitat value) (Kirsch et al. 2005). Kirsch et 

al. (2005) report the program has proven to be cost-effective 

and significantly increased the number of sites that can be 

assessed and restored. Implementing the program’s standardized documentation and mapping within ENP would provide 

a number of opportunities including a proven and cost-effective method of estimating restoration needs and costs, particularly at larger blowholes, wheel ditches, and berms. Sharing 

of information, systematic trends analyses, and other seagrass 

protection experiences in common formats with other management agencies will be invaluable. For example, Whitfield et 

al. (2002) has reported that without physical regrading, deeply 

scarred blowholes are unstable and vulnerable to continued 

degradation. 

In addition, increased public awareness of the financial 

penalties associated with damaging natural resources in 

ENP may also serve as a deterrent to careless boating. An increase in law enforcement officers patrolling ENP may also 

be effective in curbing seagrass damage. Costs associated 

with augmenting protection of seagrass beds are warranted, 

especially when considering the annual value of ecosystem 

services provided by seagrass beds (Costanza et al. 1997). A 

recent benefit-cost analysis suggests that the overall cost of 

additional law enforcement is far outweighed by the benefit 

of protecting seagrass beds, which were valued at $140,752/ha 

(Engeman et al. 2008). 

Appropriate management strategies depend largely on the 

objectives for managing benthic resources. Managers should 

consider how well management strategies are likely to meet 

the following objectives: 

1. reducing the likelihood of new scars in scarred 

areas, 

2. allowing for recovery of seagrass in scarred areas, 

and 

3. reducing the likelihood of scarring in pristine or 

relatively unscarred areas. 

Objective 3 is desirable because the cost of restoring seagrass is high; hence, preservation of existing seagrass bed integrity is most efficient (Milano and Deis 2003) and often far 

more cost effective than restoring seagrass damage. Existing, 

heavily scarred areas should also be considered and our analysis of the most impacted areas, the upper 10% of intensely 

scarred areas, suggests that these defined areas (~20) could be 

the basis for implementing on-the-water management measures and monitoring (Fig. 20). 

Applying effective management measures to existing 

scarred areas is likely to result in meeting the first two objectives, but consideration of relatively unscarred areas is 

necessary to achieve objective 3. Meeting all three of these 

objectives would be consistent with the Wilderness Act which 

states that a wilderness area should be “protected and managed so as to preserve its natural conditions. “

It is even possible that limiting access to some currently 

scarred areas could displace existing visitor use and reallocate 

impacts to areas that are currently not scarred. Reallocation 

of benthic impacts during seasonal area closures has been 

observed in marine commercial fisheries (e.g., Dinmore et al. 

2003). Many options are available to assist with meeting these 

objectives. To meet objective 3, consideration could be given 

to management measures in areas identified using a modelbased approach (Fig. 19 and 20). This would allow managers 

to identify for protection pristine areas that have a high likelihood of scarring based on physical and visitor use factors. In 

addition, this approach may assist in protecting areas that are 

heavily scarred, but could not be mapped due to poor image 

quality. The logistical challenges, e.g., signage, education, and 

enforcement, associated with implementing boating restrictions in all areas indicated in Fig. 17, would require substantial 

park resources. In contrast, implementing restrictions in the 

model-identified areas indicated in Fig. 18 would provide an 

option to manage a subset of areas that are most likely to have 

high levels of scarring, though practical challenges will still 

exist. Managers will need to consider a suite of strategies to 

address these areas. 

Balancing seagrass conservation with recreational boating 

may be accomplished most effectively by combining multiple 

strategies (Cole 1993). We recommend that a multi-faceted 

and adaptive approach be considered to manage and mitigate 

prop scarring in Florida Bay (Table 5). Examples of experiment-driven management actions include tests of channel 

markings with different spacing and sizes, restoration trials in 

scars with relatively faster-growing H. wrightii supplemented 

with bird roosting stakes to facilitate nutrient enrichment from 
Patterns of Propeller Scarring of Seagrass in Florida Bay 25

bird defecation (Kenworthy et al. 2000), and assessment of 

scar injury response to wave and storm disturbance at different scar depths. Future monitoring of the benefits associated 

with management strategies will require continued efforts to 

provide aerial interpretation of seagrass scars, combined with 

more detailed, in-water analyses of prop-scar recovery at a 

number of indicator sites. 

Restoration trials and annual monitoring of scarring distribution and density would allow park managers to assess 

the effectiveness of various management strategies and techniques in order to protect park resources and visitor experiences and take future action as warranted by implementing 

new approaches and by increasing or relaxing restrictions as 

needed to mitigate seagrass scarring. 

Table 5. Adaptive steps for using management tools to reduce seagrass prop scar impacts. Management tools are hypothesized to 

have higher potential efficacy from left to right.

Figure 20. Clusters of the most densely scarred areas, as outlined above, are possible sites for implementation of management measures, such as a pole and troll zone. Areas are approximate and only drawn as examples.

 g g g g g g g g g g g Protective Gradient g g g g g g g g g g g

Tools Education Channel Marking Boat Characteristics Pole+Troll No Access

Steps

1 X

2 X X

3 X X X

4 X X X

5 X X X X

6 X X X X X
26 South Florida Natural Resources Center Technical Series (2008:1)

Literature Cited 

Andorfer, J. and C.J. Dawes. 2002. Production of rhizome meristems 

by the tropical seagrass Thalassia testudinum: the basis for slow 

recovery into propeller scars. Journal of Coastal Research. 37: 

130-142.

Ault, J.S., S.G. Smith, D. McClellan, N. Zurcher, A. McCrea, N.R. 

Vaughan and J.A. Bohnsack. 2008. Final Report: Aerial survey of 

boater use in Everglades National Park marine waters – Florida 

Bay and Ten Thousand Islands. University of Miami, Miami, 

Florida. 97 pages.

Bell, S.S., M.O. Hall, S. Soffian and K. Madley. 2002. Assessing the impacts of boat propeller scars on fish and shrimp utilizing seagrass 

beds. Ecological Applications. 12(1): 206-217.

Burfeind, D.D. and G.W. Stunz. 2006. The effects of boat propeller 

scarring intensity on nekton abundance in subtropical seagrass 

meadows. Marine Biology. 148: 953-962.

Burfeind, D.D. and G.W. Stunz. 2007. The effects of boat propeller 

scarring on nekton growth in subtropical seagrass meadows. 

Transactions of the American Fisheries Society. 136: 1546-1551.

Cole, D.N. 1993. Minimizing conflict between recreation and nature 

conservation. In D.S. Smith and P.C. Hellmund, eds., Ecology of 

Greenways: Design and Function of Linear Conservation Areas. 

University of Minnesota Press, Minneapolis, MN. pp. 105-122.

Costanza, R., R. d’Àrge, R.D. Groot, S. Farber, M. Grasso, B. Hannon, 

K. Limburg, S. Naeem, R.V. O’Neill, J. Paruelo, R.G. Raskin, P. 

Sutton and M. van der Belt. 1997. The value of the world’s ecosystem services and natural capital. Nature. 387: 253–260.

Dawes, C.J., J. Andorfer, C. Rose, C. Uranowski and N. Ehringer. 

1997. Regrowth of the seagrass, Thallasia testudinum, into propeller scars. Aquatic Botany. 59: 139-155.

Dawes, C.J., R.C. Phillips and C. Morrison. 2004. Seagrass communities of the Gulf Coast of Florida: Status and Ecology. Florida 

Fish and Wildlife Conservation Commission Fish and Wildlife 

Research Institute and the Tampa Bay Estuary Program. St. 

Petersburg, FL. iv+ 74 pages.

Dinmore, T.A., D.E. Duplisea, B.D. Rackham, D.L. Maxwell and S. 

Jennings. 2003. ICES Journal of Marine Science. 60: 371-380. 

Duarte, C.M., J. Terrados, N.S.R. Agawin, M.D. Fortes, S. Bach and 

W.J. Kenworthy. 1997. Response of a mixed Philippine seagrass 

meadow to experimental burial. Marine Ecology Progress Series. 

147: 285-294.

Duarte, C.M., W.C. Dennison, R.J.W. Orth and T.J.B. Carruthers. 

2008. The charisma of coastal ecosystems: addressing the imbalance. Estuaries and Coasts. 31: 233-238. 

Durako, M.J., M.O. Hall, F. Sargent and S. Peck. 1992. Propeller scars 

in seagrass beds: an assessment and experimental study of recolonization in Weedon Island State Preserve, Florida. In: Webb F.J., 

Jr. (ed) Proc 19th Annual Conference Wetlands Restoration and 

Creation. Hillsborough Community College, FL, pp. 42–53.

Engeman, R.M., J.A. Duquesnel, E.M. Cowan, H.T. Smith and S.A. 

Shwiff. 2008. Assessing boat damage to seagrass bed habitat in a 

Florida park from a bioeconomics perspective. Journal of Coastal 

Research. 24(2): 527-532.

ESRI 2006. ESRI® ArcMAPTM 9.2. Copyright 1999-2006 ESRI Inc. All 

Rights Reserved. URL: http://www.esri.com.

Everglades National Park. 2008. General Management Plan. 40001 

State Road 9336, Homestead, Florida. http://parkplanning.nps.

gov/documentsList.cfm?parkId=374&projectId=11170.

Florida Fish and Wildlife Conservation Commission - Fish and 

Wildlife Research Institute (FWC-FWRI). 2004. Metadata for 

ISM_CSA_Florida_Bay_2004_imagery. Unpublished.

Florida Fish and Wildlife Conservation Commission, Fish and Wildlife 

Research Institute (FWC-FWRI). 2005. Dataset for Florida Bay 

Benthic Communities prepared by Avineon, Inc for FWC-FWRI. 

Available online at http://ocean.floridamarine.org.

Florida Fish and Wildlife Conservation Commission, Fish and Wildlife 

Research Institute (FWC-FWRI). 2006. Dataset for Florida marine facilities. Available online at http://ocean.floridamarine.org; 

metadata available online at http://www.fgdl.org/metadata/fgdc_

html/marfac.fgdc.htm.

Florida Fish and Wildlife Conservation Commission. 2007. Florida 

boating safety education program. Available online at: http://myfwc.com/Boating/Docs/BoatingSafetyEdExSummOct07.pdf.

Fonseca, M.S. and S.S. Bell. 1998. Influence of physical setting on 

seagrass landscapes near Beaufort, North Carolina, USA. Marine 

Ecology Progress Series. 171: 109-121.

Fonseca, M.S., P.E. Whitfield, W.J. Kenworthy, D.R. Colby and B.E. 

Julius. 2004. Use of two spatially explicit models to determine the 

effect of injury geometry on natural resource recovery. Aquatic 

Conservation: Marine and Freshwater Ecosystems. 14(3): 

281-298.

Hammerstrom, K.K., W. J. Kenworthy, P. E. Whitfield and M. F. 

Merello. 2007. Response and recovery dynamics of seagrasses 

Thalassia testudinum and Syringodium filiforme and macroalgae in experimental motor vessel disturbances. Marine Ecology 

Progress Series. 345: 83–92.

Hansen, M. and N.T. DeWitt. 1999. 1890 and 1990 Bathymetry 

of Florida Bay. South Florida Place Based Studies OpenFile Report 00-347. USGS Center For Coastal and Regional 

Marine Geology, St. Petersburg Field Office, United States 

Geological Survey, St. Petersburg, Florida. URL http://sofia.usgs.

gov/publictions/ofr/00-347/.

Hastings, K., P. Hesp and G.A. Kendrick. 1995. Seagrass loss associated with boat moorings at Rottnest Island, Western Australia. 

Ocean Coastal Management. 26: 225–246.

Kenworthy, W.J., M.S. Fonseca, P.E. Whitfield, K.K. Hammerstrom 

and A.C. Schwartzschild. 2000. A comparison of two methods 

for enhancing the recovery of seagrasses into propeller scars: 

Mechanical injection of a nutrient and growth hormone solution 

vs. defecation by roosting seabird. Final report submitted to the 

Florida Keys Environmental Restoration Trust Fund, 40 pages.

Kenworthy, W.J., M.S. Fonseca, P.E. Whitfield and K.K. Hammerstrom. 

2002. Analysis of seagrass recovery in experimental excavations 

and propeller-scar disturbances in the Florida Keys National 

Marine Sanctuary. Journal of Coastal Research. SI 37: 75-85.

Kirsch, K.D., K.A. Barry, M.S. Fonseca, P.E. Whitfield, S.R. Meehan, 

W.J. Kenworthy and B.E. Julius. 2005. The Mini-312 Program - 

An expedited damage assessment and restoration process for seagrasses in the Florida Keys National Marine Sanctuary. Journal of 

Coastal Research. SI 40: 109-119.
Patterns of Propeller Scarring of Seagrass in Florida Bay 27

Koch, E.W. 2002. Impact of boat-generated waves on a seagrass habitat. Journal of Coastal Research. 37: 66-74. 

Kruer, C.R. 1994. Mapping assessment of vessel damage to shallow 

seagrasses in the Florida Keys. Report to the Florida Department 

of Natural Resources and the University of South Florida/Florida 

Institute of Oceanography, St. Petersburg, Florida. 23 pages.

Kuss, K.M. 1991. Lignumvitae Key Aquatic Preserve Survey of 

Boating Impacts/ Propeller Scarring in Shallow Seagrass Beds 

within LVKP. Report to State of Florida, Department of Natural 

Resources, Division of State Lands. 31 pages.

Marba, N., M.E. Gallegos, M. Merino and C.M. Duarte. 1994. Vertical 

growth of Thalassia testudinum: seasonal and interannual variability. Aquatic Botany. 47: 1–11.

Milano, G.R. and D. R. Deis. 2003. Biscayne Bay seagrasses and recent restoration efforts. In Seagrass restoration: success, failure, 

and the costs of both. Eds S. F. Treat and R.R. Lewis III. Lewis 

Environmental Services Inc. Valrico, FL. 115-129.

National Park Service (NPS) and Florida Keys Fishing Guides 

Association (FKFGA). 2006a. Florida Bay Map and Guide. 

Published online by the National Park Service at http://www.nps.

gov/ever/planyourvisit/upload/FloridaBayMap&GuideSmall.

pdf.

National Park Service (NPS). 2006b. Year 2005 annual marine fisheries report Everglades National Park. South Florida Natural 

Resources Center, Everglades National Park. Homestead, FL. 56 

pages.

Orth, R.J, T.J.B. Carruthers, W.C. Dennison, C.M. Duarte, J.W. 

Fourqurean, K.L. Heck, Jr., A. R. Hughes, G.A. Hendrick, W.J. 

Kenworthy, S. Olyarnki, F.T. Short, M. Waycott and S.L. Williams. 

2006. A global crisis for seagrass ecosystems. BioScience. 56 (12): 

987-996.

R Development Core Team. 2007. R: A language and environment for 

statistical computing. R Foundation for Statistical Computing, 

Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org.

Rodriguez, R.W., R.M.T. Webb and D.M. Bush. 1994. Another look at 

the impact of Hurricane Hugo on the shelf and coastal resources 

of Puerto Rico, USA. Journal of Coastal Research. 10: 278–296.

Sargent, F.J., T.J. Leary, D.W. Crewz and C.R. Cruer. 1995. Scarring 

of Florida’s Seagrasses: Assessment and Management Options. 

Florida Marine Research Institute Technical Report TR-1. Florida 

Department of Environmental Protection. St. Petersburg, FL. 66 

pages.

Stowers, J. F., E. Fehrmann, A. Squires. 2002. Seagrass scarring in 

Tampa Bay: impact analysis and management options. Pages 

47-54 in H.S. Greening, ed. Seagrass Management: It’s Not Just 

Nutrients! 2000 Aug 22-24; St. Petersburg, FL. Tampa Bay Estuary 

Program. 246 pages.

Uhrin, A.V. and J.G. Holmquist. 2003. Effects of propeller scarring 

on macrofaunal use of the seagrass Thalassia testudinum. Marine 

Ecology Progress Series. 250: 61-70.

USGS. 2007. National Map Accuracy Standards. Published online at 

http://rockyweb.cr.usgs.gov/nmpstds/nmas.html.

Whitfield, P.E., Kenworthy, W.J., Fonseca, M.S. and K. Hammerstrom. 

2002. The role of a hurricane in expansion of disturbances initiated by motor vessels on subtropical seagrass banks. Journal of 

Coastal Research. 37: 86-99.

Woolpert, Inc. 2007. Report of Topographic Survey Minimum 

Technical Standards. Monroe County, Florida 2006 Orthophoto 

Project. Report to Florida Department of Transportation, District 

VI. 41 pages.

Zieman, J.C. 1976. The ecological effects of physical damage from 

motor boats on turtle grass beds in southern Florida. Aquatic 

Botany. 2: 127-139. 

South Florida Natural Resources Center

Everglades National Park

950 N. Krome Ave, 3rd Floor 

Homestead, FL 33030-4443

www.nps.gov/ever/naturescience/sfnrc.htm