Scarring of Florida's Seagrass: Assessment and Management Options

Propeller scarring, one of the fastest growing types of marine habitat degradation, is caused by motor boats traveling in waters shallower than the draft of the vessel. A statewide assessment was completed to document the extent of the problem.

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Scarring of Florida's Seagrass:
Assessment and Management Options
FMRI Technical Report TR-1

_________________________1995________________________

F.J. Sargent, W.B. Sargent, T.J. Leary, D.W. Crewz, and C.R. Kruer

EXECUTIVE SUMMARY

Seagrasses are submerged, grass-like plants that inhabit the shallow coastal waters of Florida. Seagrasses are a vital component of Florida's coastal ecology and economy. They provide nutrition and shelter to animals important to marine fisheries; they provide critical habitat for animals such as wading birds, manatees, and sea turtles; and they improve the water quality.

Marine-habitat degradation in Florida is continuing at an alarming rate as the coastal residential population and the number of seasonal visitors increase. Habitat degradation has many sources (e.g., pollution, dredge and fill), but an increasingly common cause of habitat degradation is the scarring of seagrasses. In this report, scarring can refer to either the activity of scarring or to a group of scars in a seagrass bed.

Seagrass-BoatSeagrass beds can be scarred by many activities, but scars are most commonly made when a boat propeller tears and cuts up seagrass roots, stems, and leaves, producing a long narrow furrow devoid of seagrasses. Boats operating in shallow waters are severely scarring and sometimes completely denuding to seagrass beds throughout the state.

The Florida Department of Environmental Protection recognized the need to reduce scarring of seagrasses by boats and committed resources to address this issue. As one component of this effort, the Florida Marine Research Institute (FMRI) investigated the distribution of scarred seagrass beds in the shallow marine waters of Florida's coastal counties. Aerial photography was used to locate seagrass scarring. Aerial surveys were then conducted in 1992 and 1993 to confirm the location of scarred seagrasses. We did not attempt to distinguish among the different specific causes of seagrass scarring. During aerial surveys, observations of scarred seagrasses were recorded on National Oceanic and Atmospheric Administration nautical charts and U.S. Geological Survey quadrangle maps.

Scarring intensity was categorized as light, moderate, or severe. Areas with substantial scarring recognizable on 1:24,000-scale aerial photography were delineated on the maps with polygons that were assigned a scarring intensity. Polygons categorized as light contained less than 5 percent scarring; those categorized as moderate contained 5 to 20 percent scarring, and those categorized as severe contained more than 20 percent scarring. The information acquired in this survey was incorporated into the FWRI's Marine Resources Geographic Information System (MRGIS), which produces maps and tabular products so that geographically based data can be effectively disseminated to resource managers, appropriate regional and county governments, and other interests (e.g., conservation groups and private citizens).

Seagrass-Prop-ScarsScarred seagrasses were observed in all areas of the state, mostly in shallow coastal waters less than six feet deep. More than 173,000 acres of Florida's 2.7 million acres of seagrasses were scarred, most of it lightly. This is a conservative estimate of scarring because we mapped groups of scars, not isolated, individual, propeller scars. The total seagrass acreage in Florida (2.7 million acres) includes areas in the Florida Keys that have sparse seagrass and hardbottom with dense-seagrass patches. Excluding these areas, seagrasses totaled approximately 1.9 million acres. Also, these totals do not include sparse, deep Halophila beds offshore in the Big Bend region.

The greatest acreage of moderate and severe (M/S) scarring occurred in areas having denser human populations and more registered boats. The Florida Keys (Monroe and Dade counties), Tampa Bay (Hillsborough, Manatee and Pinellas counties), Charlotte Harbor (Lee County), and the north Indian River Lagoon (Brevard and Volusia counties) had the greatest M/S scarring. Monroe County, which includes most of the Florida Keys, had the greatest M/S-scarred acreage of all the counties in the survey. The Panhandle and Big Bend regions had little M/S-scarred acreage, but in the western Panhandle embayments, M/S scarring was prevalent in the few acres of seagrasses. If an area has little seagrass acreage, then any scarring may have a critical effect on habitat functions.

All boating user-groups are responsible for scarring seagrasses. Although we did not attempt to identify each user-group's role in scarring, we believe general statements about the situations that lead to scarring are valid. The most severe, single instances of scarring are caused by large commercial vessels, but most seagrass disruption results from widespread scarring by smaller boats. Our discussions with boaters, as well as our own personal experiences, suggest that scarring of seagrasses could result when one or more of these situations occur:

  • When boaters misjudge water depth and accidentally scar seagrass beds
  • When boaters lacking navigational charts or the skill to use them stray from poorly marked channels and accidentally scar seagrass beds
  • When boaters intentionally leave marked channels to take shortcuts through shallow seagrass beds, knowing that seagrass beds may be scarred
  • When boaters carelessly navigate in shallow seagrass beds because they believe scars heal quickly
  • When inexperienced boaters engage in recreational and commercial fishing over shallow seagrass flats, thinking that their boat's designed draft is not deep enough to scar seagrasses or that the design will prevent damage to their boat
  • When boaters overload their vessels, causing deeper drafts than the boaters realize
  • When boaters anchor over shallow seagrass beds, where their boats swing at anchor and scar seagrasses
  • When boaters intentionally prop-dredge to create a channel
  • When inexperienced boaters, ignorant of what seagrasses are and the benefits they provide, accept as the behavioral norm local boating customs that disregard the environment

Management programs that address scarring of seagrasses should be based on an approach that involves (1) education, (2) channel marking, (3) increased enforcement, and (4) limited-motoring-zones. Aerial monitoring and photography of the managed area are essential in evaluating the effectiveness of a program. Management programs that use this multifaceted approach have been instituted by a few local governments and at several state parks. Initial results of the programs indicate that in some areas seagrass scarring has been reduced and that in other areas emphasis may need to be increased on one or more of the components of the four-point approach. A statewide management plan is needed to address the most egregious scarring over large areas that may be difficult to regulate at the local-government level.

Population of Florida* Vessel registrations**
1970-6,791,418 1970-235,294
1990-12,937,936 (1990) 715,516
2000-15,524,000 projected 2000-829,971

Seagrass calculations (based upon data created in 6/2000)
Nearshore seagrass: 919,962 hectares (2,273,226 acres)
Continuous seagrass: 399,916 hectares (988192 acres)
Patchy seagrass: 5,200,464 hectares (1,285,034 acres)

Propscar calculations (revised 11/2000)
Total statewide scarring of seagrasses within 0-6ft deep = 61752 hectares (1523590 acres) or 7%.

Light Damage = 38978 hectares (96,315 acres)
Moderate Damage = 17122 hectares (42,309 acres)
Severe Damage =
5652 hectares (13,966)


* Florida Statistical Abstract, 1999.
** Florida Department of Highway and Motor Vehicle Safety, 10/2000



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