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Boat Propeller Scar Damage - Greater Charlotte Harbor

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The Fish and Wildlife Research Institute (FWRI) of the Florida Fish and Wildlife Conservation Commission (FWC) performed this assessment of extent and severity of boat propeller scars in seagrass habitats under cooperative agreement with the Charlotte Harbor National Estuary Program (Charlotte Harbor NEP).  Data compiled by the Florida Marine Research Institute for the 1995 report Scarring of Florida’s Seagrasses: Assessment and Management Options (Sargent, F.J, et al.) found that Charlotte Harbor has been one of the most severely scarred areas of Florida. A 2004 report serves as an update to the 1995 work for the coastal portion of the Charlotte Harbor NEP study area and provides area resource managers with an analysis of the current extent, location and severity of boat propeller scarring.   The survey area encompassed all estuarine waters within the Charlotte Harbor NEP boundary.   The 2004 report available as a .PDF on this webpage is the second evaluation of prop scars in the Charlotte Harbor area.  

Submerged aquatic vegetation (SAV) is an integral part of the Charlotte Harbor estuarine system and an important natural resource that performs a number of significant functions.  For example, seagrasses help to maintain water clarity, stabilize bottom sediments, provide habitat for many fishes, crustaceans, and shellfish, and they make up the food for many marine animals. Most importantly, these areas are the nursery grounds for most of Charlotte Harbor’s recreationally and commercially important fisheries.  The Charlotte Harbor NEP’s Management Conference developed 2 goals specific to the preservation and restoration of submerged aquatic vegetation within the greater Charlotte Harbor watershed.

 The Greater Charlotte Harbor region has experienced a 29% decrease in seagrass coverage since the 1940s (Harris et al, 1983).  This seagrass loss is mostly within the southern portion of the study area and is believed to have originated from various causes, such as Sanibel Causeway Island construction, Intercoastal Waterway dredging activities, changes to water flow and quality characteristics (Harris et al, 1983).  Boat propeller scars are also a cause of seagrass loss in the Charlotte Harbor system.  Lee and Charlotte counties ranked 3^rd and 4^th among 31 coastal counties for the amount of scarred seagrass in data collected in the early 1990’s (Sargent et al. 1995).  As the amount of people settling in the coastal counties and the number of registered vessels continue to increase, the Charlotte Harbor NEP stakeholders need to know how the increase in boating activity may be affecting the SAV of the Charlotte Harbor region. 

Propeller scarring of seagrasses usually occurs when boaters motor through water that is shallower than the drafts of their boats.  The propellers tear some combination of the seagrass leaves, stems and roots, managing at times to remove the sediments, creating unvegetated, linear troughs of varying lengths (Figures 2 and 3).  The amount of destruction from a scar-producing event depends on water depth and the size, speed, and path of the vessel.  Some vessels create scars in areas at low tide that would not do so at high tides.  Although linear features are most often associated with the term “prop scar,” some areas of seagrass habitats have been completely denuded by repeated scarring.  In other instances, a linear scar can become a larger feature if the sediments are scoured to undercut the seagrass bed.  This erosion can result in detachment of large sections of seagrasses that then float away leaving behind patches of bare sediment wider than the original prop scar.

Seagrass habitats are especially susceptible to prop scarring because they exist in shallow depths, generally less than 2 meters (6.6 feet).  Sunlight is needed by seagrasses for photosynthesis, thus the affinity of the plants for shallow locations.  Averaging only 2.1 meters (7 feet) in depth, Charlotte Harbor is relatively shallow and susceptible for high levels of prop scarred habitat (Stoker 1986).

 The slowest Florida seagrass species to recover from prop scar damage, turtle grass (Thalassia testudinum Banks ex König), reportedly regrows into the scarred area within a range of 2-8 years in Florida (Dawes and Andorfer, 2002) with complete recovery reaching 10 years (Lewis and Estevez, 1988).  Areas subject to repeated boat impacts may never have the opportunity to recover.  Because seagrass habitats are known to be critical feeding and sheltering areas for wading birds, juvenile finfish, and shellfish, cumulative scar damage results in reduction of valuable habitat.   Decreasing productive habitat for wildlife to use may affect the condition of wildlife populations.