Manatee Movements in the Northern Gulf of Mexico and the Potential for Exposure to the Deepwater Horizon Oil Spill

Manatee Movements in the Northern Gulf of Mexico and the Potential for Exposure to the Deepwater Horizon Oil Spill Daniel H. Slone1, James P. Reid1, Allen Aven2 and Ruth H. Carmichael2 1U.S. Geological Survey Southeast Ecological Science Center 7920 N.W. 71st Street Gainesville, FL 32653 2Dauphin Island Sea Lab 101 Bienville Blvd. Dauphin Island, AL 36528 U.S. Department of the Interior U.S. Geological Survey - 1 - MANATEES IN THE NGOM 1 Introduction 2 The West Indian Manatee (Trichechus manatus) is found throughout the coastal waters of the Gulf of 3 Mexico, with the Antillean subspecies (Trichechus manatus manatus) found in Mexico, and the 4 Florida subspecies (Trichechus manatus latirostris) found from Texas through to Florida (Deutsch et 5 al. 2008, Gunter, 1941; Fig. 1). Florida manatees have in recent years been documented in increasing 6 numbers in the northern Gulf of Mexico (NGOM), including Mobile Bay, the Wakulla River, and 7 other water bodies (Fertl et al. 2005, Pabody et al. 2009, Butler et al. 2011). Their habitat and 8 resource use in these areas have not been well documented, but as with other areas, they must access 9 seagrasses or other vegetation for forage, fresh water for drinking, and warm water in the winter for 10 refuge. Recently, oil from the Deepwater Horizon spill has impacted a large area of the NGOM, 11 including seagrass beds and nearshore areas that manatees are likely to use. 12 13 Since 2007, researchers from the USGS Southeast Ecological Science Center have been conducting 14 pilot studies on manatee movements and habitat use in the NGOM to document extent of migrations 15 and determine patterns of habitat use. We have been radiotagging and tracking wild manatees 16 throughout Florida since the late 1970’s (e.g. Deutsch et al. 2003, Reid et al. 1995), but this was the 17 first effort directed at understanding movement patterns of manatees in the NGOM, prompted by 18 increasing winter use in the Wakulla Springs watershed. Similarly, in 2009 researchers with Dauphin 19 Island Sea Lab (DISL) with Sea to Shore Alliance started tracking studies by tagging two manatees in 20 Mobile Bay to determine summertime habitat use in that estuary and migration routes to Florida 21 (Table 1). 22 - 2 - MANATEES IN THE NGOM 23 On April 20, 2010, the Deepwater Horizon oil drilling platform exploded, and over the next 86 days 24 the breached well released 4.9 million barrels of oil into the NGOM (NOAA 2010a). Reaction to the 25 spill from the marine mammal community was swift, with monitoring and rescue activity for several 26 species commencing before the flow of oil had stopped (MMC 2011). Researchers from the USGS 27 and DISL continued to track manatees in the NGOM during the time of the spill, and tagged 28 additional individuals 29 30 The tracking data from tagged manatees provided a foundation for us to analyze the movements of 31 manatees relative to the measured distribution of oil from the Deepwater Horizon as it related to travel 32 routes, seagrass beds, and other manatee habitats. The primary objective of this report is to compile 33 and evaluate those data for insights on possible impacts of the Deepwater Horizon spill on manatees 34 and their habitat in the NGOM. The study area includes nearshore areas that manatees may traverse or 35 utilize from Mobile Bay, AL to Crystal River, FL (Fig. 2) 36 Methods 37 For this study, we identified the following tasks: 38 1) Assemble existing background GIS datasets relating to manatee habitat in the NGOM and 39 spatial data on shoreline oiling from the Deepwater Horizon spill. 40 2) Compile available telemetry data from manatees tracked in the NGOM. 41 3) Analyze manatee movements, identify use of resources, and determine areas of potential 42 manatee interactions with oil, either directly on travel corridors, or indirectly, on seagrass 43 beds. - 3 - MANATEES IN THE NGOM 44 GIS data 45 Pursuant to task 1, the following datasets were compiled: 46 1) Background images and polygon layers were obtained from NOAA and ESRI to depict the 47 shoreline and nearshore waters of the NGOM (Table 2). 48 2) From the USGS National Wetlands Research Center, We obtained polygon layers of seagrass 49 beds found in Alabama and Florida. These areas were of interest because by feeding on 50 seagrasses, manatees would be exposed to any toxins that were adhered to the grass, and 51 because they would tend to linger in those areas, they would also be exposed to toxins in the 52 water column. 53 3) From the Environmental Response Management Application website (ERMA, 54 http://gomex.erma.noaa.gov/erma.html), we downloaded the latest spatial data from Shoreline 55 Cleanup and Assessment Technique (SCAT) surveys that were performed along the Gulf 56 coast (NOAA 2010b; Table 2). The SCAT surveys provide a standardized measure of 57 shoreline oiling conditions for comparisons among locations. These datasets were used as a 58 proxy to indicate nearshore waters and habitats that may have been affected by oil. 59 Manatee Tagging 60 A USGS study was initiated in 2007 to identify manatee use patterns and habitats in the NGOM by 61 tagging and tracking manatees known to use the region. In September 2007, USGS researchers 62 attached a tracking tag to the tail of a manatee that had been rescued from a Citgo petroleum refinery 63 near Corpus Christi, Texas the previous winter. It was hoped that the manatee, named “Texas”, would 64 travel north in the Gulf of Mexico from its release location in Crystal River, FL, however Texas did 65 not venture much further north than Cedar Key, FL, while he was tagged. In the spring and summer - 4 - MANATEES IN THE NGOM 66 of 2008, three manatees were tagged in the Wakulla River. FL. Three manatees identified from photo 67 records as regular inhabitants of the northern Gulf were tagged in Crystal River the following winter. 68 During the summer of 2009, researchers at DISL began their tracking program by tagging two 69 manatees in Mobile Bay. A total of eight manatees were being tracked in the NGOM when the 70 Deepwater Horizon spill occurred, and an additional five were tagged and tracked in the following 71 months. Combined with the three manatees that were tracked prior to the oil spill, there were data 72 from a total of 17 manatees to analyze (Table 1). 73 74 The tracking devices used by both USGS and DISL were satellite-linked Global Positioning System 75 (GPS) tags (Telonics TMT-460 and TMT-462; http://www.telonics.com/) that were coupled with 76 System Argos (https://www.argos-system.org/) satellite transmitters. These were encased in floating 77 housings that were attached to a belt around their caudal peduncle of manatees via a 2 m long tether 78 (Deutsch et al. 1998) (Fig. 3). Both belt and tether were engineered with specific tensile strengths that 79 would allow an animal to break free should it become entangled. The tags relayed GPS locations with 80 a positional accuracy within 5 m through an Argos satellite link every 30 minutes, along with 81 temperature and information about tag movement and diving rates. The actual distance of the satellite 82 antenna from the manatee varied with the size of the manatee and behavior, but at a minimum, the tag 83 would be at the surface at all times when the manatee was <2m below the surface at slow travel 84 speeds, such as during feeding or resting. In deeper water, the tag could fix a GPS location and/or 85 transmit when the manatee surfaced to take a breath, typically every 2 to 10 minutes depending on 86 behavior (Hartman 1979). - 5 - MANATEES IN THE NGOM 87 Data Analysis 88 To delineate specific habitat and movement corridor use by manatees, GPS points from all manatees 89 were combined into one GIS point layer in ArcMap 9.2 (Redlands, CA: Environmental Systems 90 Research Institute). First, the GPS point data were retrieved from each tag, and filtered temporally to 91 exclude times when the tags were not attached to the animals. Speed and positional filters were then 92 applied to eliminate spurious locations. 93 Travel speed 94 The GPS points were converted to travel paths using the ArcMap utility Points to Lines (Rathert 95 2004). The length of each line in meters was measured and added to the attribute table using ArcMap 96 utility Xtools Pro (V.7.0.0; Data East LLC, Novosibirsk, Russia). The polyline file was then merged 97 with the original point file, so that the attributes of each travel line were based on the second of the 98 two points used to draw the line. This second-point assignment associated the information of the 99 manatee’s destination with the travel. A final two columns were then added and calculated – travel 100 time (GPS fix time of the second point minus the GPS fix time of the first point) and travel speed 101 (length divided by time). 102 103 Based on our knowledge of manatee behavior, the travel paths were subdivided into three groups. 104 Slow speeds <1kph were designated as local use movements, typically foraging, drinking, 105 thermoregulating at warm water sites, or resting. Moderate speeds ≥1kph and <2kph were 106 designated as slow travel, which might occur during transit between local use sites. Fast speeds 107 ≥2kph were designated as fast travel, which typically occur during long-distance movements. 108 Each group of travel speed lines were converted to a density surface using the ArcGIS function 109 “Line Density”, which is based on the quadratic kernel function described in Silverman (1986, p.

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