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Dredging and Sea Turtles:
Potential Impacts, Assessing Risk, &
Protection Methods
DENA DICKERSON
Engineer Research & Development Center
U.S. Army Corps of Engineers
Vicksburg, Mississippi
Coastal
Dredging
- Remove sediment
- Construct land
Coastal
Dredging
Cutter-suction Pipeline
Bucket or Clamshell
Trailing-suction Hopper
Coastal
Sea Turtle
Habitat
- Feeding
- Reproduction
7 species
Dredging Impacts on
Sea Turtles?
Entrainment - Incidental take
Incidental Sea Turtle Take by
Trailing Suction Hopper Dredge
Kemp’s ridley US - Endangered
World – Critically
Lepidochelys kempii Endangered
Photo: Erin Seney
Olive ridley US – Threatened /
Endangered
Lepidochelys olivacea
World – Vulnerable
Photo by Bill Curtsinger
National Geographic
Olive ridley – Arribada (Mass nesting)
Mexico
India
Flatback
Natator depressus World – Data
Deficient
Photo: Sea Turtle, Inc
Hawksbill US - Endangered
Eretmochelys imbricata World - Critically
Endangered
Photo: Gerry O’Mara
Green US – Threatened/
Endangered
Chelonia mydas World - Endangered
Photo: Dena Dickerson
Loggerhead US - Threatened
Caretta caretta World - Endangered
Photo: Dena Dickerson
US - Endangered
Leatherback World - Critically
Dermochelys coriacea Endangered
Photo: Matthew Godfrey
Southeast U.S. Sea Turtles
Loggerhead
Green
Kemp’s ridley
Leatherback
Hawksbill
Dredge Observer Program
Trailing Suction Hopper Dredges
1980 - Began
1995 - Consistent methods
90+ - Sites monitored
Dredge Observer Program
Dredged Material Sampling
Inflow screening Overflow screening
Dredge Observer Program
- Sea turtles
- Marine mammals
- Shark / fish
- Bycatch
Documented
Sea Turtle Takes
450
1995- 2008
400 401 Turtle Takes
Sea Turtle Incidental Takes
350 (28/year)
300
250
200
150
100
50
280 63 52 6
0
Loggerhead Kemp's ridley Green Unidentified
Species
http://el.erdc.usace.army.mil/seaturtles/
Annual Sea Turtle Take Limits
(National Marine Fisheries Service)
North Atlantic – North of North Carolina
Varies by Project
REGIONAL Limits
Gulf South Atlantic
Loggerheads 40 35
Kemp’s ridley 20 7
Greens 14 7
Hawksbills 4 2
Leatherbacks None designated
Risk Assessment Factors for
Potential Dredging Impacts
1. Dredging Project
• Location
• Timing
• Duration
• Dredge & equipment type
• Dredge operations
2. Biological Data
• Species occurrence & behavior
• Habitat utilization at dredging site
3. Entrainment Data
- Available monitoring methods
- Effective documentation
- Data Analyses (temporal, spatial, vessel)
4. Protection Methods
• Availability
• Effectiveness
Risk Assessment Factor
1. Dredging Project
• Location
• Timing
• Duration
• Dredge type
• Dredge operations
Bucket or Clamshell
Cutter-suction Pipeline Trailing-suction Hopper
Risk Assessment Factor
2. Biological Data
• Species occurrence and behavior
• Habitat utilization at dredging site
Risk Assessment Factor
3. Entrainment Data
Available monitoring methods
- Dredge Observer Program
- Dredged material screening
- Bridge watch, strandings
Effective documentation
Data analyses
- Spatial, temporal, vessel
0.80
0.70
0.70 Atlantic
Turtle Takes per Dredge Project
0.60 0.58
0.50
0.42
0.39 0.38
0.40
0.33 0.35
0.29
0.30
Takes per Project
0.20
0.11
0.10 0.08 0.08
0.06
0.00
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
0.70
0.59 0.59 Gulf of Mexico
Turtle Takes per Dredge Project
0.60
0.53
0.49
0.50 0.45
0.40 0.38
0.30
0.30
0.24
0.21 0.19
0.20 0.15 0.17
0.10
0.00
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov* Dec
Month
Month
401 Turtle Takes NA
(1995 - 2008) NA
53
CA
CA
64
NEG
NWG SA
SA
18
NEG
71
NWG
108
29° 00”
Atlantic
WGWG EG Atlantic Ocean
EG
Gulf 225
69 Gulf of Mexico
18
176
Takes per Project NA
(1995-2008) NA
0.77
CA
CA
0.76
NEG
NWG SA
SA
0.43
NEG
1.11
NWG
0.96
29° 00”
Atlantic
WG
WG
EG Atlantic Ocean
EG
Gulf 0.85
1.82 Gulf of Mexico
1.50
1.14
Risk Assessment Factor
4. Protection Methods
a. Availability
b. Effectiveness
Available Protection Methods
(Implemented 1992)
Minimize hopper use
Bucket or Clamshell
Cutter-suction Pipeline
Trailing-suction Hopper
Available Protection Methods
(Implemented 1992)
Environmental Window
Based on Water Temp. & Nesting Timeline
Hatching
Nesting
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Available Protection Methods
(Implemented 1992)
Draghead turtle deflector
Reduce pumps in water column
Available Protection Methods
(Implemented 1992)
Capture and relocate turtles
Effectiveness of Protection Methods
(Annual Turtle Takes)
Sea Turtle Incidental Takes
Projects Monitored
Effectiveness of Protection Methods
(Annual Turtle Takes per Project)
71
Protection Methods
Turtle Takes per Dredge Project
Implemented 1992
-Dredge type
-Dredging window
-Draghead turtle deflector
-Reduce pumps in water column
-Capture and relocate turtles
Newly Available Protection Methods
Implemented 2008 – Dhamra, India
Water-injection to move turtles
Implemented 2008 - USA
Non-capture seabed sweeping
Newly Available Protection Methods
To be implement 2009 - USA
“Turtle friendly” bed-leveler
UNavailable Protection Tools
Acoustic Dispersal Acoustic Detection
or = ?
Olive ridley
Lepidochelys olivacea
Photo by Bill Curtsinger
National Geographic
Dhamra Port,
India
Protection Methods Turtle deflector
- Minimized hopper use
- Reduced pumps in water
- Moved turtles with: Water-injection
- Water-injection
- Capture netting
Capture netting
Dhamra Port,
India
Effectiveness of
Protection Methods
- 3 Hopper dredges working
- 17 Months (Mar 08-Jul 09)
- > 200,000 Olive ridleys nesting nearby
- 8 Documented turtle takes
Global
Dredging
Impacts on
Sea Turtles??
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