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Recovery Strategy for the Leatherback Turtle

2.8 Threats [1]

Researchers have observed a decline of over 70% in the leatherback turtle population on its nesting beaches.  While there are known (and probably unknown) threats to leatherbacks in migratory and feeding habitat, these are not well understood.  Threats occur both in nesting habitat and at sea.  Because this strategy focuses on those known and potential threats that occur in Atlantic Canadian waters, it more specifically addresses threats that occur at sea. 

2.8.1 Threats in the Marine Environment

Entanglement in fishing gear

Leatherback turtles are incidentally captured in nets and entangled in lines in fisheries operating in pelagic and coastal foraging areas and in migratory corridors.  Of all the Atlantic sea turtle species, leatherbacks seem to be the most vulnerable to entanglement in fishing gear such as pelagic longlines, lines associated with fixed pot gear and gillnets, buoy anchor lines, and other ropes and cables (e.g., Chan et al., 1988; Goff & Lien, 1988; NMFS, 1992; Cheng & Chen, 1997; Godley et al., 1998).

Interactions between leatherback turtles and fishing gear are expected to differ depending on gear type.  Although little observer data exist to document leatherback interactions with different gear types in Atlantic Canadian waters, O’Boyle (2001) identifies the gears with high potential for interactions (Table 1).

Table 1.  Summary of the gear types with high potential for sea turtle interactions

 

GearTargeted SpeciesArea/SeasonComment

Longline

Groundfish All areas and seasons Hooks set close to bottom but entanglement a concern
  Pelagic Atlantic Coast Observations available
Gillnet Herring Newfoundland Bait fishery; not regularly tended
  Groundfish 5Z Cod fishery
  Mackerel 4X Bait fishery all year
Trap Lobster 4VWX5Z Offshore Turtles in this area
  Groundfish/Pelagic All areas and seasons Entanglement a concern
Pot Snow Crab 3L (April-September) Entanglement occurred in 2004
  Snow Crab 4VW (April- September) Entanglement a concern

Table 1.  Summary of the gear types with high potential for sea turtle interactions.  In many cases there is little or no observer data to document the incidence of sea turtle interactions with these gear types (O’Boyle 2001).

Incidental interaction of marine turtles in pelagic longlines is evident from observer data for the Canadian pelagic longline fisheries.  These fisheries have implemented the broadest observer coverage to date among Atlantic fisheries that have been identified as posing a risk of interaction with leatherback sea turtles.

Turtle interactions do not appear to occur in Canadian pelagic longline fisheries targeting shark (Javitech 2003C), but are well documented in longline fisheries targeting swordfish and tunas (28 individuals – swordfish 2001; 33 individuals – swordfish 2002; 4 individuals – offshore tuna 2002).  During a two-year programme of enhanced observer coverage levels of 20%, live release was observed in all cases for leatherback turtles in the swordfish fisheries.  Similar results were observed in the offshore tuna fishery where observer coverage levels were 100% in 2002.

From observations in the swordfish fishery, hooks and gangion line remained attached to turtles in 48.8% of all cases in 2001 and 74.5% of all cases in 2002.  Just hooks remained attached in 5.6% of all cases in 2001 and 24.1% of all cases in 2002.  All hooks and gangion line were removed from 33.3% of all cases in 2001 and 1.4% of all cases in 2002.  In all of the above cases, post-release mortality is not known (Javitech 2002, 2003A and 2003B).

Unfortunately, no observer information exists regarding interactions between the leatherback turtle and fixed gear.  However, valuable information is available through strandings.  The Nova Scotia Leatherback Turtle Working Group  reported 87 records of stranded leatherbacks from 1995-2002 – turtles entangled in fixed fishing gear and turtles found floating dead in shelf waters off Atlantic Canada.

Of the 87 records, 74% provided direct or indirect evidence of leatherbacks interacting with fixed fishing gear and 62% were associated with specific types of gears.  Snow crab, rock crab, inshore lobster, offshore lobster and whelk fisheries were associated with 29% of the records, 22% of the records involved mooring or buoy lines associated with bottom gill nets, bait nets and pound nets of other fish traps.  Three percent were associated with vertical lines in the groundfish longline gear.

Leatherback turtles are also entangled in U.S. Atlantic waters.  For example, 92 leatherbacks were entangled in fixed pot gear from New York through Maine for the period 1990-2000 (Dwyer et al., 2002).  Additional leatherbacks are stranded with line wraps or evidence of prior entanglement (Dwyer et al, 2002).  Further, leatherback interactions have been observed in the shrimp trawl and other bottom trawl fisheries.  Historically, interactions were observed in the drift gillnet fishery for swordfish.  However, in January 1999, the U.S. National Marine Fisheries Service (NMFS) issued a Final Rule to prohibit the use of driftnets (i.e. permanent closure) in the North Atlantic swordfish fishery (50 CFR Part 630).

Although NMFS promulgated regulations requiring the use of turtle excluder devices (TEDs) in shrimp trawl fisheries in 1990, Epperly et al. (2002) in a review of sea turtle stranding data, found that the TED openings were much too small to exclude leatherbacks and larger loggerhead and green turtles.  In 2003 NMFS amended the regulations to require larger TED openings in U.S. Atlantic and Gulf of Mexico waters.  In addition to the TED regulations, the U.S. also established a leatherback turtle Conservation Zone in 1995 to restrict trawl activities on the Atlantic coast during periods when leatherbacks are concentrated.

The susceptibility of leatherbacks to entanglements may result from their large body size, long pectoral flippers and soft shell.  Entanglement of leatherbacks in lines or cable can result in serious injuries, infection, necrosis or death.  These entangled turtles are generally limited in their ability to feed, dive, breathe or perform any other behaviour essential to survival (Balazs, 1985).

 Collisions

While no incidences of collisions with boats are documented in Atlantic Canada, they have been known to occur in some areas of the U.S. and may have an impact on the leatherback turtle population that also uses Canadian waters.  In areas where recreational boating, commercial fishing and ship traffic are concentrated, propeller and collision-related injuries may represent a source of mortality (NMFS, 1992).  However, in situations where there is evidence of a collision, it is difficult to infer whether the collision itself led to the death of the turtle in question, or if the turtle was hit after it died of other causes.  Leatherback turtles are known to bask at the surface for extended periods of time when foraging in temperate waters and, therefore, may be vulnerable to collisions with marine traffic.

Marine Pollution

The effect of marine pollution on sea turtles is not well quantified, and therefore the magnitude of pollution-related mortality is unknown.  Leatherback sea turtles may be more susceptible to marine debris ingestion than other turtle species due to their pelagic existence and the tendency of floating debris to concentrate in convergence zones that adults and juveniles use for feeding areas and migration (Lutcavage et al., 1997; Shoop & Kenney 1992).

Leatherbacks are known to ingest a variety of anthropogenic marine debris, including plastic bags, balloons, plastic and Styrofoam pieces, tar balls, plastic sheeting, and fishing gear (e.g., Sadove, 1980; Hartog &Van Nierop, 1984; Lucas, 1992; Starbird, 2000).  Ingestion of such materials may interfere with metabolism or gut function and lead to blockages in the digestive tract, which could result in starvation or in the absorption of toxic byproducts (Plotkin & Amos, 1989).

Leatherbacks may serve as an indicator of the degree of contamination of the oceanic food web by bio-accumulating substances such as heavy metals and polychlorinated biphenyls (PCBs) found in plankton-feeding jellyfish (Davenport & Wrench 1990).  Metal and PCB levels in the leatherback are expected to represent a biomagnification of concentrations found in their prey; however, to date, tissue samples derived from leatherbacks in European waters have not revealed evidence of significant chemical contamination (Davenport et al., 1990; Godley et al., 1998).

Acoustic disturbances

Little is known about the hearing ability of the leatherback turtle and its response to acoustic disturbance.  Studies involving adult green, loggerhead and Kemp’s ridley turtles suggest that sea turtles detect sounds in the low frequency sound range, with the greatest hearing sensitivity between 250-700 Hz (Ridgway et al., 1969; Lenhardt et al., 1983; Bartol et al., 1999).

The effects of exposure to increased noise, based largely on studies involving marine mammals, may include habituation, behavioural disturbance (including displacement), temporary or permanent hearing impairment, acoustic masking, and mortality (Richardson el al., 1995).  Studies on sea turtles have shown that certain levels of exposure to low frequency sound may cause displacement from the area near the sound source and increased surfacing behaviour (O’Hara & Wilcox, 1990; Lenhardt et al., 1983).  This raised the concern that turtles may be displaced from preferred foraging areas (e.g., O’Hara & Wilcox, 1990; Moein et al., 1994).

There are a range of sources of anthropogenic noise in the marine waters of Atlantic Canada that produce underwater sounds within the frequency range detectable by sea turtles.  These include oil and gas exploration and development, shipping, fishing, military activity, underwater detonations, and shore based activities (Davis et al., 1998; Greene & Moore, 1995; Lawson et al., 2000).  Concerning the exposure to seismic airguns used in exploration, studies to date describe behavioural responses such as; increased swimming speed, increased activity, change in swimming direction and avoidance (DFO, 2004).  Startle responses and erratic swimming behaviour was observed by McCauley et al. (2000).  A study by Moein et al., (1994), noted a temporary reduction in hearing capability and temporarily increased physiological parameters (e.g., glucose, white blood cells and creatinine phosphokinase) which is suggestive of damaged tissues or altered physiology.  .Overall, based on the available information, it is considered unlikely that sea turtles are more sensitive to seismic operations associated with oil and gas exploration than cetaceans or some fish (DFO, 2004). Seismic operators currently use mitigation techniques, such as “ramp-up” procedures to encourage species such as marine mammals to move away from survey areas, and use “shut down” procedures when a species is identified as too close to survey.  However, mitigation focused on detection are expected to be less effective for turtles given that they are more difficult to identify both visually and acoustically.  Noise from offshore hydrocarbon production platforms and exploration drilling generally tend to be of low frequency (<500 Hz) (Richardson et al., 1995); however there are no published studies on the potential impacts of production or drilling operations on sea turtles.  Sea turtles may react to noise from vessel traffic and helicopter overflights with a startle response (NRC, 1990; NOAA, 2002).  Although it is assumed that turtles close to the surface can hear aircraft noise and may subsequently change their behaviour, there are no published studies to confirm this (NOAA, 2002). 



[1] SARA requires that the recovery strategy identify “threats to the survival of the species that is consistent with information provided by COSEWIC.” [SARA, s.41(b)].