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Recovery Strategy for Leatherback Turtles
2.10 Biologically limiting factors
Identifying biological factors that may limit recovery is difficult for a species about which so little basic biological information is known. Because the leatherback’s life span and age at maturity are unknown, it is difficult to assign any special risk to either characteristic. If, as some researchers believe, the leatherback is long-lived and slow to mature it is clearly at greater risk than rapidly maturing species. The time between nesting periods may also work against recovery, since these turtles appear to nest only every 2 to 3 years.
High risk behaviours other than reproductive behaviour include the leatherback’s preference for long distance swimming just under the surface (risk of collision); its proclivity for ingesting floating objects; its preference for sandy beaches that are also attractive to humans for development; and the hatchlings’ orientation to light, which can lead them away from the ocean rather than toward it.
The list of threats to leatherback turtles is long and reflects their unique behaviours and wide geographic distribution. While many of the threats are not present along the Pacific coast of Canada, the fact that leatherback recovery will only occur as the result of a concerted international effort means that a Canadian recovery plan cannot ignore threats that occur outside Canadian waters. In the discussion below, threats are organized according to where they occur (foraging environment vs nesting environment). Threats in the foraging environment include those that are well known and those whose importance, especially in B.C., remains to be determined. One of the main thrusts of the Recovery Strategy is to evaluate the threats described below.
Threats in B.C. occur only in the foraging environment and are hard to quantify because they occur over a large area and the number of animals is low. Both factors make observation and recording much more difficult than at a nesting beach. Nevertheless, because of the precarious status of the Pacific populations of leatherbacks, the loss of even a few mature animals anywhere in the world, including B.C. coastal waters, may be significant to the viability of the Pacific stock. Adults foraging in Canadian waters are the largest, most cold-tolerant and most fecund individuals, and thus are more significant to the viability of the species than their numbers alone would suggest.
2.11.1 Threats in the foraging environment
Accidental capture and entanglement
Leatherback turtles are caught accidentally in nets and on lines, especially in fisheries in pelagic and coastal foraging areas and in migratory corridors. Leatherbacks are especially vulnerable to entanglement in fishing gear because of their massive front flippers (James 2001) and are vulnerable not only to gear in use (especially un-monitored gear), but also to abandoned gear. Entangled turtles will drown if unable to free themselves, but may also lose limbs or become more vulnerable to predation. Turtles that break free may still be encumbered by trailing gear (NMFS 2001).
The potential for accidental capture and entanglement in Canadian Pacific waters is currently unknown due to the limited amount of sightings that occur in this region. Crab and other pot fisheries may pose the threat of entanglement in vertical lines, and both large and small mesh gillnets can trap the animal. The main period of fisheries interception is probably between July and September, when the hake mid-water trawl and salmon gillnet and troll fisheries coincide with leatherback appearance in B.C. waters. However, the extent of accidental capture-related threats from fisheries in B.C. waters is likely low due to the rarity of the species occurrence but is presently impossible to quantify because of the limited sightings data.
Information from other regions in the Pacific indicate that many types of fisheries pose threats, with pelagic (floating) longline, gillnet and high seas driftnet fisheries prominent (now prohibited, some driftnet fisheries continue illegally). Leatherbacks are intercepted by pelagic longline gear (McCracken 2000) and may be attracted to bait or simply snagged. There are large pelagic longline fisheries operated by many nations on both sides of the Pacific and in the South China Sea (although not presently in B.C.). Pelagic longlining for swordfish, shark and tuna results in significant bycatch of adult leatherbacks, although per cent mortality is not always reported (Balazs and Pooley 1994) and may be delayed after turtles are released (Witzell 1984).
Reduction of longline bycatch is a high priority and new approaches are being rapidly developed. The risk to leatherbacks presently appears to be highest where longlines are set at night in shallow water and lights are used to attract the target species, most commonly swordfish and sharks. Although there is some interest in developing a pelagic longline tuna fishery off B.C, the Recovery Team believes such a fishery would present a threat to leatherbacks.
The U.S. trawl fishery for shrimp also creates a significant turtle bycatch. Turtle Excluder Devices (TEDs) can reduce the number of turtles caught in shrimp trawl nets by giving them an escape route (US Environmental Protection Agency 1999), and TED regulations were amended in 2003 to increase the size of the escape opening, a change that will benefit leatherbacks (www.mslabs.noaa.gov/teds.html).
Ingestion of debris
In B.C. coastal waters, debris arises from many sources, including coastal development and vessel traffic. Leatherbacks will eat inedible objects such as plastic bags, balloons, and tar balls that may resemble jellyfish, their intended prey (Mrosovsky 1981). They will also eat fishing nets (Starbird 2000). The effects of plastic bag ingestion on leatherback physiology and behaviour, including impaction and death, are reviewed by Fritts (1982).
Collisions with boats
Turtles can be injured or killed if struck by boats and propellers. Leatherbacks may be particularly at risk because of their habit of swimming just beneath the surface. Perhaps the largest concern in B.C. arises from transiting vessels. It is not known whether offshore collisions with large ships occur. However, given the slow swimming of leatherbacks and the often high speeds of vessels, these types of impact could cause mortality.
Leatherbacks have been sighted in several popular fishing, transportation and recreational boating areas in B.C., including near-shore waters. In 1999 a leatherback was hit by a sport fishing vessel (Lisa Fairley 2003, pers. comm.). No damage appears to have resulted to either party; nevertheless, the incident suggests significant potential for ship strikes in B.C. waters, especially during the busy summer months and fishing seasons.
In addition to the threats noted above, a number of additional potential threats exist. Severity of the following threats can only be assigned after further research. They include:
Diseases and parasites
Little is known about diseases and parasites in leatherbacks, including in Canadian waters. Fibropapillomatosis is a neoplastic disease that primarily affects green turtles. The etiologic agent has not been isolated or characterized. Fibropapilloma tumours have recently been observed in leatherbacks in Mexico (Huerta et al. 2002; Murakawa and Balazs 2002).
Sharks and killer whales have been reported to attack adult leatherbacks (Sarti et al. 1994; Caldwell and Caldwell 1969).
Oil exploration and extraction
Oil extraction from the seabed carries risks of spills, blowouts, and increased marine traffic. Oil exploration may also pose indirect threats to foraging habitat, including the effects of drilling, anchoring, explosives and pollution.
Leatherbacks visiting Pacific Canadian waters are exposed to the same pollutants as are other forms of marine life. In B.C. these include sewage and agricultural and industrial chemicals. Bioconcentration of chemical pollutants in the prey of leatherbacks has not been studied and their impact is not known. Accumulation of heavy metals and PCBs has been demonstrated (Davenport et al. 1990).
Salmon farms are concentrated in the inside passage between Vancouver Island and the mainland. Environmental threats posed by salmon farms include noise from predator-scaring devices, fecal pollution, anchoring systems and the possibility of parasite transmission. However, the potential for leatherbacks to interact with salmon farming operations in coastal B.C. cannot be estimated without a more complete record of sightings.
2.11.2 Threats in the nesting environment
Threats in the nesting environment are relevant to international projects and conventions involving Canada. Despite their remoteness from Canada, threats in the nesting environment may outweigh those in foraging areas, and thus may be critical to any Canadian collaborative actions. The population of leatherback turtles most likely to frequent B.C. waters nests in the tropical Western Pacific Ocean. However, it is possible that leatherback turtles from the Eastern Pacific population also visit B.C. waters.
Fisheries on adults and juveniles
Adult leatherbacks nesting in Malaysia and Indonesia are subject to incidental take in various fisheries throughout their habitat, and possibly the directed take by the villagers of the Kai Kecil Islands who have traditionally hunted leatherbacks for food and ritual purposes (Suarez and Starbird 1996). However, there is limited harvest of adult and juvenile turtles and the extent to which populations are affected is unknown.
Harvest of eggs
The eggs of leatherback turtles, like those of other sea turtles, are aggressively harvested for subsistence and sale. Continued harvesting ensures reduced recruitment. In Malaysia, for example, decades of excessive egg collection have decimated turtle populations, and their collection in Terengganu, the most productive state, is now illegal (Liew 2002). Egg harvest is controllable through social programs and beach protection as practiced, for example, in Mexico (Sarti 2002).
Nest predation and parasitism
Many natural predators, such as rats, mongoose, birds, monitor lizards, snakes, crabs, and other invertebrates eat turtle eggs. Domesticated species such as cats, dogs, and pigs also pose a threat. Nest destruction by feral pigs is one of the biggest problems for Western Pacific leatherback populations, especially in Papua (NMFS 2000). Nesting beach parasite loads (i.e., insects such as fly larvae and crickets) are another natural threat.
Increased human presence
Human activities on nesting beaches can disturb nesting females and their eggs. Females may abort nesting attempts, shift nesting beaches, delay egg-laying and select poor sites. Compaction of sand from people walking over nests can slow hatchling emergence.
Light sources such as flashlights and campfires can disorient hatchlings and females, making it more difficult for them to find their way to the sea. Vehicles driving on the beaches compact sand and nests, unearth nests, and create ruts that hatchlings can get trapped in on their seaward migration.
A variety of activities result in elimination or degradation of nesting habitat. They include:
§ Construction and mining: Buildings, piers, jetties, etc. are obstacles for turtles and can increase natural erosion. Sand and coral rubble removal and other beach mining severely affect a nesting beach.
§ Beach armouring: sea walls, rock revetments, riprap, sandbags, groins, and jetties affect nesting by preventing females from reaching good nesting grounds, and can trap or delay hatchlings and females on the journey back to sea, increasing exposure to predators. Armouring may also increase beach erosion.
§ Beach nourishment: Attempts to replace sand lost to erosion can cause problems for leatherback nesting. Nests may become too deeply buried. New sand may be unsuitable for nesting. Heavy machinery used to clean and rake beaches can destroy nests. The machinery used to haul and distribute sand can compact the beach, destroy nests and cause difficulties in digging new ones.
Hatchlings and adults, once on land, rely on illumination to orient toward the sea. Land-based lights from buildings, streets and vehicles can cause turtles to migrate inland rather than back to the sea. Whitherington (1992) found that white mercury vapour (MV) lights and other broad-spectrum lights could disrupt nesting of loggerhead and green turtles and recommended yellow, low-pressure sodium vapour (LPS) lights as an alternative. These same recommendations may apply to leatherbacks.
Introduced plants can displace natural vegetation and proliferate on nesting beaches. Increased shade from introduced plants can result in cooler temperatures within nests and may alter sex ratios of hatchlings (see Section 2.4.2). Roots may entangle eggs and hatchlings. Nesting females can also become tangled in vegetation, slowing or preventing their return to the sea.
Contamination and pollution
Beaches tend to concentrate some of the same kinds of debris and pollution as are hazardous at sea. Examples include plastics, abandoned netting, and spilled oil.
Sharma (2000) provides a recent discussion of destruction of nesting habitat in peninsular Malaysia including the once-significant rookery at Rantau Abang.
2.12 Knowledge gaps
Within the leatherback’s Pacific Canadian range, the main knowledge gaps concern the animal’s occurrence, distribution, behaviour and vulnerability to specific threats. One important outcome of the research needed to fill these gaps will be predictive models that will help set and prioritize management goals (Chaloupka 2003, pers. comm.).
2.12.1 Survey requirements
Reports of leatherbacks in B.C. waters are few and anecdotal. Those that exist need to be collated and analyzed, and new reports need to be systematically gathered (see Section 3, Recovery). Collaboration with the international research community is needed to confirm or disprove the hypothesis that most of the leatherbacks off B.C. originate in Western Pacific rookeries. One outcome of such research may be predictive models of leatherback occurrence. Questions related to abundance and migration include:
· Where, when and for how long do leatherbacks occur in Pacific Canada?
· How many leatherbacks utilize Pacific Canadian waters and what proportion of the total population does this represent?
· What migration corridors do the turtles use to enter and leave Pacific Canada?
· Do the leatherbacks in Pacific Canada contribute significantly to the viability of the populations to which they belong?
2.12.2 Biological/ecological research requirements
Inferences about mortality rate, the relative importance of nesting versus marine hazards, and predictions about population trends depend on assumptions about the turtle's age distribution and life span. Inferences on something as basic as life span are currently hampered by a lack of data. Gaps in our knowledge of population biology include:
· life expectancy;
· lifetime reproductive potential (age of first nesting, frequency of nesting, fecundity and egg survival);
· population viability assessment: How many individuals can a population lose and still be expected to recover?
To determine the critical habitat of the leatherback in Canadian waters we must investigate:
· location of foraging activity;
· metabolic rate and food requirements;
· relationship with principal prey species - distribution, species eaten and their caloric value;
· water quality in foraging areas.
2.12.3 Threat clarification research requirements
Systematic collection and analysis of sightings information will help define critical leatherback habitat, clarify threats to leatherbacks in Canadian waters, and contribute to overall understanding of the life cycle of this population. Currently there is little information about occurrence or incidental capture of sea turtles off the coast of British Columbia. Studies are needed to determine:
· number and kind of interactions with fisheries or maritime activities in Pacific Canada;
· mortality rate from interactions with fisheries or marine activities;
· potential for boat collisions and damaged caused;
· potential impact of oil and gas pollution (spills, leakage, etc);
· threats from disease (information from necropsies);
· potential for interaction with aquaculture operations.
2.12.4 Critical Habitat
See General Habitat Section 2.7 and Appendix I.
SARArequires that the recovery strategy identify “…threats to the survival of the species that is consistent with information provided by COSEWIC.” [SARA s.41(1)(b)].
SARA requires that the recovery strategy identify “a statement about whether additional information is required about the species” [SARA s.41(1)(f)’].
- Date Modified: