Recovery Strategy for Leatherback Turtles (Dermochelys coriacea) in Pacific Canadian Waters
2. Background (cont'd)
- 2.6 Population Status and Trends
- 2.7 General Habitat Requirements
- 2.7.1 Critical Habitat
- 2.8 Ecological role
- 2.9 Social and economic considerations
- 2.10 Biologically limiting factors
- 2.11 Threats
- 2.12 Knowledge gaps
2.6 Population Status and Trends
Population estimates for leatherbacks are based on numbers of nesting females, hence are relative rather than absolute and may be affected by skewed sex ratios. Nevertheless, the trends are clear. When both Pacific and Atlantic stocks are considered, the global number of nesting female leatherbacks as estimated by Prichard (1982) had fallen to 115,000 in 1980. A 1995 study (Spotila et al. 1996) estimated that the global population, using data from 28 known nesting beaches, was approximately 34,500 nesting females. Although both studies employed slightly differing calculations and assumptions in their estimates, this remains an alarming global decline. The data reflects more severe nesting declines in Pacific populations, where some beaches had annual adult mortality as high as 33% (Spotila et al. 2000). Large numbers of turtles were killed in high seas fishing operations (Wetherall 1993; Eckert and Sarti 1997), while egg harvest, the killing of nesting females and nesting beach habitat destruction were also important factors (Chan and Liew 1996). Nesting activity in the Atlantic appears to be more stable, but still has periods of increase and decrease, making the trends more difficult to decipher.
The leatherback turtle is listed by IUCN as Critically Endangered (80% reduction in numbers in ten years or three generations). It is also listed under CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora), an international agreement that ensures trade in wild animal and plant species does not threaten their survival, and in Appendix 1 of the Convention on Migratory Species, which classifies the species as endangered.
The following sections present status and trends for Eastern and Western Pacific populations; the latter may be more directly relevant to Canadian waters.
Eastern Pacific nesting populations
Until recently the largest known population of leatherbacks nested on the Pacific coast of Mexico. These turtles, whose known foraging waters include areas to the south off Peru and Chile, have severely declined in recent years. Mexiquillo Beach, an index beach on the Mexican coast, has seen a decline in nests from 5,000 in the 1980s to less than 100, and this decline is believed representative of the entire Eastern Pacific population (Sarti 2002). The decline is believed due primarily to egg harvest and accidental capture in fisheries.
Western Pacific nesting populations
The Western Pacific population of leatherbacks, which is the presumed source of most of the adults foraging off Pacific Canada. includes populations that nest in Malaysia, Indonesia (Papua), Papua New Guinea and the Solomon Islands, with lesser contributions from beaches in Vanuatu, Fiji, China and Australia. The most important sites are Indonesia and Papua New Guinea. Unfortunately, the population trends are not as well known as for the Eastern Pacific populations, and published reports are few and conflicting.
Terengganu, one of the east coast states of peninsular Malaysia, was once a major leatherback nesting area but has declined drastically. The Rantau Abang population has decreased from about 10,000 nests in the 1950s to fewer than 20 in recent years (Liew 2002). The declines appear to have occurred in two waves, one coinciding with rapid development of the fishing industry in Terengganu in the mid-1970s, and the second with introduction of the Japanese high seas squid driftnet fishery in the North Pacific in 1978. The nesting beaches were also subject to severe overharvest of eggs (often approaching 100%) since the 1940s. Since these events, the decline has averaged 16% annually.
The other major nesting sites in the Western Pacific are on beaches in Papua (formerly Irian Jaya), a part of the Indonesian archipelago that shares a land mass with New Guinea. In the 1980s, smaller areas of nesting in Indonesia occurred on western Sumatra and southeastern Java (Suarez and Starbird 1996). Most of the turtles in Papua nest at Jambursba Medi Beach (Hitipeuw 2002; Putrawidjaja 2000), where over 80% of the nests are affected by poaching, predation by wild pigs, and erosion (Hitipeuw 2002; Suarez and Starbird 1996). The number of leatherback clutches deposited at Jambursba Medi beach was reported to be stable between 1993 and 1996 (Hitipeuw 2002; Dermawan 2002). however, the long term trends are unclear and possibly declining (Hitipeuw 2002).
2.6.2 Pacific Canada
Information on sightings in Pacific Canadian waters is extremely limited and it is not currently possible to draw any conclusions on population trends.
2.7 General Habitat Requirements
Habitats that need to be considered include nesting, breeding and foraging habitats. Very little is known about distribution patterns in foraging habitats and migration routes and about the years between hatching and sexual maturity.
In nesting habitat, females require a sandy beach with a deep ocean approach and few obstructions like rock or coral (Pritchard 1971; Ernst and Barbour 1989). Habitat requirements for hatchlings and juveniles appear to be almost exclusively tropical until the turtles exceed 100 cm in carapace length (Eckert 2002a). Large juveniles and subadults probably share habitats with adult leatherbacks.
Adults frequent cooler waters, including the continental shelves off Canada and the northwestern United States (Bowlby et al. 1994; Stinson 1984). They follow oceanic frontal systems where productivity is high and results in high concentrations of prey (Lutcavage 1996). The Pacific coast off B.C. provides foraging habitat; however, no studies have verified specific foraging habitat areas important to leatherback turtles. Therefore it is impossible to identify either the habitat currently occupied by leatherback turtles in Pacific Canadian waters, or the amount of critical habitat needed to recover and support a viable population.
2.7.1 Critical Habitat
SARA, under section 2, defines critical habitat as the "habitat necessary for the survival and recovery of a listed wildlife species and that is identified as the species' critical habitat in the recovery strategy or in an action plan for the species." It is difficult to define critical habitat for turtles, as each life stage has different requirements distributed over large ocean basins. Although the knowledge base to help determine critical habitat is increasing with new research projects, at this time it is not possible to identify critical habitat for this species. As set out in SARA, if information is inadequate to identify critical habitat within the recovery strategy, a schedule of studies must be prepared. This schedule, once implemented, will yield new information that will help to identify the species' critical habitat in the future.
The schedule of studies, which is essentially a list of habitat research projects, recommended to identify critical habitat important to the recovery of leatherbacks in Pacific Canadian waters can be found in Appendix 1. Upon completion of these projects, it is hoped that the results will allow Fisheries and Oceans Canada to be able to identify critical habitat for this species in an action plan.
2.8 Ecological role
Adult leatherbacks feed voraciously on jellyfish and other soft-bodied pelagic invertebrates that consume large quantities of zooplankton and fish larvae. They thus occupy an important position in the marine food chain. It may be assumed that leatherbacks play an important ecosystem role, helping maintain the balance between the numbers of their prey and the organisms that feed on that prey. They are also important components of terrestrial ecosystems in providing nutrients through unhatched and broken eggs and eggshells, and the eggs themselves are food for terrestrial animals who carry the nutrients inland (Eckert 2002c).
2.9 Social and economic considerations
There is very limited directed fishery on adult leatherbacks anywhere in the world as the flesh is not generally considered palatable. However, the people of the Kai Kecil Islands in Indonesia have a ritual hunt for adult leatherbacks (Suarez and Starbird 1996). Many people that live near nesting areas eat and sell leatherback eggs.
In Pacific Canadian waters, leatherbacks probably do not occur close enough to land or in sufficient numbers to be of any importance to tourism. Although coastal First Nations are familiar with the animal, very limited anecdotal information from Clayoquot Sound, centrally located in the Pacific Canadian foraging range, suggests no evidence of any special use or significance (A. Webster 2002, pers. comm.). If an ethnographic study of the significance of leatherbacks to coastal First Nations were to be done it would need to include groups throughout the known range of sightings.
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 off B.C., remains to be determined. One of the main thrusts of the Recovery Strategy is to evaluate the threats described below.
Threats in Pacific Canadian waters occur only in the foraging environment and are hard to quantify because they may 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 coastal waters off B.C., 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 waters off B.C.. However, the extent of accidental capture-related threats from fisheries in these waters is likely low due to the rarity of the species occurrence but is presently impossible to quantify because sightings are limited.
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 percent mortality is not always reported (Balazs and Pooley 1994) and may be delayed after turtles are released (Witzell 1984).
Global 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.
Ingestion of debris
In coastal waters off B.C., 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 Pacific Canada 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 off the coast of B.C., including near-shore waters. In 1999 a leatherback was hit by a sport fishing vessel (L. Spaven 2003, pers. comm.). No damage appears to have resulted to either party; nevertheless, the incident suggests potential for ship strikes in these 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 parasite
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, pollution, and noise.
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 in leatherback turtles (Davenport et al. 1990).
Salmon farms and shellfish aquaculture are concentrated in the inside passage between Vancouver Island and the mainland. Environmental threats posed by salmon farms include noise from predator-deterrent devices, fecal pollution, entanglement in net pens and anchoring systems and the possibility of parasite transmission. However, the potential for leatherbacks to interact with aquaculture 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 waters off the coast of B.C. nests in the tropical Western Pacific Ocean. However, it is possible that leatherback turtles from the Eastern Pacific population also visit waters off B.C.
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 egg
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 parasitis
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 nests.
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 waters off the coast of B.C. are few and anecdotal. Those that exist have needed 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 Pacific 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 B.C. 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
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