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Recovery Strategy for Leatherback Turtles (Dermochelys coriacea) in Pacific Canadian Waters

2. Background

2.1 Current status

Common name: Leatherback turtle

Scientific name: Dermochelys coriacea

Assessment summary: May 2001

COSEWIC status: Endangered

Reason for Designation: The leatherback turtle is undergoing a severe global decline (> 70 % in 15 years). In Canadian waters, incidental capture in fishing gear is a major cause of mortality. A long lifespan, very high rates of egg and hatchling mortality, and a late age of maturity makes this species unusually vulnerable to even small increases in rates of mortality of adults and older juveniles.

Occurrence in Canada: Pacific and Atlantic coasts

Status history: First listed as endangered in 1981

2.2 Taxonomy and description

2.2.1 Species and populations

The leatherback turtle Dermochelys coriacea is one of only two species of sea turtle regularly seen in Canadian waters (the other is the green sea turtle Chelonia mydas).

The leatherback is the only member of the family Dermochelyidae (Bustard 1972). Although Atlantic and Pacific subspecies of leatherback turtles have been described, morphologic distinctions are questionable (Pritchard 1979). More recent analysis of mitochondrial DNA sequence divergence confirms that Atlantic and Pacific populations should be considered as genetically distinct lineages of a single species (Dutton et al. 1999). However, the same study indicates there is much less genetic differentiation among leatherback populations than has been observed among populations of other sea turtles.

Atlantic leatherbacks nest primarily on beaches in Central and South America, the southern Caribbean and in Western Africa and are sighted year-round off Atlantic Canada. For management purposes, the main nesting populations of the Pacific leatherback belong to two genetically distinct populations (Dutton et al. 1999):

  • Eastern Pacific, nesting primarily in Mexico and Costa Rica and including Guatemala, Nicaragua and Panama;
  • Western Pacific, nesting primarily in Papua (formerly Irian Jaya), Malaysia, the Solomon Islands and Papua New Guinea.

Preliminary evidence suggests that leatherback turtles seen foraging in the North Pacific are from the Western Pacific population (Dutton et al. 1999), so it is likely that most leatherbacks in Pacific Canadian waters also belong to the western Pacific population. However, this conclusion is based on small sample size and needs to be confirmed through additional research. In the interim, this Recovery Strategy considers the viability of both Western and Eastern Pacific populations of leatherback turtles.

2.2.2 Appearance

The leatherback is the only sea turtle that lacks a bony shell. It is the largest living reptile and can grow as long as 2 m and weigh as much as 900 kg (Zug and Parham 1996). The adult's carapace is a leathery, oil-saturated connective tissue layer, ridged and tapered at the rear and overlying a mosaic of loosely interlocking dermal bones (Pritchard 1971). The front flippers are proportionately longer than in other sea turtles (half the carapace length and up to 270 cm) and are roughly three times the length of the back flippers (Brown 1976).

The leatherback has no scales or claws and lacks the crushing and chewing plates characteristic of other sea turtles that feed on hard-bodied prey (Pritchard 1971). The jaw of the leatherback has two tooth-like projections on the upper jaw that interlock with a single "tooth" on the lower mandible. The edges of the mandibles are adapted for cutting soft tissue and the long esophagus has backward-pointing spines that help the animal swallow the hydromedusae that are their main diet (Bleakney 1965).

Dorsally, adult leatherbacks are almost completely black with some white spotting, while the ventral surface is usually whitish. Hatchlings are black with lighter undersides, but they also have small bead-like scales that are lost within the first six months of life (Pritchard 1971). Each leatherback turtle has an apparently unique, patterned pink spot on the dorsal surface of its head, directly over the pineal organ, which has been used to identify individuals (McDonald and Dutton 1996).

There are no major differences in the average size of mature males and females. However, the sexes can be differentiated by tail length. The male's tail usually extends beyond the end of the rear flippers while the female's does not (Pritchard 1971).

2.2.3 Physiology

Leatherbacks differ from other turtles in being facultative homeotherms that can maintain a core body temperature higher than the surrounding water temperature (Frair et al.1972). This unusual ability may result from a number of anatomical and physiological adaptations including thermal inertia in retaining the heat generated by constant swimming, large body mass, insulating and brown adipose tissue, and counter-current exchange in flippers (Mrosovsky and Pritchard 1971; Davenport 1998; Eckert 2002b). The leatherback's broad temperature tolerance allows it to range as far north as the Barents Sea and as far south as Chile and New Zealand (Eckert 1993).

Leatherbacks are highly adapted for diving, and adults have been recorded deeper than 1200 m in tropical latitudes (Eckert et al. 1996; Lutcavage et al. 1992). Such deep diving may be for nocturnal foraging on soft-bodied invertebrates that migrate vertically (Eckert et al. 1989), and probably increases their chances of contact with fishing gear.

2.2.4 Swimming and migration

Adult leatherbacks swim continuously at a constant rate of around 0.65 m/sec. They cover roughly the same distance each day, although horizontal distance depends on how much diving they do. Swim speed and distance studies indicate that leatherbacks average 45-65 km per day. Adult leatherbacks do not appear to bask or rest during the day, and continue to move forward even when near the surface. When moving long distances they tend to swim just below the surface (Eckert 2002b).

Leatherbacks appear to migrate to feed, and are locally more abundant where jellyfish, their primary prey, are at higher densities (Grant et al. 1996). After nesting, the turtles appear to migrate from tropical to temperate waters, following thermal fronts and oceanic gyres and seeking areas of high productivity for soft-bodied invertebrates (Lutcavage 1996).

The movements of turtles are studied using a variety of tagging methods. Earlier methods of flipper tagging suffered because of poor tag retention but nonetheless demonstrated prodigious migrations. For example, a leatherback tagged in French Guiana was recorded 128 days later at Fox Harbour, Newfoundland, a straight line distance of more than 5,000 km at 39 km/day (Goff et al. 1994). More recently, radio transmitters whose signal can be received by mobile VHF antenna or satellite have yielded more information, especially when combined with swim speed and dive recording data-loggers (Eckert 1995). Newer tracking methods mean that tagging is no longer restricted to females that can be captured and tagged as they come ashore to nest, and have showed that these turtles make very long annual migrations with a predetermined destination. The longest post-nesting satellite tracks of leatherbacks to date were collected by Eckert (1998), in which two female leatherbacks were followed for over a year after tagging in Trinidad.

Attaching tracking instruments at sea is more difficult than tagging turtles on the nesting beach. However, the technique has the advantage of allowing males, non-nesting females and juvenile leatherbacks to be tracked. The first adult male leatherback turtle was satellite tagged off Cape Breton Island in 1999 (James and Eckert 2002). Satellite tagging efforts continue in Eastern Canadian waters (M. James 2002, pers. comm.) and in the Pacific off Monterey Bay, California (S. Eckert 2002, pers. comm.).

The distribution and habitats of juvenile leatherbacks were poorly known until recently. Eckert (2002a) analyzed the available data on sightings and found that turtles smaller than 100 cm (carapace length) were found only in waters 26°C or warmer, but that larger turtles were found in waters as cold as 12°C. In Canada, however, leatherbacks often occur in northern waters off Newfoundland in temperatures from 0 to 15°C (Goff and Lien 1988). Leatherbacks thus appear to spend the first part of their lives in warm waters, and may venture into cooler climates as they develop thermoregulating capacity. Frair et al. (1972) observed that a captive leatherback in 7.5°C water maintained a body temperature of 25.5°C, suggesting considerable cold water tolerance. Much more research is needed to determine the movements of juvenile leatherback turtles during the years between hatching and their first foraging migrations as adults.

2.3 Distribution

2.3.1 Global

The adult leatherback is not only the most migratory of all sea turtles but also the widest-ranging reptile (Pritchard and Trebbau 1984). It is found in tropical and temperate waters of the Atlantic, Pacific, and Indian Oceans (Gulliksen 1990; Ernst and Barbour 1989). Adults swim more than 15,000 km per year for foraging and, as in the case of the leatherbacks that occur off B.C., can traverse entire oceans (Eckert 2002b). In addition to the major Pacific nesting sites in Mexico, Costa Rica, Papua, Papua New Guinea, Solomon Islands and Malaysia (James 2001), additional nesting occurs throughout Central America and in the western insular areas around the Solomon Islands, Vanuatu, Fiji, as well as in Australia (NMFS and USFWS 1998). See Figure 1.

Figure 1. Pacific Ocean showing place names mentioned in the text.

Figure 1. Pacific Ocean showing place names mentioned in the text.

2.3.2 Pacific Canada

In contrast to Atlantic leatherbacks, which are seen year-round off the east coast of Canada, adult Pacific leatherbacks are most often seen foraging off the coast of B.C. between July and September (Goff and Lien 1988). This timing correlates well with sightings of leatherbacks from California to Washington in the U.S. (Stinson 1984; Starbird et al. 1993). Despite a growing database of sightings, there are few areas where leatherback turtles are routinely observed (Figure 2).

Sightings records are anecdotal and generally made by fishermen, either through accidental capture or as a vessel passes close enough to a turtle near the surface. In recent years, reported sightings by pleasure boaters have become more frequent likely due to an active campaign to increase awareness to report sightings. The leatherback's size, distinctive features and rarity mean that sightings are generally well recalled.

Figure 2. Leatherback turtle sightings in Pacific Canadian waters from 1931 to 2005.

Figure 2. Leatherback turtle sightings in Pacific Canadian waters from 1931 to 2005.

The first leatherback turtle recorded in Pacific Canadian waters was seen in 1931 at Bajo Reef, Nootka Sound, on the west coast of Vancouver Island (MacAskie and Forrester 1962). Other west coast Vancouver Island sightings include waters from Pachena Point to the Scott Islands, including near the town of Bamfield in Barkley Sound and on La Perouse Bank, an important commercialfishing area about 15 km offshore. Offshore sightings range as far as the boundary of the Canadian Exclusive Economic Zone (EEZ).

Leatherback turtle sightings have more frequently been reported in recent years throughout the Queen Charlotte Islands. Multiple observations have been made off the southern Queen Charlotte Islands (Froom 1976), as well as at Langara Island, Rennel Sound and Skidegate Inlet.

Leatherbacks have also been observed in Hecate Strait and the protected waters of Georgia Strait (Gregory and Campbell 1984), including waters off Victoria (L. Spaven 2003, pers. comm.).

2.4 Natural history

2.4.1 Diet

Leatherbacks are pelagic animals that consume a variety of edible (and, unfortunately, inedible) slow-moving objects (Ernst and Barbour 1989). Their diet includes soft-bodied pelagic invertebrates like jellyfish (medusae) and tunicates (Bleakney 1965; Den Hartog 1980; Lutcavage and Lutz 1986; Davenport and Balazs 1991). This diet places them at the top of a marine food chain based on microscopic plankton that are responsible for more than half the primary productivity of pelagic waters (Eckert 2002c).

Adults forage both at the surface and during dives (Eckert et al. 1989). Adults foraging in the North Pacific have been observed feeding on the jellyfish Cyanea and Aurelia (Starbird et al. 1993; Eisenberg and Frazier 1983), the latter massing in large shoals that are ideal for grazing. Leatherbacks also consume other schyphomedusaespecies such as Chrysaora (Den Hartog and Van Nierop 1984) and possibly Phacellophora, two species known to occur in North Pacific waters (Starbird et al. 1993). Because jellyfish are very high in water content and low in nutritional value, leatherbacks must range widely to find areas rich in jellyfish, which are often along coastal upwelling areas and oceanic frontal systems (Lutcavage and Lutz 1986; Shoop and Kenney 1992).

The diet and foraging behaviour of hatchling and juvenile leatherbacks is very poorly known, and the low survival of captive juveniles makes their study difficult. Their rapid growth rate, however, suggests that they feed as voraciously as adults.

2.4.2 Reproduction and early development

The life span of the leatherback turtle is not known, nor is its age at sexual maturity. Estimates based on growth rate (which is expected to be much higher than other marine turtles; Rhodin 1985) and analysis of ocular ossicles (Zug and Parham 1996) range from maturity at 2-3 years (earlier than other sea turtles) to 14 years – a clear indication of the incomplete nature of our understanding of the biology of this species.

It is unclear where mating occurs. For most sea turtle species, mating can occur off the nesting beaches as well as along migration corridors (Meylan et al. 1992). Leatherback mating is probably similar, though scientific observation is rare (Eckert and Eckert 1988; Godfrey and Bareto 1998).

Pacific leatherbacks lay their eggs on sandy tropical beaches with a deep-water approach that reduces the amount of travel on land (Eckert 1987). They prefer beaches with little or no offshore reefs or rock. Nesting is long, laborious and usually nocturnal. Inseminated females return several times to the beach during nesting season, making short feeding excursions in between (the so-called inter-nesting period).

Females excavate a suitable site for egg deposition using their rear flippers (Pritchard 1971). Once the nest is deep enough a clutch of around 100 eggs is deposited, including up to half as many yolkless eggs whose function is unknown. Eggs are buried and the nest compacted followed by the female's return to the ocean. The whole process takes 80 to 120 minutes (Pritchard, 1971). Females average 4-6 clutches per season, at 8-12 day intervals.

The incubation time is 60-65 days (Ernst et al. 1994) and sex determination is temperature-dependent. The temperature producing roughly equal numbers of males and females has been established for some nesting sites used by Atlantic leatherbacks and is within a very narrow range (29.25-29.5°C; Mrosovsky et al. 1984). Temperatures below and above this range produce all-male and all-female clutches respectively (Chan and Liew 1996). Temperature-dependent sex ratio has also been investigated by Binckley et al. (1998). Hatchlings are 5-6 cm long and emerge at night, orienting toward the ocean because of its higher illumination relative to land.

Leatherbacks of the Western Pacific population nest at different times of the year, depending on the beach. Two different beaches in Papua, for example, are active in May-August and November-January (NMFS 2001). Nesting occurs every two to three years and the females return to the same beach every nesting season (NRC 1990; Hughes 1996).

2.5 Nationally Significant Populations

Leatherback turtles frequenting Pacific Canadian waters are considered to be genetically distinct from the turtles occurring in Atlantic Canadian waters. This distinction was first suggested by Pritchard (1979) and confirmed through analysis of sequence divergence in mitochondrial DNA by Dutton et al. (1999). The Pacific stock comprises (at least) two reproductively isolated populations, namely the Western and Eastern Pacific populations described in Section 2.2.1. Each of these may warrant recognition as a nationally significant population, but it is not yet known whether turtles from the Eastern Pacific population frequent Pacific Canadian waters. Both populations are considered in this Recovery Strategy.