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Recovery Strategy for the Leatherback Turtle

2.6 Population Size and Trends

2.6.1 Global Population

 As above, the leatherback turtle is difficult to census in the marine portion of its life cycle, as it is largely pelagic.  Therefore, current population estimates are based on surveys of adult females encountered on monitored nesting beaches.  Pritchard (1982) estimated that the overall world population was approximately 115,000 nesting females in 1980.  In 1995, a study incorporating information from 28 nesting beaches throughout the world yielded a revised estimate of approximately 34,500 females; the lower limit was 26,200 and the upper limit was 42,900 (Spotila et al., 1996).

These figures reflect dramatic declines at several nesting locales, particularly in the Pacific (Chan & Liew, 1996; Steyermark et al., 1996; Eckert & Sarti, 1997) where recent trends suggest that this population is facing imminent extinction (Spotila et al., 2000).  For example, there were 3103 leatherbacks nesting at Terengganu, Malaysia in 1968, 200 turtles in 1980, and only 2 in 1994 (Chan & Liew, 1996).  Similar declines are occurring in Playa Grande, Costa Rica, where annual mortality of nesting females is over 30% (Spotila et al., 2000).

Although some nesting populations (e.g. St. Thomas, etc.) have been extirpated, the status of existing nesting population in the eastern Atlantic and in the Caribbean appears to be stable.  Data collected in southeast Florida indicate an increasing in nesting, although it is important to note that there was an increase in survey effort (rather than area).

The largest leatherback rookery in the western Atlantic remains along the northern coast of South America in French Guiana and Suriname, and the nesting population in the trans-boundary region has been declining since 1992 (Chevalier & Girondot, 1998).  Recent information suggests that western Atlantic populations declined from 18,800 nesting females in 1996 (Spotila et al., 1996) to 15,000 nesting females by 2000 (Spotila, pers. comm.).

While leatherback turtles may have shifted their nesting from French Guiana to Suriname due to beach erosion, it appears that the overall area trend of nests has been negative since 1987 (NMFS SEFSC 2001).  Without information to determine whether turtles are nesting elsewhere, it can be assumed that that the western Atlantic portion of the population is being subjected to mortality beyond sustainable levels.

A number of studies have used aerial and shipboard surveys to estimate the seasonal occurrence of leatherbacks in waters off the continental United States (e.g., Hoffman & Fritts, 1982; Shoop & Kenny, 1992; Epperly et al., 1995).  Shoop and Kenney (1992) found (after three survey years) that an average of 6.85 turtles are located in every 1000 km from near Nova Scotia to Cape Hatteras, North Carolina.  The mean sighting latitude for leatherbacks was 40° 05’N and the mean sea temperature was 20.4 ° C. Total study area population during the summer was estimated to be 100-900 leatherbacks; this is a minimum surface estimate.  Similar abundance estimates are not yet available for Canadian waters, as the limited linear aerial or transect-based shipboard surveys undertaken have been focused on cetaceans.  Data have been gathered opportunistically from volunteer commercial fishers, who record sightings of leatherbacks while fishing or travelling to and from fishing grounds.  Sightings and entanglement data have also been collected through phone and mail surveys, and through the entanglement and stranding networks.  

2.6.2 Population in Atlantic Canada

 Existing data on leatherback distribution reveal relatively large numbers of sightings in several popular fishing areas along the Scotian Shelf (James, 2000; James et al., 2005a & 2005b) and along the southeast coast of Newfoundland (DFO, 2005), however these sightings are biased toward areas where fishing activity occurs.  Therefore, sightings and incidental captures of leatherbacks are most likely to occur in the heavily fished areas off the Scotian Shelf and the Newfoundland south coast.  General baseline data about the abundance and distribution of the species throughout the region are lacking.  With these limitations, it is not possible to precisely assess abundance in eastern Canadian waters.

In 1998 and 1999, 300 leatherback turtle sightings were documented by a fisher-scientist collaborative venture entitled the Nova Scotia Leatherback Turtle Working Group (NSLTWG).  The NSLTWG group was initiated in Atlantic Canada to investigate the distribution of leatherback turtles in the northwest Atlantic (James, 2000).  These numbers suggest that summer leatherback densities in eastern Canada may be higher than the estimate of 100 to 900 leatherbacks per summer reported by Shoop & Kenney (1992) for a much larger study area along the coast of the northeastern United States.

Moreover, abundance estimates based on aerial or shipboard surveys must be considered conservative, as these only include observations of turtles at the surface; they do not account for those turtles present at various depths (Shoop & Kenney, 1992).  Given the lack of offshore aerial survey data and fishery bycatch data on leatherbacks in Atlantic Canada, leatherback population size and trends in this area have yet to be determined.