Leatherback turtle (Dermochelys coriacea) COSEWIC assessment and status report: chapter 7

POPULATION SIZES AND TRENDS

As this is a largely pelagic species and, as such, is difficult to census in its marine environment, population estimates are currently based on the abundance of adult females encountered on nesting beaches.  It is generally believed that all major nesting sites for this species have been identified, and nesting activity has been intensively monitored at most of these sites for several years (Spotila et al., 1996).

Pritchard (1982) estimated the overall world population to be about 115,000 nesting females in 1980.   In 1995, a revised estimate incorporating information from 28 nesting beaches throughout the world yielded approximately 34,500 females, with a lower limit of about 26,200 and an upper limit of about 42,900 (Spotila et al., 1996).  These figures reflect dramatic declines at several nesting locales, particularly in the Pacific (e.g., Chan & Liew, 1996; Steyermark et al., 1996; Eckert & Sarti, 1997); 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 at other rookeries, including Playa Grande, Costa Rica, where annual mortality for nesting females is over 30% (Spotila et al., 2000).  A recent evaluation of trends at these and other nesting beaches suggests that the Pacific population of leatherbacks is facing imminent extinction (Spotila et al., 2000).

Nesting activity in the Atlantic has been more stable, although it can fluctuate considerably from year to year; as a result trends can be difficult to discern.  For example, the annual number of nests deposited in French Guiana has fluctuated between 10000 and 50000+ (females lay an average of 6 clutches per season) for the period 1978-1995 (Girondot & Fretey, 1996).  Leatherbacks do not nest annually; inter-nesting intervals are 2-3 years.  This can account for some of the annual variation in nesting population size.

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 & Kenney, 1992; Epperly et al., 1995).  Shoop and Kenney (1992) calculated a mean summer leatherback density of 18.3 turtles/1000km after 3 years of surveying continental shelf waters from the Gulf of Maine to North Carolina.  This was translated into abundance estimates of between 100 and 900 leatherbacks in the study area each summer.  These overall ranges of abundance do not provide statistical confidence intervals, but are simply summaries from a series of point estimates.  Similar abundance estimates are not available for Canadian waters, as dedicated line-transect aerial surveys for marine turtles have not been conducted, nor have true transect-based shipboard surveys.  Instead, data have been gathered opportunistically from volunteer commercial fishers, who record sightings of leatherbacks while fishing or traveling to and from fishing grounds.  The potential for observing or incidentally capturing leatherbacks in these areas and other areas of the Scotian Shelf is related to fishing effort.  There are little data concerning the presence or absence of leatherbacks from areas where there is little or no fishing activity.

With these limitations, it is not possible to precisely assess abundance in eastern Canadian waters.  Estimates may be suggested, however, relative to those made for other areas.  For example, Shoop and Kenney (1992) recorded 128 turtles over 3 years and 454 dedicated aerial surveys of shelf waters from the Gulf of Maine to Cape Lookout, North Carolina, while over 300 turtles were opportunistically sighted by a sample of commercial fishers operating in waters off Nova Scotia during the summer and fall of 1998 and 1999 (James, 2000).  Since, when compared to opportunistic sightings made from vessels, aerial surveys provide superior opportunities for spotting leatherbacks, it can be inferred that summer leatherback densities in eastern Canada may be higher than the estimate of 100 to 900 leatherbacks/summer reported by Shoop & Kenney (1992) for the much larger study area along the coast of the northeastern United States.  In addition, abundance estimates based on aerial or shipboard surveys must be considered minimal, as these only include observations of turtles at the surface; they do not account for those turtles present at various depths (Shoop & Kenney, 1992).

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