Leatherback turtle (Dermochelys coriacea) COSEWIC assessment and status report: chapter 11
EVALUATION AND PROPOSED STATUS
Leatherback decline is not fully understood, however several anthropogenic impacts are suspected to account for a high level of mortality. Foremost among these is incidental capture in fishing gear (e.g., Eckert & Sarti, 1997; Spotila et al., 1996, 2000). On nesting beaches, low recruitment rates due to high natural hatchling mortality and excessive human harvesting of eggs constitutes a threat to this species. Suspected impacts associated with global climate change may be underestimated. As sex determination in this species is determined by nest incubation temperature, even subtle temperature changes associated with global warming could potentially bias sex ratios.
The leatherback has exhibited a precipitous global population decline from 115,000 nesting females in 1980 (Pritchard, 1982) to 34,500 females in 1995 (Spotila et al., 1996). This represents a 70% decline in only 15 years, or less than one generation. Long-term tagging studies on some nesting beaches have revealed annual rates of adult female mortality of up to 33% (Spotila et al., 2000). Life history traits of sea turtles include a long life, delayed sexual maturity, and reproductive intervals of two to three years (Crouse et al., 1987), and high subadult and adult survival rates (Congdon et al., 1993). However, in the case of the leatherback, widespread incidental capture in fisheries is now believed to account for a high level of mortality among adult and subadult turtles (Spotila et al., 1996, 2000). Popular management techniques aimed at recovering sea turtle populations have traditionally focused on the protection of eggs on nesting beaches (Crouse et al., 1987). In contrast, recent population models based on demographic data for loggerhead sea turtles (Caretta caretta) reveal that conservation practices aimed at enhancing the survival of juveniles and adults may be far more effective (Crouse et al., 1987). The importance of focusing conservation efforts on later life-history stages has also been argued in the case of leatherbacks (e.g., Eckert & Sarti, 1997; Spotila et al., 2000). In fact, recent demographic modeling of this species suggests that without increased human intervention to reduce adult and subadult mortality arising from fisheries interactions, the leatherback faces extirpation throughout extensive parts of its range (Spotila et al., 1996, 2000).
Based on skeletochronological analyses, Zug and Parham (1996) have described a relatively short maturation time for leatherbacks. Rapid maturation, coupled with this species’ high fecundity, may enable some leatherback populations to recover if egg poaching and incidental capture and killing of large juveniles and adults can be dramatically reduced (Zug & Parham, 1996). Conversely, if these analyses have underestimated age at maturity, recovery of populations will be quite difficult and slow.
The leatherback was originally assigned status as endangered in Canada in 1981 (Cook, 1981). At that time, very little was known about the species’ distribution and movements in Canadian waters. Subsequent work in Newfoundland (e.g., Goff & Lien, 1988) and Nova Scotia (e.g., James, 2000) has revealed that a portion of the Atlantic population range into waters off Canada’s east coast each year. Leatherback presence in eastern Canada is related to seasonally abundant cnidarians (particularly Cyanea sp.), their principal prey (Bleakney, 1965; Goff & Lien, 1988; Shoop & Kenney, 1992; James, 2000). Although leatherbacks do not nest in Canada, waters off Atlantic Canada provide important seasonal foraging habitat for these turtles. Human activity in and degradation of the marine environment continue to result in an unknown level of annual mortality among leatherbacks in Canadian waters. It is recommended that the leatherback retain its status as endangered in Canada.
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