Leatherback turtle (Dermochelys coriacea) COSEWIC assessment and status report: chapter 8

LIMITING FACTORS

Nesting Beaches

Both natural processes and human activities on leatherback nesting beaches have been implicated in this species' decline.

The collection of leatherback eggs for sale in local and foreign markets is a serious and widespread problem for this species on its nesting grounds (e.g., Campbell et al., 1996; Leslie et al., 1996).

As leatherbacks prefer to nest on open beaches, adjacent to deep water (and typically unprotected by fringing reefs), in some years large numbers of nests are lost to flooding and erosion (e.g., Whitmore & Dutton, 1985; Leslie et al., 1996).

Although leatherback meat is considered unpalatable by most, poaching of free-swimming and nesting turtles does occur in some areas, most notably by indigenous peoples in the Indian Ocean and western Pacific Ocean (e.g, Chan & Liew, 1996; Suarez & Starbird, 1996). 

Increasing beach development and use discourages females from nesting in many areas and may prevent females from reaching nesting sites (this is particularly true when retaining walls are erected as part of beach armouring projects) (NMFS, 1992).  The mechanical raking of beaches and use of off-road vehicles may disturb nest sites, resulting in decreased hatching success and/or increased mortality among emergent hatchlings (Hosier et al., 1981).  Artificial lighting in the vicinity of nest sites can cause disorientation of both adults (Witherington, 1992) and hatchlings (Witherington & Bjorndal, 1991).  This can result in failed nesting attempts and, in the case of hatchlings, failure to move towards the water and high mortality.

Many leatherback conservation programs operating on nesting beaches have collected eggs from unprotected nests and incubated them artificially, usually indoors in styrofoam boxes and at lower temperatures than in natural nests (Davenport, 1997).  As leatherback embryos are subject to temperature-dependent sex determination, this widespread practice typically yields male-biased sex ratios amongst resulting hatchlings.  Several authors have identified this as a possible conservation problem (Morreale et al., 1982; Mrosovsky, 1982; Dutton et al., 1985); however the implications of this practice for leatherback populations have not been quantified.  Clearly, if eggs are to be incubated artificially, equal numbers of clutches should be incubated above and below the pivotal temperature of 29.5°C.

Global warming may interact with temperature-dependent sex determination to affect leatherback populations negatively.  Even minute changes in climate could potentially alter the sex ratios across entire nesting beaches by increasing mean nest temperatures and favouring the production of females (Davenport, 1997).  Global warming is predicted to have other deleterious effects on marine turtles.  Alterations in ocean current patterns may accompany climate change, thereby affecting the migration and dispersal of marine turtles (Davenport, 1997).  Increased hurricane activity also may be associated with global climate change.  This could, potentially, result in increased nest loss due to amplified wind and wave erosion on leatherback nesting beaches (Davenport, 1997).

The Marine Environment

A number of widespread threats to leatherbacks have been identified in the marine environment. Principal among these is entanglement in different types of fishing gear.  Leatherbacks become entangled in longlines, buoy anchor lines, and other ropes and cables (e.g., Chan et al., 1988; Goff & Lien, 1988; NMFS, 1992; Cheng & Chen, 1997; Godley et al., 1998).  These incidents can result in serious injuries (rope or cable cuts on the shoulders and front flippers) or death by drowning.  Although incidental take of marine turtles on pelagic longlines is common, the vast majority of turtles is released alive (Witzell, 1984); however, the post-capture mortality of these turtles is not known.  Leatherbacks, like all sea turtles, demonstrate physiological tolerance to extended periods of anoxia (Shoop & Schwartz, 1992).  This ability presumably enables some entrapped turtles to survive long periods of forced submergence (Shoop et al., 1990).

The effects of marine pollution on sea turtles are not well understood.  Therefore, the magnitude of pollution-related mortality is not known.  There are many documented cases, however, of leatherback mortality associated with ingestion of and entanglement in marine debris.  Leatherbacks are known to ingest a variety of anthropogenic marine debris, including plastic bags, tar balls, plastic sheeting, and fishing gear (e.g., Sadove, 1980; Hartog &Van Nierop, 1984; Lucas, 1992; Starbird, 2000).  Ingestion of such materials may interfere with metabolism or gut function, or lead to blockages in the digestive tract and subsequent starvation (Plotkin & Amos, 1990).

As its diet of jellyfish is high in water, and low in organic content, the leatherback must consume large quantities of food (Lutcavage, 1996).  Davenport and Wrench (1990) have suggested that the leatherback, as a presumably long-lived species (not confirmed), should serve as an ideal indicator of the degree of contamination of the oceanic food web by accumulating substances such as heavy metals and polychlorinated biphenyls (PCBs).  Metal and PCB levels in the leatherback should represent a biomagnification of concentrations found in plankton-feeding jellyfish.  However, tissue samples derived from leatherbacks in European waters have not revealed evidence of significant chemical contamination (Davenport et al., 1990; Godley et al., 1998).

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