The Leatherback Sea Turtle: A Marine Mammal at Sea

 Samuel Sadove

 Abstract

 Dramatic declines in leatherback populations have been recently documented at almost all ofthe world's nesting beaches. These declines have increased international interest in understanding the leatherback's biology, in hopes of reversing these declines. As with most sea turtles, harvesting of females and eggs on nesting beaches, along with habitat destruction of these beaches, contributes significantly to their declining populations. Even with well protected nest sites, leatherback numbers continue to decline.

Introduction

Telemetric studies have demonstrated a highly pelagic movement pattern for leatherbacks. Until recently these studies have been conducted on animals emerging from nesting beaches. Movement patterns observed for leatherbacks are quite different than those observed for other sea turtle species. Besides telemetry, physiological studies on leatherbacks have also demonstrated a dramatic difference between the leatherback and other marine turtles. Numerous studies by authors such as Balasz, Paladino, Morsovsky, Spotilla, Standora and others, have demonstrated striking physiological differences between this species and other marine turtles. These physiological differences have enabled the leatherback to be the only turtle, or reptile to regularly travel to and survive Arctic or sub-Arctic habitats. The leatherback has been regularly sighted off Newfoundland, Canada and observed of the shores of Greenland as far north as 69^°N (personal observations).

The only other large marine vertebrates to have life cycles involving regular movements from tropical habitats to Arctic or subarctic habitats are cetaceans. It is for these characteristics that I suggest viewing the leatherback form a wholly different perspective - as a marine mammal in a reptile body.

Methods

During the late 1970's and early 1980's a study was conducted along the eastern seaboard of the United States and Canada to determine the population and distribution of marine mammals and sea turtles (Cetacean and Sea Turtle Assessment Program/CeTAP). One of the most surprising observations of that 4-year study were the regular sightings of large annually recurring concentrations of leatherbacks found at two sites; one off the eastern portion of Long Island, New York and the other off the southern part of Newfoundland, Canada. At the time little was known of the leatherback outside of its nesting beaches and such concentrations had not been reported for other locations. Little was done with this information, but it provided an interest to the author to begin incorporating leatherbacks as a part of a broader marine mammal study. Coincident with data collection on fin whales in the New York Bight during this time, sightings were also collected for leatherback sea turtles. In addition, data from stranded animals were collected and leatherbacks were necropsied as specimen condition allowed. Approximately 15-25 adult leatherbacks stranded a year, while approximately 15+ sightings at sea were recorded annually. As the data was collected it became obvious that large numbers of adult leatherbacks were occurring in the region. This indicated that these animals were not part of any developmental migration, as with other sea turtle species for this region.

Results and Discussion

Studies on large whales have demonstrated that these animals go from limited geographic breeding areas to broad geographic feeding areas on an almost annual basis. Although none of the large whales can really be considered social, large aggregations are observed in limited areas. Although these aggregations are usually associated with prey abundance they are not completely dependent upon the presence of prey. Whales have been observed waiting in areas until prey becomes abundant in that area. One of the most interesting phenomena is that for whales, these broad feeding areas seem to involve a significant genetic conservatism in regard to population structure. Yet the mating grounds do not seem to demonstrate the same features as often.

Features such as diving physiology and habitat that are described for large baleen whales are interestingly some of the same types of features found for leatherbacks. Unfortunately, what is missing for leatherbacks is information associated with the non-nesting habitats and their distributions at sea. The pelagic nature of the leatherback's life has similarities to that of some whales. Sightings surveys from vessels and aircraft of both a dedicated and opportunistic nature have led to a much clearer understanding of whale biology. I suggest that the same be applied to leatherbacks.

During the last three years dedicated surveys have begun for leatherbacks utilizing some of the techniques also used for whales. This has involved collecting sighting information from user groups such as fishermen, commercial vessel traffic (i.e. ferries) and aircraft. This identifies the presence or absence of leatherbacks in non-nesting areas. Other methods involve following animals when sighted.. Respiration rates, direction and speed of movement can be noted along with other environmental parameters such as depth, water temperature, prey presence and sea states. Photographic identification, based upon the coloration patterns seen on the head and shell can also be initiated to determine whether individuals return to similar areas over time.

The work conducted in the New York region can be easily utilized at any location in the world. We have now begun to see results that indicate a great deal of site fidelity, regional concentration, and seasonal timing based upon these studies. Relationships to prey species, types of prey species and non-nesting behaviour have begun to be elucidated. Modification of techniques associated with whale studies are enabling us to begin to study leatherbacks in a more productive manner. I would strongly suggest that methods known better for other species such as marine mammals be utilized to help better understand and protect this very endangered species.

Perhaps the leatherback might be best understood if we look away from the nesting beach toward the open sea for the leathery skinned marine mammal.