Nesting of green turtles (Chelonia mydas) at ... - Seaturtle.org

152 B.J. Godley et al. / Biological Conservation 97 (2001) 151±158
mortality occurring away from the island may have
important impacts on the population size.
The life cycle of marine turtles involves movements
over great spatial scales (see Musick and Limpus, 1996
for review), probably taking decades for turtles to reach
adulthood. Although adults from the Ascension population
are thought largely to forage in Brazilian coastal
waters (Carr, 1975; Luschi et al., 1998), the life cycle of
juveniles involves extended periods in pelagic ocean
current systems (Musick and Limpus, 1996) and they
probably share coastal foraging areas with juveniles
from other populations (Lahanas et al., 1998). This life
cycle may expose this population to a number of ®sheries.
Although small-scale traditional ®sheries for marine turtles
once existed in Brazil (Pritchard, 1976), they are no
longer in operation (Marcovaldi and Marcovaldi, 1999),
and all marine turtle species are legally protected. Turtles
are, however, still incidentally caught in ®shing
gear. A national programme of marine turtle conservation
now exists in the Brazilian feeding grounds where
the mortality resulting from catch by artisanal ®sheries
has been reduced (Marcovaldi et al., 1998).
The fundamental goal of our study was to update
information on the number of green turtles nesting on
Ascension Island and hence to identify whether there
have been any dramatic changes in nesting numbers
over recent decades. We describe the design and implementation
of a rigorous survey regime which has produced
a robust estimate for the number of clutches laid
in a season and which will provide a template for future
monitoring work.
2. Methods
2.1. Study site
Ascension Island (7 57 0 S, 14 22 0 W) is an isolated
volcanic peak on the mid-Atlantic ridge that has 32
beaches and coves (Mortimer and Carr, 1987). The
location of beaches around the Island are shown in Fig.
1. Many of the beaches are backed by uninhabited
beach huts (maximum of one per beach) which are
used occasionally by island residents for recreational
purposes.
2.2. Enumeration of marine turtle nesting activities
Surveys of nesting beaches are often used to assess the
status of marine turtle populations (Schroeder and
Murphy, 1999). These utilise the fact that each time a
female turtle emerges from the water to attempt nesting,
(a ``nesting activity'') it creates a distinctive set of tracks
on the sand: with a track ascending to any aborted digging
attempts or successful nest, and a further track
descending to the sea. By counting tracks to and from
Fig. 1. Map of Ascension Island (7 57 0 S, 14 22 0 W) illustrating marine
turtle nesting beaches. Numbering system follows that of Mortimer
and Carr (1987). Key to common names: (1) South West Bay; (2)
Turtleshell; (3) Clarke's; (4) Payne Point; (5) Mitchell's Cove; (6)
Blowhole; (7) POL South; (8) POL North; (9) Deadman's; (10) Fort
Hayes; (11) Georgetown; (12) Long Beach; (13) Comfortless Cove;
(14) English Bay (EB); (15) EB Cove 1; (16) EB Cove 2; (17) EB Cove
3; (18) EB Cove 4; (19) Ladies Loo West; (20) Ladies Loo East; (21)
Porpoise Point (PP) Cove 1; (22) PP Cove 2; (23) PP Cove 3; (24) PP
Cove 4; (25) PP Cove 5; (26) PP Cove 6; (27) Northeast Bay; (28)
Beach Hut; (29) Hannay; (30) Pebbly West; (31) Pebbly East; (32)
Spire. In addition, the major settlement, Georgetown, and the Air-
Field, where meteorological observations were made, are marked.
the sea (and dividing by two) it is possible to infer how
many nesting activities have occurred.
Based upon the work of Mortimer and Carr (1987),
beaches on Ascension Island were divided into two
classes: those of major importance (beaches 1, 12, 27
and 29; Fig. 1); and those of minor importance (all
other beaches). From 1 December 1998 until 1 October,
1999, major beaches were visited on 3 successive days
each week. On day 1, all visible tracks were destroyed
using large rakes. On the morning of day 2 all new
activities were counted and the associated tracks
destroyed. On day 3, again all new activities were
counted. Minor beaches were visited every 1±3 weeks,
but only on 2 successive days, with tracks being raked
on day 1 and new activities being counted on day 2.
An estimate of the number of turtle activities on each
beach for each inter-survey day was generated by interpolation.
For major beaches, the mean daily count of
two successive 3-day survey bouts was attributed to
each intervening day. For minor beaches, a mean of the
two successive counts was used to generate an interpolated
value for intervening days. Thus an estimated
number of activities was attributed to every beach for
each day.
Track raking and counting was undertaken by a diverse
group of workers including the authors, local turtle wardens
and volunteers. Several beaches were removed
from the surveying regime as a result of both hosting