Hawksbill Turtle Nesting at Manatee Bar, Belize, 1991

Greg W. Smith
Belize Audubon Society and WIDECAST-Belize, c/o Hol Chan Marine Reserve, San Pedro, Ambergris Caye, Belize

Manatee Bar, a regionally important nesting beach for the endangered hawksbill turtle (Eretmochelys imbricata), is located on the mainland coast of Belize approximately 20 miles south of Belize City. Manatee Bar received 108 hawksbill nests (>15,000 eggs) on 8 km of beach in 1991. Nesting occurred from May to October, with a peak in late August and early September. Despite high productivity there is nearly 100% depredation of eggs by raccoons and coatimundis; thus, no significant numbers of hatchlings are produced at this site. Protection of nests from depredation is needed to preserve this rookery. Fencing individual nests while also chemically masking olfactory clues appears a promising means of egg protection. Gillnets set in waters adjoining the beach during the nesting season are a potential threat to turtles and should be restricted. Beach-front development should be limited to low impact schemes that encourage 'eco-tourism' and the long term protection of this important nesting area.

Introduction

In 1990, a national ground survey for sea turtle nesting in Belize located a previously unrecorded concentration of 160 hawksbill nests on an 8 km section of mainland beach south of the Manatee River Bar. These nests were easily counted by the presence of eggshells in and around each nest due to depredation. Typically, the nest chamber was penetrated from directly above and eggshells were scattered around the excavated chamber. This pattern is typical of raccoon depredation (Cornelius, 1986). Although we were able to count 160 nests, the actual tally for 1990 remains unknown for two reasons. First, nesting was still occurring when the patrol was conducted on 23 September; second, a few nests appeared to be from a previous nesting season. As the hawksbill is highly endangered worldwide (Groombridge and Luxmoore, 1989) and there are relatively few known nesting concentrations in the Caribbean Sea (Meylan, 1989), a follow-up study was planned for 1991 to determine the length of the nesting season, the total number of nests laid, and a means of controlling egg depredation by small mammals.

Methods

The 8 km beach south of Manatee Bar was patrolled every 3-4 days from 15 May to 15 August 1991 by a local resident hired to record sea turtle nesting activity. Thereafter the site was patrolled approximately every two weeks (19-20 August, 3-5 and 22-24 September, 8-9 October, and 31 October-1 November) by the author. Where destruction of eggs was not evident, possible nesting crawls were extensively probed and hand searched in an attempt to locate and protect eggs. Three methods (transplantation, exclusion, chemical marking) found to have been successful elsewhere in protecting eggs were applied to nests missed by predators. Nest transplantation has been successful in South Carolina (USA) against raccoons, but has not been particularly effective at Tortuguero (Costa Rica) where the major predator is the coatimundi (Stancyk, 1990). Exclusion, which requires placing a metal screen over the nest and taking care to bury the screen deeply enough so that raccoons cannot dig beneath it, has also been found to be successful against raccoons (Sally Murphy, South Carolina Wildlife and Marine Resources, pers. comm.). This exact method did not appear applicable to hawksbill nests in Belize (which are shallow nests, often concealed in vegetation), but we were able to adapt it to exclude native predators by positioning sticks 1-2 inches apart around the nest in a cone tied at the top and extending 6-12 inches underground. A third method (suggested by Steve Cornelius, WWF-US, who had heard of its success elsewhere), was to chemically mark around a nest site with human urine. These three methods were used to protect eight undisturbed nests: one nest was transplanted only, two were transplanted and chemically marked, two were chemically marked only, one had all visual cues removed and was chemically marked, and two had protective fences built around the nest chamber. Five more nests which had been disturbed but still contained eggs also had fences built around them.

Results

A total of 108 confirmed hawksbill nests were counted on 8 km south of Manatee Bar in 1991. Nesting occurred from May through October; peak nesting was in late August and early September (Table 1). All 100 unprotected nests (100/108=92%) were disturbed by predators. Although some disturbed nests still contained eggs when first located and were reburied in situ, predators usually returned to these nests and only one was observed to produce hatchlings (46 hatched of 106 eggs remaining). In contrast, seven of the eight nests where protection was attempted remained undisturbed throughout incubation (hatch success was not determined). The eighth nest, transplanted without chemical masking, was disturbed by predators. Predator attempts to dig around the two undisturbed nests protected by fences were also observed.

Of six disturbed nests still containing eggs and around which fencing was later applied, four were revisited by predators and lost all remaining eggs. The remaining two nests were successfully protected. First night depredation was not observed. Nest destruction typically showed penetration from directly above the egg chamber, with eggs scattered uniformly around the egg chamber. Only one likely case of poaching was documented, despite almost daily visits to this uninhabited beach by hunters, coconut collectors, and people beach-combing. In this one case a nest that appeared disturbed by raccoons was found to have fewer than 20 eggshells around it and a perfectly clean but empty egg chamber. Usually, a predator-disturbed nest is full of sand and discarded shells.

In addition to the 8 km regularly patrolled, 3-4 km of low sandy beach north of Manatee Bar was patrolled periodically. Nesting and nest depredation were reported, but nest density was very low. Only two old nest sites were located along a small section (0.5 km) north of Manatee Bar on 21 November when a proposed subdivision development site was surveyed.

Gillnets pose a threat to sea turtles in the Manatee Bar area. Although it is illegal to set nets within 100 yards of nesting beaches to catch sea turtles, it is not prohibited to set nets for fish adjacent to nesting beaches. Gillnets were observed set in waters adjoining the nesting beach on 3 September and on 10 October.

Table 1. Seasonal hawksbill turtle nesting rate on 8 km of beach south of Manatee Bar, Belize, in 1991. The number of new nests counted on each patrol date was divided into the number of days since the last patrol to calculate a mean number of nests per day.

Date

New Nests

Interval

Nests/Day

18 May

02

--

--

18 June

03

31

0.10

18 Aug

47

61

0.77

03 Sept

35

16

2.18

22 Sept

17

19

0.89

04 Oct

04

12

0.33

01 Nov

01

28

0.03

Discussion

With 108 nests confirmed in 1991, Manatee Bar appears to be an important regional nesting site for the endangered hawksbill sea turtle. A few nests were probably not counted due to the infrequency of patrols, but the tally of 108 is thought to be very close to the actual number. The nearly 100% destruction of nests by predators meant that nests were easily identified by exhumed and scattered eggshells. An intensive study of hawksbills at Jumby Bay, Antigua (West Indies), found average annual clutch frequencies of 4.8 and 4.4, with average clutch sizes of 157 and 147 in 1987 and 1988, respectively. The females nesting at Jumby Bay deposited about 700 eggs per season and, with predator-free hatch successes of some 80%, each female was able to produce about 560 hatchlings (Corliss et al., 1989). From these data I estimate that 22-25 hawksbills produced >15,000 eggs at Manatee Bar in 1991.

Despite the large number of eggs laid annually at Manatee Bar, only a few hundred hatchlings result. The loss of eggs to predators must be the major priority of any future work at this site. Based on the appearance of destroyed nests and tracks, as well as an absence of first night depredation, the major predator at Manatee Bar is believed to be the raccoon (cf. Stancyk, 1990). However, the similarity in tracks between raccoons and coatimundis raises the possibility that coatimundi could also be responsible for a portion of the nest destruction. So few undisturbed nests were located (n=8) that is was not possible to statistically test the three methods of nest protection. However, our success (judging by seven of eight protected nests remaining undisturbed) in chemically masking olfactory cues and excluding predators by fencing indicates that it may be possible to protect turtle eggs without resorting to killing or relocating native predators. The absence of first night depredation would allow morning beach patrols to protect nests by combining exclusion and chemical masking, paying particular attention to burying fences deep enough to preclude predators from digging under them.

Poaching does not appear to be a threat to sea turtles nesting at Manatee Bar at this time. However, gillnets, even though they are not directed at sea turtles, do pose a threat because of the potential for incidental capture and drowning. Recent high mortality in sea turtles due to drowning in gillnets caused the Florida Department of Natural Resources to enact emergency legislation regulating gill-netting (Barbara Schroeder, FLDNR, pers. comm.). The presence of gillnets along the Manatee Bar nesting beach during the hawksbill nesting season, therefore, deserves consideration. Our preliminary data indicate an annual nesting population of only 22-25 hawksbills; thus, the loss of even one to drowning could have adverse effects on the population. When dealing with depleted, long-lived species such as sea turtles, the exclusion of threatening agents like gillnets during the nesting season is a necessary management decision.

The observed nesting period of May-October corresponds with reports from fishermen throughout Belize (Smith, 1990). Sea turtles are protected in Belize between 1 June and 31 August, but the Minister of Agriculture and Fisheries recently announced the Government's intention to extend the closed season from April through October, specifically to ensure the protection of nesting hawksbills. In addition, the Minister announced a moratorium on the taking of hawksbills on land or at sea, the protection of nesting beaches (which is expected to include lighting and construction restrictions), a requirement that shrimp fishermen use turtle excluder devices (TEDs) in their trawls, and the enforcement of a maximum harvestable size, as opposed to the current minimum size limit that allows the take of large juveniles and adults. The latter decision will protect large turtles of all species. These laws are expected to be enacted soon and are evidence of the Belize Government's commitment to recovery of sea turtle populations in Belize and throughout the region, particularly the hawksbill.

Recently a proposal for a subdivision to be located on the beach north of Manatee Bar was submitted to the Government but was rejected due to the presence of nesting sea turtles. All of the beach both north and south of the Manatee River Bar is privately owned land; therefore, it is to be expected that pressure to develop this area will continue. Turtle nesting and coastal development are not necessarily mutually exclusive. However, any form of beach development that would remove or alter the beach or nearshore bottom (e.g., dredging, break-waters, groins, sea walls) could destroy nesting habitat. Similarly, beach clearing, such has already occurred on a small section of the proposed subdivision at Manatee Bar, destroys vegetation that provides preferred hawksbill nesting habitat. Artificial lighting is another problem for sea turtles because lights attract and disorient hatchlings and can deter nesting. Therefore, the bright pier lights, beach lights, street lights, and security lights which often accompany development are generally considered incompatible with sea turtle nesting.

The type of development that would be compatible with sea turtle nesting is already a growing part of Belize's development. Small, environmentally conscious hotels located on large tracks of private lands and reserves using kerosene lanterns and catering to 'eco-tourists' would not only be compatible with sea turtles, but could actually enhance sea turtle recovery by helping to protect nests and increasing public awareness. There are hundreds of kilometers of shore-front property in Belize where sea turtles don't nest that are suitable for large hotels, condominiums, and subdivisions. But there are very few known sites in the Caribbean where a person can walk a beach and find over 100 hawksbill nests. This is a special treasure in Belize, and one which should be earnestly safeguarded.

Acknowledgements: I would like to thank Jack Woody (U. S. Fish and Wildlife Service) and Marydele Donnelly (Center for Marine Conservation) for financial support, the members of the Belize Audubon Society (BAS) Reef Preservation Committee for their support and interest in sea turtle research and protection, and Delores Godfrey and Magda Avery of BAS for their handling of finances and administration.

Corliss, L. A., J. I. Richardson, C. Ryder, and R. Bell. 1989. The hawksbills of Jumby Bay, Antigua, West Indies, p.33-35. In: Proc. Ninth Annual Workshop on Sea Turtle Conservation and Biology (S. A. Eckert, K. L. Eckert, and T. H. Richardson, Compilers). NOAA Tech. Memo. NMFS-SEFC-232. U. S. Dept. Commerce.

Cornelius, S. E. 1986. The Sea Turtles of Santa Rosa National Park. Fundacion de Parques Nacionales, Costa Rica. 64 p.

Groombridge, B. and R. Luxmoore. 1989. The green turtle and hawksbill (Reptilia: Cheloniidae): world status, exploitation and trade. A Publication of the CITES Secretariat. Lausanne, Switzerland. 601 p.

Meylan, A. B. 1989. Status Report of the Hawksbill Turtle, p.101-115. In: Proc. Second Western Atlantic Turtle Symposium (Larry Ogren, Editor-in-Chief). NOAA Tech. Memo. NMFS-SEFC-226. U. S. Dept. Commerce.

Smith, G. W. 1990. Ground surveying for Sea Turtle Nesting in Belize, 1990. Annual Report submitted to U. S. Fish and Wildlife Service. 23 p.

Stancyk, S. 1990. Nest transplantation as a deterrent to mammalian predation at Tortuguero, Costa Rica, p.59-62. In: Proc. Tenth Annual Workshop on Sea Turtle Biology and Conservation (T. H. Richardson, J. I. Richardson, and M. Donnelly, Compilers). NOAA Tech. Memo. NMFS-SEFC-278. U. S. Dept. Commerce.

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