Research activities

Nesting Beach Monitoring 2005

All beaches of Bonaire were periodically surveyed for sea turtle nesting activity, with emphasis on the most actively used turtle-nesting beach "No Name" on Klein Bonaire. No Name beach was visited with greatest frequency and is Bonaire’s index beach for measuring annual fluctuations in nesting activity.

Turtle nesting activity was first registered on May 3, when a loggerhead nest was found on No Name beach, Klein Bonaire, followed the next day by a hawksbill nesting on the same beach. The last nest of 2005 was found on November 12 and was laid by a hawksbill. For 2005, a total of 29 hawksbill and 14 loggerhead nests were recorded on No Name beach, with the months of May and June showing the greatest nesting activity for loggerheads, whereas hawksbills were most active in August and September (Figure 2). Hawksbill nests were fairly uniformly spread out along No Name beach, whereas loggerheads came ashore to lay only in areas with substantial amounts of beach sand.

^ Figure 1. Number of nests recorded by month laid by loggerheads and hawksbills on No Name beach, Klein Bonaire.

^ Figure 2. Locations of individual hawksbill nests (open circles) and loggerhead nests (dots) along No Name beach, Klein Bonaire.

In addition to Klein Bonaire, other beaches showing nesting activity include Slagbaai and Playa Chikitu within the Washington Park, Boca Onima on the east coast of Bonaire, and several small beaches along the south-west coast (see Appendix III). The 10 green turtle nests recorded in a period from July14 to October 5 at Playa Chikitu were probably laid by at least two individual turtles. Similarly, two loggerheads were likely responsible for the five nests found along the south-west coast. The Boca Onima nests were apparently green turtle nests, but no further post-hatching confirmation could be carried out since the nests were destroyed by sand mining activities (see below).

Nest size and productivity was measured through nest revisions after hatching. At No Name beach, revision of 20 nests yielded an average hawksbill nest size of 138.7 eggs (range 71 – 181) and average hawskbill hatching success was 76.8%. Revision of 15 loggerhead nests yielded an average nest size of 120.2 eggs (range 51 - 143) and hatching success of 81.9%. Both hawksbill and loggerhead hatching success rates resulted higher in 2005 than in 2004, but are slightly below 2003 values. The extreme rainfalls experienced during 2004 were mostly absent in 2005, likely resulting in drier beach sand conditions and contributing to higher hatching success rates.

In 2005, the estimated number of hatchlings produced at Klein Bonaire can be calculated from the total number of nests, average nest size and average hatching rate. The 28 hawksbill nests laid along No Name resulted in approximately 3000 live hawksbill hatchlings emerging, plus almost 1500 loggerhead hatchlings emerging from their 15 nests. The total of 4500 hatchlings produced in 2005 is substantially lower than the 7000 hatchlings reported for 2004 and reflects the lower number of nests deposited by fewer reproducing turtles. Whereas this apparent decline may be of concern, it should be realized that substantial annual variation in nesting activity is common in marine turtles, especially where populations are relatively small. Only nesting trend information collected longer-term (more than 10 years) will be truly indicative of population status.

Foraging Ground Surveys

Foraging ground surveys were conducted by snorkeling along the entire west coast of Bonaire and all around Klein Bonaire. In addition, turtle surveys using the netting technique were done at Lac Bay. The purpose of these surveys is to tag, sample and measure individual turtles, and to establish catch-per-unit-effort measures of turtle abundance. Combined, the surveys yielded a total of 28 hawksbills and 74 green turtles, of which 9 and 11 were recaptures, respectively (Figure 4). Five of the captured hawksbills were breeding adults.

^ Figure 3. Size distribution of hawksbill and green turtles captured, tagged and measured at Bonaire. Black bars indicate recapture of turtles tagged in previous years.

For primarily logistical reasons, Klein Bonaire was the most consistently surveyed area both during 2005 and previously, thus providing a basis for detecting trends in turtle abundance on these foraging grounds. East and West Klein Bonaire turtle habitats were separated in this analysis because of the great differences in local turtle abundance, with East Klein Bonaire defined as the area from Ebo’s reef to Sampler Buoy and the remaining coast referred to as West Klein Bonaire. Catch-per-unit-effort was defined as the number of turtles sighted and/or captured by trained observers in one hour of survey time. Observations of adult turtles were excluded, since these are most likely seasonal visitors and not foraging ground residents. Table 1 shows the results of the 2003 and 2005 in-water surveys at Klein Bonaire. Total survey durations are remarkably similar between these years, which helps validate the comparison. Most significantly, East Klein Bonaire saw a substantial drop in green turtle abundance between 2003 and 2005, from 6.0 turtles sighted per hour to 2.9 per hour. Elsewhere around Klein Bonaire green turtle numbers - and those for hawksbills in both areas - remained relatively constant. It is not yet clear what the cause for the green turtle decline along East Klein Bonaire might be. The high density of juvenile green turtles along East Klein Bonaire appears related to the foraging opportunities present in the shallow fringing lagoon, where green turtles feed on sea grasses, in particular Manatee Grass (Syringodium filliforme).

^ Table 1. Comparison of 2003 versus 2005 catch-per-unit-effort survey results at Klein Bonaire.

Recaptures of previously tagged and measured turtles provide information on movement and somatic growth rates. Only one recaptured turtle had moved a significant distance from its earlier point of capture: juvenile green turtle 03-017 was first tagged at Karpata on 24 January, 2003 then recaptured at Lac Bai on 1 December, 2005.

Measured somatic growth rates varied by individual, with carapace length increasing about 2-5 cm/year for hawksbills, with the exception of turtle 03-064 from Andrea, which grew at the high rate of 6.4 cm/year (see Figure 5). Green turtles showed even greater variation, with a significant difference between the very fast growing turtles of Lac Bai and those found along the reefs at the rest of Bonaire and Klein Bonaire. Lac Bai green turtles grew between 5-9 cm/year and tended to be larger in size than other green turtles. The high growth rate of green turtles at Lac Bai indicates that the foraging conditions are exceptionally good there.

^ Figure 4. Somatic growth of hawksbill and green turtles at Bonaire.

Nesting Season 2004
On the research front, we observed sea turtle nesting in 2004 at higher levels than during 2003, with a total of 103 nests recorded for all the beaches of Bonaire and Klein Bonaire.

^ Figure 1. Monthly nesting activity by species at 'No Name' beach, Klein Bonaire. In all, we recorded 23 loggerhead nests, 50 hawksbill nests and 5 of undetermined species at Klein Bonaire during 2004. No green turtle nesting activity was observed on Klein Bonaire in 2004 and beaches along the north shore remain unsuitable for nesting turtles, with only sparse patches of sand among the coral rubble.

In-Water Surveys 2004
In-Water Surveys around Bonaire were conducted mostly during the first half of the year, when less time was required for nesting surveys. Excluding the breeding adult turtles that were also caught in the water, we captured 35 hawksbills and 41 green turtles, of which 9 and 6 were recaptures, respectively (one hawksbill included here as a recapture was actually a turtle found entangled and dead in a fishing line at the town pier in Kralendijk). For a variety of reasons, the in-water surveys conducted in 2004 were less systematic than in 2003 and yielded too few catch-per-unit-effort observations to make meaningful comparisons of turtle abundance. Turtle size distributions (figure 2) continue to indicate that the surveyed foraging habitats around Bonaire and Klein Bonaire are used by green and hawksbill turtles in a narrow size range only. Larger turtles are known to reside along the east coast of Bonaire, but we have so far been unable to extend systematic surveys to that coast.

^ Figure 2. Size distribution of immature green and hawksbill turtles caught during the 2004 in-water surveys. Black bars indicate turtles recaptured that were tagged by us in 2003.

Satellite Tracking
Satellite tracking of breeding turtles became our flagship project, with four turtles fitted with transmitters: two on hawksbills and a loggerhead turtle at Klein Bonaire, plus one on a nesting green turtle at Playa Chikitu, Washington Park. All tracked turtles made long distance migrations to their foraging grounds and their movements generated huge public interest for these animals.

A more extensive review of our research efforts for 2004 is available in our Progress Report for 2004. It can be downloaded from this website by clicking here (PDF format 1.9 mb).

Previous research information:

We are making good progress with systematic surveys on the nesting beaches and turtle reef habitats. Mid-December we purchased a boat (the "Nancy Too") suitable for visiting most turtle foraging areas during calm weather. On January 13 2003 we hired Gielmon Egbreghts, a local lobster fisherman and smoked-fish entrepreneur, to help with fieldwork and other aspects of the project. It is anticipated that he will become capable of running our turtle program in a few years.

Gielmon 'Funchi' Egbreghts measures juvenile hawksbill

On our index nesting beach, "No Name" on Klein Bonaire, we documented and confirmed 11 fresh hawksbill nests laid since early October. An additional 17 turtle nests were confirmed after hatching, including one loggerhead turtle nest. Since these 28 nests represent only the latter part of the turtle nesting season, we estimate that from 40-50 nests were actually deposited on the index beach during 2002. No nests at No Name were observed lost to major predation or poaching. Other beaches on Bonaire were also occasionally used by turtles for nesting, particularly Playa Chikitu, Playa Benge by green turtles, and Windsock Beach where two hawksbill nests were confirmed after hatching.

Capture locations of turtles examined & tagged as of Feb -03

On the foraging grounds around Bonaire we have spent 32 hours of actual in-water time and so far caught, tagged, measured and examined 43 juvenile turtles: 23 hawksbills and 20 green turtles. Most animals were caught by hand in coral reef environments, at depths from 3 to 30 meters. Individual turtles live at scattered locations along the west coast of Bonaire, but there are two known "hot spots" with particular high densities at Klein Bonaire's Ebo's Reef and at Karpata.

Size histogram of turtles examined & tagged as of Feb 7-03

The former location has a shallow coral-fringed lagoon offering good foraging on seagrasses to green turtles. However, it is as of yet unclear why turtles are particularly attracted to the Karpata location. All of the turtles found on reef habitats appeared to be healthy, with no fibropapilloma tumors evident. Anecdotal evidence suggests a large number of green, hawksbill, and some loggerhead turtles reside and forage at Lac Bay lagoon, a habitat that contains extensive shallow water seagrass beds. We are currently constructing a 200 x 5 meter net that will allow us to begin surveying this important lagoon system.