Schut, K.

With the in-water monitoring activities, STCB collects data on the following CITES species:

• (a) green turtles; (b) hawksbill turtles; (c) loggerhead turtles; (d) sharks; (e) rays.

In addition, data is collected on:

• (f) barracudas; (g) tarpons; (h) rainbow parrotfish; (i) midnight parrotfish; (j) fish schools (creole fish, creole wrasse, black durgon, blue tang/surgeon fish, palometa, chub, bar jack, black margate, horse-eye jack, school master, yellow-tail snapper).
• Time of survey: date and time.
• Environment: horizontal and vertical visibility, food availability, substrate, water temperature.
• Disturbance: presence and number of fishing boats and humans in the survey regions.
• Observers: number of observers and their level of experience.

For the in-water monitoring activities, STCB has divided Bonaire into three survey regions: Northwest Bonaire, Southwest Bonaire and Klein Bonaire. Within these survey regions, eighteen fixed systematically random transects (survey areas) have been set up. Subsequently, an additional eighteen random transects were set up in the areas between the fixed transects. Therefore, a total of 36 transects have been set.

Green turtles Chelonia mydas and hawksbill turtles Eretmochelys imbricata are neg- atively impacted by natural and anthropogenic disturbances. Unknown numbers of turtles are killed annually in the coastal waters of Bonaire and Klein Bonaire, Caribbean Netherlands. We used N-mixture models, conventional distance sampling and the multiple Lincoln-Petersen method to estimate abundance from transect-count and net-capture surveys. Maximum likelihood and Bayes- ian generalised linear models were used to assess trends in annual abundance in 2003−2018, and a Bayesian state-space logistic model was developed to generate the posterior distributions of pop- ulation parameters and make abundance predictions for 2019−2030. Mean ± SE annual abun- dance was 555 ± 149 green turtles (2.5th and 97.5th percentiles = 337, 943) and 70 ± 13 hawksbill turtles (49, 101), and there were no trends in western Bonaire and Klein Bonaire in 2003−2018. Mean annual abundance was 348 ± 135 green turtles (171, 731) and there was a positive trend inside Lac Bay, southeast Bonaire, 2003−2018. Green turtles have higher population growth rate and carrying capacity, and therefore can sustain higher human-induced mortality than hawksbill turtles. However, under low mortality rates (<0.100), both species can fluctuate stably between the lower and upper limits of the carrying capacity. The methodology implemented can be adapted to estimate sea turtle abundance, monitor and model their population dynamics, and assess the neg- ative impact of human-induced mortality in other Caribbean islands.

KEY WORDS: Chelonia mydas · Eretmochelys imbricata · Abundance · N-mixture model · Distance sampling · Multiple Lincoln-Petersen method · Generalised linear model · Bayesian state-space logistic model

Sea Turtle Conservation Bonaire (STCB) has been protecting sea turtles on Bonaire since 1991, so this year represents the 26th Anniversary of our organization. In 2002, we standardized our research and monitoring efforts, following the appointment of a Scientific Officer. Annually we monitor our nesting beaches around Bonaire, conduct intensive in-water netting and snorkel surveys, and we regularly track sub-adult and post-breeding migrations using satellite telemetry. These techniques provide us with a better understanding of Bonaire sea turtles’ breeding success, abundance, health, growth rates, migratory paths and distant feeding grounds, residency duration, habitat quality, and threats. In 2017, Scientific Advisor Dr. Frank Rivera-Milán analyzed in-water transect counts, net captures, and nesting data that STCB has collected over the years. In cooperation with STCB, Rivera-Milán will produce scientific publications in the coming years, as well as review STCB’s methodology for netting (net captures), in-water surveys (transect counts) and nesting. 

During the 2017 season, we recorded 78 nests at our index beach on Klein Bonaire. A total of 61 hawksbill and 17 loggerhead nests and suspected nests were documented on “No Name Beach”. On the beaches on Bonaire and Klein Bonaire combined, we observed three sea turtle species (hawksbills, loggerheads and green turtles) crawling 228 times, which included a total of 128 confirmed or suspected nests. 21 green turtle nests were recorded in northeastern Bonaire. Hawksbills and loggerheads mainly nested on Klein Bonaire and the beaches of southern Bonaire. That said, three loggerhead and three hawksbill nests were recorded in northeastern Bonaire. The nesting period on Bonaire in 2017 ran from April to December with the highest number of nests laid between mid-June and mid-September. 

Estimates of clutch size and hatch success suggest that around 12,155 turtles hatched from nests on Klein Bonaire and Bonaire in 2017, including approximately 7,988 hawksbills, 2,033 loggerheads, and 2,134 green turtles. Sea turtle nesting activities across Klein Bonaire and Bonaire have been increasing since monitoring began in 2002. 

During in-water snorkel surveys, we counted and, when possible, captured green turtles and hawksbills in all regions sampled, including Klein Bonaire, along the west coast of Bonaire, and near the reef bordering Lac. Netting in Lac was conducted in three weekly sessions across the year. The aggregation of green turtles near Lac remains much larger than sites along the west coast, and greens captured there were bigger than conspecifics elsewhere, perhaps a result of the composition and high densities of sea grasses in Lac. 

The total occurrence of fibropapillomatosis (FP) among green turtles captured in nets at Lac declined considerably in 2017, continuing the downwards trend observed in 2015. This year only 5.5% of green turtles captured in and around Lac had visible FP tumors. 

STCB co-authored an important research paper in 2017: “Ecological regime shift drives declining growth rates of sea turtles throughout the West Atlantic” together with researchers led by Karen A. Bjorndal.

During 2017, there were 35 sea turtle hotline incidents reported, 32 of which were directly related to turtles in trouble; one involved the general public harassing sea turtles; and one call was related to poaching. The fishing industry and associated by-catch, one of the biggest threats Caribbean-wide, was implicated in approximately 23% of the turtles in trouble. A total of nine incidents were related to a large mass of seaweed sargassum that drifted inside the Lagoen and Sorobon area at the end of December. In the open ocean, these floating mats are extremely diverse, providing important habitat for over 250 species of fish and invertebrates, many of which are not found anywhere else. Young sea turtles often spend their tender years finding refuge and a plentiful food supply in these floating seaweed mats. However, when it enters coastal areas and starts rotting, it can cause mortality. Unfortunately, climate change has brought warmer temperatures, which increases algal growth rates, and possibly stronger currents/shifting currents, which combined with more and more land-based nutrients flowing into our oceans, are thought to be the reason why we are seeing more and more massive ‘strandings’ of these floating seaweed mats.

Sea Turtle Conservation Bonaire (STCB) has been protecting sea turtles on Bonaire since 1991, so this year represents the 25th Anniversary of our organization. In 2002 we standardized our research and monitoring efforts, following the appointment of a Scientific Officer. Annually we monitor our nesting beaches around Bonaire, conduct intensive in-water netting and snorkel surveys (capture-mark-recapture), and we regularly track sub-adult and post-breeding migrations using satellite telemetry. These techniques provide us with a better understanding of Bonaire sea turtles’ breeding success, abundance, health, growth rates, migratory paths and distant feeding grounds, residency duration, habitat quality, and threats. 

Sea Turtle Conservation Bonaire (STCB) was formed in 1991 in order to protect the island’s marine turtle populations. In 2002 we standardized our research and monitoring efforts, following the appointment of a Scientific Officer. Annually we monitor our nesting beaches around Bonaire, conduct intensive in-water netting and snorkel surveys (capture-mark- recapture), and track post-breeding migration using satellite telemetry. These techniques provide us with a better understanding of Bonaire sea turtles’ breeding success, abundance, health, growth rates, migratory paths and distant feeding grounds, residency duration, habitat quality, and threats.

During the 2015 season, we recorded 56 nests at our index beach on Klein Bonaire. Total hawksbill (46) and loggerhead (10) nests documented there were once again similar to numbers observed during recent years. Across Bonaire and Klein Bonaire, we observed three sea turtle species crawling 233 times, which includes 77 confirmed or suspected nests. Eight green turtle nests were recorded in northeastern Bonaire with a further five green turtle nests on Klein Bonaire. Hawksbills and loggerheads exclusively nested on Klein Bonaire and the beaches of southern Bonaire. Total nesting activities peaked from mid-June through to mid-September, with nests being laid between April and the end of December 2015.

We continue to be concerned about false crawls (unsuccessful nesting attempts) for both hawksbills and loggerheads as we again documented a much higher number of false crawls in 2015 than in 2013 when our concerns first developed. This phenomenon may result from a small number of individuals which were inefficient nesters (i.e., false crawled multiple times before successfully laying a nest), disturbance to turtles during nesting, and/or indicate deterioration in the quality of particular nesting sites, perhaps due to factors such as presence of an invasive vegetation species on our index beach. Estimates of clutch size and hatch success suggest that nearly 8,170 sea turtles hatched on the beaches of Bonaire and Klein Bonaire during 2015, including some 5,170 hawksbills, 1,700 loggerheads and 1,300 green turtles.

During in-water snorkel surveys, we observed and captured green turtles and hawksbills in all regions sampled, including Klein Bonaire, along the west coast of Bonaire, and near the reef bordering Lac. This year netting in Lac was conducted in four weekly sessions across the year, as a result of research in 2014 that suggested this may be a more effective sampling strategy. The aggregation of green turtles near Lac remains much larger than sites along the west coast, and greens captured there were bigger than conspecifics elsewhere, perhaps a result of the composition and high densities of sea grasses in Lac.

The total prevalence of fibropapillomatosis (FP) among green turtles captured in nets at Lac and Lagoen marginally declined in 2015 for the first time in five years, although there were still nearly a third of captures in the nets observed to have external tumors present (30%). The highest incidence was at Lagoen where six of the seven green turtles captured in the net there were observed with external FP tumors.

During 2015 STCB began the fieldwork component of a five-year research program in partnership with a research team funded by the Dutch National Research Organization (NWO) to study connectivity among sea turtles between the Dutch Caribbean Islands. As part of this research, satellite tags were fixed to four sub-adult green turtles; 19 nests were equipped with temperature loggers; and a long term experiment was set up in Lac Cai to learn more about turtle grazing and seagrass productivity there.

There were 27 sea turtle hotline stranding incidents reported during 2015, involving 35 individual sea turtles. Once again, one of the biggest threats Caribbean-wide to sea turtles was the fishing industry and associated by-catch. There were two turtles in trouble in separate incidents requiring rehabilitation early in 2015, including an olive ridley sea turtle which was successfully returned to the wild post rehabilitation.