Aspects of the reproductive biology and early life stage survival of sea turtles

Nicolas J. Pilcher

Most species of marine turtles have declined in abundance and are threatened with extinction. This thesis describes aspects of the reproductive biology of three distinct green turtle (Chelonia mydas) populations and two hawksbill turtle (Eretmochelys imbricata) populations, spanning three different geographical regions: the Saudi Arabian Gulf, the Red Sea, and the Sulu Sea off Sabah, Malaysia. It also describes research into specific life history stages and their related conservation issues. This information is fundamental to enhancing the conservation of these populations and may be useful to the conservation of these species elsewhere. A total of 1391 green turtle females and 33 males were tagged in the Arabian Gulf, 194 females were tagged in the Red Sea, and 2063 females were tagged in Sabah. Turtles were studied on the Saudi Arabian Gulf islands during the nesting seasons in 1991 and 1992, at Ras Baridi on the Saudi Arabian Red Sea between 1989 and 1992, and on the most important turtle rookery in Sabah, Malaysia, during 1996 and 1997, along with records of eight years of pre-existing data (1990 to 1997). The reproductive biology of hawksbills nesting on four Arabian Gulf islands during the 1991 and 1992 seasons was examined, and the initial effects of the 1991 oil spill were investigated. A second population of hawksbills on Pulau Gulisaan, Sabah, was studied between 1996 and 1997, and long-term records collected during 1985 to 1997 were analysed, during which nearly 2200 hawksbill females were tagged. In the Arabian Gulf, greens turtles shared the beaches with hundreds of hawksbill turtles on four small islands. Karan island hosted the largest green turtle population, while Janaisland hosted the largest hawksbill population. On these islands terrestrial predation was virtually non-existent, and Saudi nationals did not eat turtle eggs. However, foreign fishing crews were found to collect eggs illegally. The squid Sepioteuthis lessoniana was observed to prey on hatchlings as they entered the sea. The 1991 oil spill did not have any apparent effects on adult nesting behaviour, with most parameters being similar to other populations world-wide. The immediate effects on eggs and incubation were not apparent, but it is believed that future populations may be prone to effects of hydrocarbon accumulation, which might possibly lead to reduced growth rates and infertility. In the Red Sea, most nesting took place at a small rookery called Ras Baridi, where the nesting season extended from July to November with a peak during August. Most terrestrial predation on hatchlings and eggs was by foxes (Vulpes rueppelli) but was not widespread. Similar to the Gulf region, Saudi nationals did not eat turtle meat due to religious custom and rarely collected turtle eggs. The major cause of mortality to hatchlings came from cement dust fallout and lights from the nearby cement factory. In Sabah, most turtle nesting took place at the Turtle Islands Park, off the Eastern coast. Over 100,000 green turtles nested at the Turtle Islands Park since 1980. Hawksbills differed in having long individual nesting seasons lasting up to 130 days, although a significant number of turtles nested for periods lasting up to 24 months without an apparent remigration interval. A new predator was the Black-capped Kingfisher (Halcyon pileata), which preyed on hatchlings caught in hatchery enclosures. Nesting populations were believed to be under threat from the trawl fishery operating nearby. In general, all five nesting populations nested at approximate two-week intervals with the exception of the Sabah hawksbills, of which a significant proportion nested at 30-day intervals. This population was also distinct in having long (4-6 month) nesting seasons and short (< 2 years) inter-season (remigration) intervals. In general green turtles remigrated after 1-3 years. Morphometrically, both the Saudi Arabian and the Sabah adult turtles were among the smallest in their respective regions, but eggs and hatchlings were among normal global ranges. Cement dust and lighting at Ras Baridi had an adverse impact on hatchling survival. These impacts were investigated in relation to the unique nature of the Ras Baridi population and the importance of habitat protection. Emission of cement dust since I 979 was estimated to be over 120 tonnes output daily, which had accumulated to depths of up to more than 100 cm. A relatively hard dome was created above the clutch during the 'filling-in' process and during incubation, resulting in less than 40% survival of total egg deposition. Eggs were found to develop naturally but not hatch. Conservation and management practices raised emergence success to more than 85 % by marking all nests and partially excavating these after the incubation period. Additionally, lights attracted hatchlings away from the shoreline and up to IO per day were found desiccated along the factory's perimeter, which in addition to the low emergence success, further reduced the reproductive potential at the site. Shielding the security lights and switching off unnecessary lights would greatly reduce these negative effects on hatchling orientation. In Sabah, the turtles' open sea finding mechanism was believed to vary from normal practices as the hatchlings must account for the numerous small islands and reefs on their offshore migration, and face higher rates of predation than elsewhere in the world. Hatchlings were fitted with harnesses and lighted floats and were followed as they left the natal beach. Swimming speed and offshore headings were noted in each trial and the predation rate and location were noted. Hatchlings were found to orient in the nearshore in the absence of waves, but suffered 40 - 60 % mortality within the first two hours at sea before the 10 m depth contour. The management implications of this research are discussed, in particular as they affect conservation programmes that include the use of hatcheries. It was believed that releases toward direct, straight-line courses to the Sulu Sea may improve survival rates, and releases at random locations would deter nearshore predation. Turtle hatcheries have been established in many countries, but the protocols employed at these hatcheries typically do not allow for the immediate release of newly emerged hatchlings. It was not known whether prolonged captivity would reduce swimming performance and therefore elevate mortality from predation. Swimming speed and style were monitored in a raceway system that catered for the hatchling' s normal swimming characteristics. These controlled laboratory experiments indicated that swimming performance decreased with prolonged captivity, dropping by over 11 %, from 0.494 m·sec·' to 0.701 m·sec·', with six hours of detention. Average swim rates and swimming style during the tests varied with prolonged captivity (x = 0.578 m·sec·1n = 609) which, coupled with decreased swimming distances, could have negative effects on overall hatchling dispersion patterns and survival rates. In terms of conservation and hatchery management practices, it was concluded that enforced captivity decreased the chances of survival as compared with immediate releases. This finding is of great importance to hatcheries across the globe, where delayed releases of hatchlings is commonplace. Our understanding of the long-term trends in populations sizes for the Saudi Arabian turtle populations is lacking. The data collected during these studies provide an initial assessment of the status of each population and a baseline upon which future findings may be compared. Similarly, the data on turtle morphometrics, clutch sizes and frequency, and hatchling production will complement the growing database on populations across the globe. The effects of cement dust had not been documented prior to these studies. Nearshore predation and orientation, although understood, was not known for the Indo Pacific region, neither for multiple small island habitats. The effects of hatchling retention in hatcheries prior to release were not documented, and have proven to reduce hatchling swimming performance. The results of the research into early stage survival of sea turtles will be invaluable in the development and improvement of management and conservation programmes throughout turtles' range.

Aspects of the reproductive biology and early life stage survival of sea turtles

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Aspects of the reproductive biology and early life stage survival of sea turtles

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