Watson DM (2006) Growth rates of sea turtles in Watamu, Kenya Earth & E-nvironment 2: 29-53 Earth & E-nvironment 2: 29-53 University of Leeds Press Growth rates of sea turtles in Watamu, Kenya Douglas M. Watson School of Earth & Environment, University of Leeds, Leeds, W. Yorkshire LS2 9JT; Tel: 0113 3436461 Abstract Green Turtles (Chelonia mydas) are at the forefront of conservation efforts due to their protected status and this relies on increased knowledge and awareness about them and their characteristics, including growth, from around the world. In Watamu, Kenya on the Western Indian Ocean coastline a capture-mark-recapture (CMR) study was carried out over a period of several years to produce recapture interval data. Growth analysis from changes in length over time found rates are non-monotonic, similar to those observed in the Pacific Ocean and ranged for Green Turtles from 0.468-10.67cm/yr (95% confidence), mean 5.18cm/yr (s.d 2.83). These rates were more rapid than many locations worldwide, with both seasonal and annual variations in growth along with recapture numbers and time. Average growth rates are slightly higher than the second most common species in Watamu: Hawksbill Turtles (Eretmochelys imbricata), which also has protected status. There is evidence for rapid individual increases in size outside the 95% confidence growth rate boundaries. The human measurement error found in association with carapace measurement highlights one of a number of limitations with such research. These findings are fundamental to a greater understanding of lifecycle, growth and morphological changes in the Western Indian Ocean, with limited comparable data from the region. They also have important implications for the successful conservation of not only the species present in Watamu, but also sea turtles in general. ISSN 1744-2893 (Online) © University of Leeds 29 Watson DM (2006) Growth rates of sea turtles in Watamu, Kenya Earth & E-nvironment 2: 29-53 1 Introduction The existence of sea turtles on earth stretches back in history to the cretaceous period with them generally found on the coast lines of mid latitude countries where they periodically visit beaches to forage and nest. Increased pressure from fishing with more technologically developed methods and impacts from urbanisation has led to a decline in populations. Hence more recently this has resulted in them being at the forefront of conservation efforts to help restore this decline to their pre-exploitation levels. Life History research is imperative to inform management techniques aimed at aiding population recovery. A number of projects have strived to achieve this with respect to the Green Turtle (Chelonia mydas) which was recognized as being globally under threat in 1990 (National Research Council, 1990). It is now listed as endangered by the World Conservation Union (IUCN; Hilton-Taylor, 2000) with populations estimated to have declined by 37-61% over the last 141 years (Seminoff, 2002). Consequently a substantial time and effort has been put into further understanding the species with specific focus on the quantification of growth of Green and Hawksbill Turtles (Eretmochelys imbricata) under natural conditions, normally with the use of capture-mark-recapture (CMR) programmes. The Hawksbill Turtle’s status is Critically Endangered (IUCN; Baillie and Groombridge, 1996) with further research being now fundamental to its future. One such study has been undertaken in Watamu Kenya, where for the last 7 years the local Non-Governmental Organisation (NGO) Watamu Turtle Watch (WTW)/ Local Ocean Trust (LOT) has been working with local people to increase educational awareness and help conserve populations. Analysis from this location is imperative due to the limited quantification of growth rates in Western Indian Ocean sea turtles. The aim of this study was to investigate the growth of sea turtles, specifically Green Turtles, in Watamu Kenya, with the use of a mixed-longitudinal capture-mark-recapture scheme. To achieve this, the following objectives were set: • Analyse growth data for Green Turtles in Watamu to determine how rates vary throughout the lifecycle, and compare the rates to corresponding values found in locations worldwide. • Quantify the human measurement error associated with the longitudinal study and asses how much impact this has on the findings. • Identify any variations in annual recapture rate/growth and evaluate the success of the CMR programme. • Compare Green Turtles growth rates of to those of Hawksbill Turtles and attempt to estimate the age of the Green Turtles using a recognised growth function. 2 Literature review 2.1 Historical context The importance in this specific area of sea turtle research is that an understanding of growth and growth-pattern variation helps develop an understanding of potential rates of population decline and recovery (Zug et al., 2002). This can help lead to successful conservation measures like one seen in Hawaii where there has been a significant increase in numbers of Green Turtles following the implementation of strategies stretching back to the 1970’s (Balazs and Chaloupka, 2004a). Further evidence that this is not just simply natural multi-decadal variability comes from the fact that the initial number recovery coincided with the Endangered Species Act 1978 (Hays, 2004). This came centuries after the first conservation legislation in the world against the killing of sea turtles below a specific size, that originated from the Bermuda Assembly in 1620 (Bustard, 1972). Consequently it can be seen that there is a long term emphasis on sea turtle conservation which can only continue to gather momentum with increased knowledge of such species. 30 Watson DM (2006) Growth rates of sea turtles in Watamu, Kenya Earth & E-nvironment 2: 29-53 2.2 Study site Watamu is a small coastal fishing town found on the eastern coast of Kenya, East Africa. Situated around 20km south of Malindi, its coast is part of the wider Watamu/Malindi Marine Parks and Reserve (WMMPR) and has a separate designated Marine Park around the shoreline encompassing three bays. The town is small, friendly and consists of numerous houses, shops, stalls, hotels and amenities spread along the sea front. 2.3 Site context It has been recognized for years that there are excellent reefs and turtle populations present on the east coast of Kenya around Watamu and Malindi (Frazier, 1995). Consequently Watamu Turtle Watch (WTW) was formed in 1997 following years of work by local naturalist Barbara Simpson to help conserve the local marine ecosystem. WTW specifically focuses on research and conservation of a variety of sea turtle species that regularly forage and nest in the area. In 2002 WTW expanded and formed the Local Ocean Trust (LOT) with a broader emphasis help conserve the WMMPR. Other LOT activities include an environmental education program which includes an information centre in the town centre and scheduled events with schools and local groups. Activities include working with the local people to inform and educate them of the importance primarily the turtles have, and their impact on the greater ecosystem. Issues related to over fishing in inshore waters, the use of illegal fishing methods impacting on the turtle populations, pollution, marine destruction and uncontrolled development are being tackled in the area (WTW, 2006). The education programme is closely connected to a cash reward scheme where fishermen present their turtle catch for measurement, tagging and release with the reward greater for a larger older turtle. This allows over time a CMR scheme to be in effect compiling data about individual turtles. 2.4 Capture-Mark-Recapture (CMR) schemes Schmidt (1916, in Balazs, 1995) carried out the first recognized attempt to quantify growth and population in the Virgin Islands using the CMR technique; previously they had not been seriously attempted due to the challenges posed with actually capturing the immature turtles (Balazs, 1995). Presently the main challenges to these studies include the fact that sea turtles are relatively slow growing, resulting in long term labour and time investment and the likelihood of recapturing individuals is relatively poor due to the nature of their movements and migration routes (Bjorndal et al., 2001). However, when successfully carried out over a period of time results from CMR studies produce direct growth measurements which can be used to model population and age specific growth rates (Lutz and Musick, 1997). Unfortunately it is difficult to measure growth in the early size range (hatchlings) in the wild due to the high natural mortality (Bjorndal et al., 2001) and the prolonged time they spend at sea before returning to foraging grounds to grow to maturity (Musick and Limpus, 1997). The juvenile to nesting stages are where most recaptures take place. Furthermore anthropogenic impacts in terms of mortality of juveniles and adults (Bjorndal et al., 2001) for meat and trade compounds the already difficult task of measuring growth in wild reptiles. In order to carry out morphometric measurements and to mark the turtles they must be actually caught, with a number of techniques used in the past. Common methods include scoop nets, tangle nets, basking, (Balazs, 1995) and bullpen or pound nets (Balazs and Chaloupka, 2004b). Also hand capture from a skiff, land (Green, 1993) or small boats and snorkelling/scuba at night (Balazs et al., 1996; Balazs and Chaloupka, 2004a) has proven to be effective. Finally, capture methods can include using turtle rodeo (Limpus and Reed, 1985) which involves jumping on them from the bow of a boat after a pursuit. Recapturing individuals over time enables profiles to be built up of morphometric data including straight carapace length (SCL) or curved carapace length (CCL) along the midline of the carapace. Also straight carapace width (SCW) or curved carapace width (CCW) at the greatest 31 Watson DM (2006) Growth rates of sea turtles in Watamu, Kenya Earth & E-nvironment 2: 29-53 point of width (Balazs, 1995) are taken at first and subsequent recaptures (Lutz and Musick, 1997).