The Olive Ridley Project, Plymouth, Devon, United Kingdom
The Maldives comprise 1,192 islands that form two narrow chains of atolls 820kilometres in length and 130 kilometres wide. The atoll chains run North-South across the East–West currents of the Indian Ocean and act as a trap for flotsam. Lost, abandoned, or discarded fishing nets (otherwise known as ghost nets) are often part of this floating debris. According to the UN, around 640,000tons of ghost gear is discarded into the world’s oceans every year. Ghost gear can be found in all of the planet’s oceans, lakes, rivers, and seas, and has a devastating effect on marine animals and their environment. Millions of marine mammals, turtles, seabirds, and other species have been injured or killed by entanglement in, or ingestion of, ghost gear. Hundreds of ghost nets are found in the Maldives alone every year. Fishing nets made of polypropylene or polyethylene is less dense than water and can float in currents for years (or even decades) catching vulnerable marine animals and mixing with other marine debris in oceanic gyres (Macfayden et al.,2009).
The Maldives’ dominant fishing techniques are pole-and-line and hand-line, neither of which use nets; therefore, it is unlikely that ghost nets are generated in the Maldives (unless they come from illegal fishing operations), and most likely originate in neighbouring countries or in international waters. Currently there is a lack of information on the amount and types of ghost gear generated annually in the Indian Ocean, from what countries and fisheries it originates, and the interactions between sea turtles and ghost nets. Despite global efforts to remove and research ghost nets, the issue remains largely unresolved (e.g. Butler et al., 2013; Timmers et al., 2005; Wilcox et al., 2012). Here, we present the results of a two year study on sea turtle entanglements in ghost gear in the Maldives and give recommendations for further studies into ghost net-sea turtle interactions in the Indian Ocean.
The Maldives, situated in the middle of the Indian Ocean, is dominated by two major monsoons (or seasons). The South West monsoon (SW or Summer Monsoon) lasts from May to October, and the North East monsoon (NE or Winter Monsoon) occurs between December and March. April and November are normally transitional periods of unsettled weather between monsoons. During these periods the winds and oceanic currents reverse directions. Currents flow mainly eastward during the summer monsoon (SW) and westward during the winter monsoon (NE) (Molinari et al., 1990; Shankar et al., 2002).
Ghost nets were collected from beaches, reefs, and open water throughout the Maldives. Some nets were recorded by citizen scientists (including recreational divers, snorkelers, local community members, and tourists) but trained marine biologists permanently stationed in the Maldives collected most of the nets. With over 100 volunteers dispersed throughout the Maldives, nets were discovered and removed in an ad hoc fashion as opposed to by systematic surveys. However, utilising a team of trained volunteers allowed us to extensively increase the amount of geographical area that could be covered in a country where it is otherwise logistically difficult to collect data. Volunteers followed a standardized protocol for removing and collecting data on ghost netsthat was designed by the Olive Ridley Project (ORP) and the International Union for Conservation of Nature (IUCN). The protocol was based on the WWF Net Kit first published in 2002 at http://awsassets.wwf.org.au/downloads/mo001_the_net_kit_1dec02.pdf.
When sea turtles were found entangled in the recovered ghost nets, their curved carapace length (CCL) was either estimated to the nearest 5cm (when removal from the water was not possible) or accurately recorded when removal of the turtle from the water was feasible. Miller (1997) estimated the CCL at maturity of a female olive ridley sea turtle in the Indian Ocean to be 66 cm, while Pandav et al. (1997) noted that most turtles nesting on the east coast of India had CCLs larger than 66 cm, with only a few nesting females measuring less than 60 cm. For the purposes of this study, we have assumed that individuals of 60 cm CCL or larger are mature adults. The species of turtle was either confirmed by the person that disentangled the turtle (if the source was deemed by the authors to be reliable) or it was confirmed by the authors from photographs. The authors recognize that gaps in their data exist, as all ghost net recoveries occurred within a few hundred meters of shore. Discussions with Maldivian fishermen revealed that they regularly see ghost nets (up to 15 per day were reported) floating offshore and many of these nets have turtles trapped inside them. However, logistical difficulties exist in accessing remote or offshore areas of the country. The authors realize that many ghost nets and entangled turtles likely float by unnoticed and unrecorded and that the results presented here represent only the minimum number of turtle entanglements that have occurred in the Maldives over the last two years.
A total of 174 sea turtles were found entangled in ghost nets in Maldivian waters between 01 June 2013 and 30 June 2015. Species specific capture rates were: Chelonia mydas (n=2), Eretmochelys imbricata (n=6), Dermochelys coriacea (n=1), and Lepidochelys olivacea (n=163) (Figure 1). A further two individuals were found entangled in ghost nets, but the species could not be confirmed as no photos were taken. Olive ridley turtles accounted for 94% of all entanglements during this time period. An additional eight olive ridley turtles were found with injuries indicative of previous entanglement, such as amputations of flippers or strangulation marks around the neck, but not actually entangled in a ghost net.
Figure 1. Number and species of sea turtles found entangled in ghost nets in the Maldives between 01 June 2013 and 30 June 2015.
Olive ridley entanglements
Juvenile turtles appear to be at the highest risk of entanglement, with 104 found entangled (64% of all olive ridleys recovered) compared to 26 (16%) females and only two (1%) males (Figure 2).
Figure 2. Age and sex class of olive ridley turtles found entangled in ghost nets in the Maldives between 01 June 2013 and 30 June 2015.
Numbers of recorded entanglements were plotted against time in an attempt to identify peak periods and recognize seasonal variations (Figure 3). During the NE Monsoon,100 olive ridley turtles were recorded (61% of total) with peak incidences in January (Nt=38)* and March (Nt=30). Comparatively, during the SW Monsoon, 35 turtles were recorded entangled (21% of total). The inter-monsoon months of April and November collectively accounted for 28 (17%) entanglements.*Nt= total number during a particular monthbased on data from 2013-2015
Figure 3. Number of olive ridley turtles per month found entangled in ghost nets in the Maldives between 01 June 2013 and 30 June 2015.
Records of entanglements appeared to be concentrated in three atolls (Baa, North Male, and Ari) (Figure 4), but the authors believe that this is because of the higher density of trained volunteers in these atolls.
Figure 4. A density map of all of the turtles found entangled in the Maldives between 01 June 2013 and 30 June 2015. Red represents high concentrations while green represents low concentrations. Higher densities of trained volunteers in certain areas may skew the data.
Of the entangled olive ridley turtles, 20 were either found dead or died shortly afterwards at a rehabilitation centre. The authors recognize that the remains of dead turtles may have been removed from ghost nets by predators and, therefore, may be under-represented in the dataset. As of July 2015, eight olive ridleys that were found entangled during the study period were currently undergoing rehabilitation or suffered injuries too debilitating to be released back into the wild. The remaining 83% of ORTs were either released immediately after disentanglement or were released back into the wild after a period of rehabilitation.
Olive ridley turtles
There have been no confirmed reports of nesting olive ridley turtles in the Maldives, although one live hatchling was found on Baa Atoll in 2007 (G. Stevens, pers. comm.) and there have been two reports of false crawls in Baa Atoll by one author (JAH, pers. obs.). Olive ridleys are very rarely observed on near-shore Maldivian reefs. Despite this, they are the most common species found entangled in ghost nets in the Maldives. Often, recovered individuals are severely emaciated, dehydrated, and suffering from injuries such as complete or partial flipper amputations and strangulation. Individuals have also been found in nets with flipper amputations that had begun to heal. These injuries suggest that the turtles spend days, if not weeks, entangled and floating in ocean currents before drifting near shore. The true number of entanglement incidences throughout the Indian Ocean is unknown, but likely to be much greater than what is reported here. These data only represent a small proportion of the true number of entanglements, as most go unnoticed or unreported. Our study suggests that if olive ridley turtles are recovered within weeks of entanglement, most can be released either immediately or after a short period of rehabilitation. However, the fate of turtles released from ghost net entanglements remains unstudied.
We can confirm that juvenile olive ridleys are at the highest risk for entanglement in ghost nets. Unfortunately, still little is known about this life stage of olive ridley turtles. It is believed that their early nomadic years are spent drifting with ocean currents (Shenoy et al., 2011). Adults are also highly migratory and spend much of their life in the open ocean. Pitman (1990) noted that the species are often seen investigating, or associated with, flotsam in the eastern tropical Pacific. Olive ridley turtles also have a habit of basking at the sea surface (Pitman, 1993), which may make them susceptible to boat strikes and entanglements.
The main breeding period for olive ridleys in India and Sri Lanka is December through April, with nesting peaking in February and March (Frazier, 1987; Pandav & Choudhury, 1998). This coincides with the NE Monsoon and the time of year when the most entanglements were recorded. Males are rarely encountered entangled in ghost nets, but the authors hesitate to speculate why this is.
Hawksbill, green and leatherback turtles
The only report of a leatherback turtle entangled in a ghost net was reported in video format with a leatherback of unconfirmed size swimming towards the cameraman while towing a large ghost net behind it. A tiger shark was shadowing the turtle, and the divers were unable to follow to disentangle the animal. The fate of the turtle remains unknown.
Hawksbill and green turtles are commonly sighted on reefs in the Maldives and nesting of both species occurs frequently (Frazier et al., 1984). Hawksbill and green turtles spend much of their time in shallow reef or sea-grass environments, so are generally only at risk of entanglement when ghost nets drift close to shore. The threat of entanglement to turtles in near-shore environments is usually short-lived as nets are either washed on to the beach, removed by humans, or snag on a shallow reef. This could explain why fewer hawksbills and greens are found entangled in ghost nets compared to olive ridley turtles, which likely encounter more ghost nets in their pelagic habitats. The threat of entanglement for an adult green or hawksbill turtle would increase during their breeding migration, when they travel hundreds to thousands of kilometers across open ocean.
The problem of ghost nets and their interactions with sea turtles remains undefined. A critical lack of evidence further complicates this issue. We recommend improved and increased efforts in data collection and improved collaboration between organisations and individuals focused on ghost net and turtle entanglements. Adult male turtles are rarely found entangled and their migratory routes or behavioral choices could give clues as to why they manage to avoid entanglements. Satellite telemetry comparing the migratory routes of adult males, adult females, and juvenile turtles could identify hotspots for ghost net encounters. Understanding where juvenile olive ridley turtles travel after being released from entanglement may identify areas in the region where juvenile turtles may be congregating. Most of the turtles were disentangled from what the authors defined to be gill nets and we would, therefore, recommend an immediate reduction in gill net fishing in the region.
That 83% of the entangled olive ridleys were released back into the wild either immediately after disentanglement or after a short rehabilitation period demands more response to the growing problem of ghost gear in the Indian Ocean. The lives of many turtles could be saved if the turtles are found within weeks of initial entanglement. The problem of ghost gear needs to be brought to the attention of the global public and residents of the Indian Ocean need to be aware of how their actions contribute to the problem.
Between 01 June 2013 and 30 June 2015, 182 sea turtles were recorded entangled in ghost nets or suffering from injuries consistent with being previously entangled in a net in the Maldives. A team of trained volunteers following a protocol designed by ORP and IUCN recorded sea turtle entanglements in ghost nets in an ad hoc fashion. Olive ridley turtles made up 94% of the turtles entangled and juvenile turtles were the most common life-stage recorded (64% of captures for the species). Entanglements peaked during the NE Monsoon, which coincides with the olive ridley breeding season in India and Sri Lanka.
Ghost nets are a threat to all species of turtles in the Indian Ocean and encroach on a number of marine habitats. However, they appear to be the most dangerous to turtles in the open ocean where they may be seen as a source of shelter or food by passing animals.
The authors highlight that the number of entanglements reported here represent only a fraction of the true number, as most entanglements likely go unnoticed or unreported. We recommend that the problem of ghost gear and its impact on marine turtle populations be brought to the attention of the general global public and that there be improved efforts in data collection in order to properly quantify the problem and eventually define mitigation measures.
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