1Wildlife Conservation and Management Bureau, Department of Environment, Tehran, Iran
2FLAME University, Pune, India
3Department of Environment, Konarak County, Iran
#amobaraki@yahoo.com
INTRODUCTION
Strandings- in which sea turtles wash ashore dead or alive- can occur for many reasons. These include accidental capture in fisheries (including abandoned, lost or otherwise discarded fishing gear, commonly known as ghost gear), and shark control gear, vessel strike, ingestion of foreign materials such as plastics, disease, illegal take, development activities such as dredging, freshwater discharge into marine environments after intense rainfall, severe storms, and low water temperatures resulting in cold stunning (Pandav et al., 1997; Orós et al., 2005; Chaloupka et al., 2008; Tómas et al., 2008; Casale et al., 2010; Corsini-Foka et al., 2013; Poli et al., 2014; Behera et al., 2016; Nicolau et al., 2016; Flint et al., 2017; Başkale et al., 2018; Sönmez, 2018; Yaghmour et al., 2018; Cheng et al., 2019; Griffin et al., 2019; Belmahi et al., 2020; Cantor et al., 2020; Kettemer et al., 2022).
Surveys for stranded sea turtles can contribute to understanding species distribution, population structure, shifts in size- and age-classes, scale of local threats, and effectiveness of management actions (see Hamann et al., 2010; Rees et al., 2016). Hence, studying stranded turtles can be useful to understand populations where in-water studies are challenging and additional knowledge is required to guide future research and inform and/or assess potential conservation or management actions.
Figure 1. Location of Bandar-e Tang (arrow in inset map) in a regional context.
Materials and Methods
The Gulf of Oman (also known as the Sea of Oman) is located in the northwestern Arabian Sea and connects the Indian Ocean to the Persian Gulf via the Hormuz Strait. Bandar-e Tang (59.8740° N, 25.3576° E; Figure 1) is a village near Chabahar and one of the most important fishing ports in the Sistan and Baluchestan province of Iran (Iran Marine Statistics, 2016) on this Gulf. The Iran fisheries organisation in Bandar-e Tang reports > 200 boats use the port (Mobaraki, unpubl.).
Low-density nesting of green (Chelonia mydas) turtles has been recorded on the surrounding coastline, and local fishers’ ecological knowledge and scientific field surveys have identified nearby foraging areas for green and olive ridley (Lepidochelys olivacea) turtles. Stranded turtles have also been observed at this location in the past (Mobaraki, 2004) and so it was selected for this study.
A snapshot survey was conducted on 1st May 2021, in which the ~1.5km of sandy beach (Figure 1) adjacent to Bandar-e Tang was physically searched for live and dead stranded sea turtles. Turtles found during the survey were identified to species and sex (by the extension of the tail beyond the carapace; Bolten 1999), and the curved carapace length (CCL) measured with a flexible tape measure to the nearest 0.1cm. Green turtles were assigned to the following size classes based on CCL: juvenile (CCL <55cm), subadult (55-93cm) or adult (CCL >93cm) (see Pilcher et al., 2015; Mobaraki et al., 2020; Al Ameri et al., 2022) and olive ridley turtles as immature (comprising juveniles and subadults; <60cm) or adult (>60cm) (Shanker et al., 2004; Tripathy, 2008; Rees et al., 2012a). Some stranded turtles were photographed on site and the stage of decomposition was later categorised according to Flint et al. (2009). Necropsies were not performed.
Results
Species and size-class
A total of 39 stranded sea turtles, comprising 38 green turtles and one olive ridley turtle, were found on the survey date; all were dead. The majority of green turtles were juveniles (52.6%) (Table 1). The sex of the single adult green turtle could be established (male), but all subadults had short tails and males could not be reliably distinguished from females using external characteristics only. The one stranded olive ridley turtle was a subadult of undetermined sex (Table 1).
Table 1. Species and size classes of stranded sea turtles found at Bandar-e Tang beach.
Carcass condition
Twenty-four dead turtles were photographed to determine stage of decomposition, which ranged from D3 (carcass in fair condition; decomposition with internal organs intact) to D6 (disarticulated bones; no soft tissue remaining), with the majority at stages D4 (carcass in poor condition; advanced decomposition with internal organs falling apart) to D5 (mummified carcass; skin holding bones together) (Table 2; Figure 2). The head, flippers, and/or internal organs had been removed from some carcasses (Figure 2), presumably by scavengers.
Table 2. Stage of decomposition (see Flint et al., 2009) of stranded sea turtles found at Bandar-e Tang beach.
Cause of mortality
None of the carcasses showed macroscopic signs of trauma or injury that could have contributed to the turtle mortality.
Figure 2. Carcass condition from left to right: D3, D4, D5, D6, scavenged. (Photo credits: Asghar Mobaraki)
Discussion
The snapshot survey described in this study reveals variation in the species and size-class of dead stranded turtles at Banda-e Tang beach on 21st May 2022. The stage of decomposition suggests that the turtles stranded as individuals. Potential cause/es of mortality were not established, but the number of stranded turtles (n=39) found on a short length of coast (1.5km) in a single day in comparison to reports (e.g., 4-45 turtles on ~26km over 1-4mo, Lagueux and Campbell, 2005; 69 turtles on ~200km coastline over 3mo, Kannan et al., 2005; 209 turtles on 180km coastline per year, Levy et al., 2015; 63 turtles on >1600km coastline over 2yr, Belmahi et al., 2020) from areas without a large nesting population in the area indicates the need for a longer study to determine the cause of mortality.
Species and size-class
All but one of the 39 stranded turtles were green turtles, with ~70% in the juvenile and subadult size classes. A study of foraging turtles in nearby waters also found predominantly juvenile and subadult green turtles (Mobaraki et al., 2020), potentially the result of size-class partitioning due to habitat requirements and/or risk of predation as observed in other green turtle populations (Bresette et al., 2010). These green turtles are likely part of the Oman nesting stock (Mobaraki et al., 2020). Oman hosts the most important nesting population of green turtles in the northern Indian Ocean (see Phillott & Rees, 2021; Pilcher et al., 2021), with ~7,000 nesting females per annum in 1979 (Ross & Barwani, 1982). However, there have been no recent estimates of the nesting population size and the population trend over time is unknown (Willson et al., 2021). Green turtles in the northern Indian Ocean are categorised as Vulnerable on the IUCN Red List (Mancini et al., 2019), and the loss of these immature green turtles should be considered in context of the regional population status.
Only one olive ridley turtle was found at Bandar-e Tang beach, which is unsurprising as reports of the species from Iranian waters in the Gulf of Oman are less common. Foraging individuals have been observed (Kami, 1997) and recorded as bycatch (Mobaraki, 2004; Mobaraki & Abtin, 2021), and a few post-nesting olive ridley turtles have been tracked on their return breeding migration from Oman into the Persian Gulf (Rees et al., 2012b). Hence, the single dead olive ridley turtle at Bandar-e Tang beach likely represents the low abundance of this species in local waters and not a lesser vulnerability to bycatch.
Similarly, the absence of stranded loggerhead (Caretta caretta) turtles likely reflects low numbers of foraging loggerheads in the Gulf of Oman (Baldwin et al., 2003) and post-nesting loggerhead turtles migrating from Oman into the Gulf of Oman and Persian Gulf (Rees et al., 2010). No stranded hawksbill (Eretmochelys imbricata) turtles were found during this snapshot survey. Hawksbill turtles are the most common nesting sea turtle in countries bordering the Persian Gulf (e.g., Iran, Kuwait, Qatar, Saudi Arabia, United Arab Emirates; see Phillott & Rees, 2021) but this species does not often migrate through the Hormuz Strait (Pilcher et al., 2014). A large population of hawksbill turtles nest on the Dimaniyat Islands of Oman in the Gulf of Oman and turtles forage around the same islands (Willson et al., 2021), but there are no in-water or nesting reports of the species from the Iranian coast of the Gulf of Oman (Mobaraki, 2004).
Carcass condition
Time of death cannot be conclusively established from carcass condition (stage of decomposition) due to variables like submersion period and drift duration, temperature, predation etc (see Santos et al., 2018; Schultz et al., 2022). The varied stages of decomposition among stranded turtles in this study from D3 to D6 suggest that mortality was likely due to individual events and not an anthropogenic activity or environmental factor that resulted in mass mortality.
Cause of mortality
As green turtles have a Type 2 life history pattern, moving from oceanic to neritic habitats at a size of ~20-35cm CCL to complete their development (Bolten, 2003), the immature turtles found in this survey were more likely to encounter the cause of their stranding in comparatively shallower waters, such as coastal fisheries. Sea turtle mortalities are often attributed to interactions with fisheries, which is one of the five major threats to the taxa worldwide (Mast et al., 2005) and a major threat in the region (Phillott & Rees, 2021), and drowning after entanglement or hooking in fishing gear is the most likely cause of mortality of the turtles found in this study. However, strandings should only be conclusively attributed to fisheries if indicative signs of entanglement, hooking, drowning, or decompression sickness are found (see Phillott & Godfrey, 2019). Any subsequent studies can look for expanded lungs containing fluid, and thick white /pale pink foam being expressed from the nostrils or present in the trachea, bronchi or lungs in fresh carcasses to indicate drowning (Wolke & George, 1981; Stacy et al., 2017), or fishing line protruding from the mouth and/or cloaca, or puncture wounds on the shoulder and/or neck as evidence of hooking (Watson et al., 2005; Archibald & James, 2018).
The cumulative contribution of coastal and small-scale fisheries to bycatch can exceed that of industrial fisheries (Peckham et al., 2007; Alfaro-Shigueta et al., 2011; López-Barrera et al., 2012). Iranian fishers in the coastal and near offshore national waters of the Gulf of Oman mainly operate small to medium-sized motorised wooden vessels locally called lench or fiberglass vessels. Depending on the species and season, drift nets, gill nets, and long lines are usually used to target mackerel species such as Spanish mackerel (Scomberomorus commerson), Indo-Pacific king mackerel (Scomberomorus guttatus), tuna species including longtail tuna (Thunnus tonggol) and yellowfin tuna (Thunnus albacares), and black promfret (Parastromateus niger). Depending on the target species and vessels, different types and sizes of nets are used. Mesh size of gill and drift nets is up to 15cm and the length may be >10km, while longlines can exceeds hundreds of metres (Iran Marine Statistics, 2016). Sea turtles worldwide are known to be vulnerable to bycatch in these type and dimensions of fisheries gear (see Lewison et al., 2013).
If stranding at Bandar-e Tang beach are the result of interaction with fisheries, the mortality probably reflects individual bycatch events and not mass-bycatch events, as observed in the Odisha state of India (Pandav et al., 1997; Behera et al., 2016). Sea turtles are classified as ‘Endangered’ in Iran and deliberate catch and consumption of bycatch is forbidden and can result in a heavy fine (Department of Environment, 2004). As a result, most fishers check their nets frequently to reduce the risk of accidental capture and drowning (Mobaraki, pers.obs.) and bycatch turtles are released at sea or, if dead, left on the landing beach such as the location of this study. There are no seasonal or spatial closures to limit fisheries activities during the sea turtle nesting season or in foraging areas due to knowledge gaps about the ecology of sea turtle in Iranian coastal waters (Mobaraki, 2021).
Studies on a larger temporal and spatial scale are required to identify the cause/s of sea turtle strandings on the Iranian coast of the Gulf of Oman and determine if conservation actions are required. If field work for this purpose is challenging, then interviews to collate fishers’ ecological knowledge, participatory mapping, and confirmation of species through fishers’ photographs of bycatch turtles would provide further information about resident and bycatch species, size class, and sex, preferred habitats, population trends, and threats (Phillott & Chandrachud, 2021).
CONCLUSION
A snapshot survey of the 1.5km beach at Bandar-e Tang, Iran, found 39 dead sea turtles, most of which were juvenile and subadult green turtles. This reflects the species and size-class of sea turtles found in local waters. The most likely cause of mortality was drowning after entanglement in fishing gear, although this was not conclusively determined. As the turtle carcasses were at different stages of decomposition, it is unlikely that the turtles died en masse. More information about the sex and cause of death of dead turtles could be obtained through necropsies by trained personnel. Interviews with local fishers could also reveal bycatch rates and the ecology of local sea turtles. Further study on a larger temporal and spatial scale is recommended to assess the size of this potential threat and determine if conservation action is required.
ACKNOWLEDGEMENTS
We thank the provincial office of Department of Environment in Sistan and Baluchestan for their kind support of the field visit, and local people for their assistance. ALan F. Rees prepared Figure 1.
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