By Andrew Smart, Conservation and Science Senior Manager.
Since it is National Marine Week (22 July to 6 August, organised by the Wildlife Trusts), it is appropriate to return to our only marine reptile species, the marine turtles, last covered by Croaking Science in April 2021: Should we count marine turtles as members of the British fauna? One species, the leatherback turtle (Dermochelys coriacea), is considered to be a member of the British fauna, while the other species, perhaps best described as hard shelled turtles, are vagrants who have moved away from their normal migration paths and, one way or another, end up stranded on UK beaches Turtles stranded on UK beaches after storms send them off course.
These strandings are generally of loggerhead turtles (Caretta caretta) and more recently, Kemp’s ridley, (Lepidochelys kempii). Occasionally green turtle (Chelonia mydas) and very rarely, hawksbill (Eretmochelys imbricata) and olive ridley turtles (Lepidochelys olivacea) (both single records) are washed up on our beaches1. Loggerheads and Kemp’s ridley originate from the Caribbean, most likely from nesting beaches on the Florida coastline and Gulf of Mexico, from where hatchlings travel across the Atlantic. The hatchlings spend their early years, often called the ‘lost years’, in the North American basin and many move into the North Atlantic gyre2, travelling across the ocean from east to west, then follow the current south past the Azores and Canary Islands before travelling westward back towards their nesting beaches as juveniles3.
Loggerheads are sometimes killed after interactions with boats, caught in longline fisheries4, tangled in pot lines or in ‘ghost nets’, but many animals suffer from ‘cold stunning’ when they move out of warm water into colder temperatures (10 – 15oC) that cause them to effectively ‘shut down’5. Turtle hatchlings will die at temperatures below 10oC6. Animals that have been ‘cold stunned’ or died are stranded along European coasts and are collected and examined. Any UK strandings of turtles (or any other marine animal) can be reported to the Marine Strandings Scheme (or in Scotland SMASS). Many stranded dead turtles have been found to have frequently ingested plastics (69% of individuals examined) which they may mistake for food 7, 8; jellyfish and salps. Animals stranded in the UK tend to be juveniles, with a carapace measurement of less than 40 cm, and tend to be stranded in the winter months1. In the last three decades numbers of loggerhead and Kemp’s ridley turtles have increased, with more than twice the number of animals stranded when compared to the 1980s1.
Genetic analysis of loggerheads stranded in France indicate that as well as animals from Florida and the Caribbean, some also come from the nesting beach on the Cape Verde Islands, from where animals are occasionally blown north by storms along the Portuguese and Spanish coasts9 and into the Bay of Biscay. Loggerheads nesting in the Mediterranean move in and out of the Straits of Gibraltar along with animals from the Caribbean10, which move inwards through the ‘Straits’ and are sometimes recorded off the coast of Algeria. Mediterranean turtles would be unlikely to move northwards into colder water around the UK. However, recent work suggests that there may be a population of juvenile loggerheads using the Bay of Biscay as feeding grounds during warmer periods11.
Kemp’s ridleys also come from the Gulf of Mexico and are being recorded more frequently as UK strandings in the last three decades. Very occasionally there are strandings of olive ridley, hawksbill and green turtle1. Green turtles stranded on French coasts have been from African populations not from the Mediterranean12, where the nesting green turtle populations in the east generally travel along the North African coast to feed before returning to nest in Turkey and Syria13.
The leatherback turtle is the largest turtle species; the biggest recorded washed up on a Welsh beach in the 1980s, weighed over 900 kg and was 2.9 m (9 ½ ft) long14. Leatherbacks feed mainly on jellyfish and other gelatinous plankton, such as salps, and spend a lot of their time feeding because this prey is not very nutritious. Strandings are often linked to plastic ingestion because of the similarity to jellyfish, their main prey species. The animal has a remarkable throat structure with backwards pointing barbs that holds jellyfish in place until it’s swallowed14.
Leatherbacks are regularly seen off the UK and Irish coasts and these North Atlantic animals have migrated from their nesting sites along the coast of northern South America and some Caribbean islands15. Females nest at these rookeries, often on a 2 yearly cycle, and animals follow one of three different migration paths based on directions taken by tracked animals. Animals from the North Atlantic population migrate northwards up to the coast of Nova Scotia in Canada, eastwards across the Atlantic towards the Azores and the coast of Africa or north eastwards up to the Atlantic coast of the island of Ireland, the Irish Sea and the Celtic Sea16, 17, where they find jellyfish swarms on which to feed18. They have been recorded as far north as Norway14 and can survive in these cold waters because they are able to generate their own body heat and being large, once warm they take longer to cool down. They also have heat conservation and heat exchange mechanisms that enable them to feed actively in waters below 15oC, temperatures that would cause ‘cold stunning’ to other turtle species. Leatherbacks that are found further north tend to be larger than those off the Spanish and French coasts19, 20.
The continental shelf around Europe and off Canada are ideal sites for their main food, the barrel jellyfish, Rhizostoma pulmo, but they will feed on other gelatinous or jelly-like plankton. Barrel jellyfish can reach 90 cm in diameter and ‘blooms’, large congregations, of jellyfish can occur when conditions are ideal and food (plankton) is plentiful21. A leatherback filmed off the Canadian coast22 was found to eat over 50% of their entire annual energy requirement in the 90 days that they spent in northern waters and this is likely to be the case off the UK in the Celtic and Irish seas. Leatherbacks are probably moving into high quality feeding areas where the location of a feeding ‘patch’ will provide them with lots of energy at low cost 23, 24. Unfortunately, some of their foraging areas in the Atlantic align with existing long line fisheries and this has an impact on the population, with estimates in the year 2000, of a leatherback bycatch of between 30,000 and 60,000 animals from both north and south Atlantic populations combined4, 25.
After feeding through the summer the leatherbacks head back south-westerly and generally follow a straight line migration path back to their nesting beaches26. Scientists have been able to track animals leaving nesting beaches and also animals that have been caught in fishing gear off the UK coast and found that when they’re feeding in northern waters they tend to stay close to the surface, feeding in localised patches, suggesting feeding in jellyfish swarms in the surface waters21, 27, 28, 29. Tracking results indicate that when they travel to and from their nesting beaches they swim much faster and tend to be more prepared to undertake deeper dives to find food29, 30.
How has this leatherback migration pattern developed? One suggestion is that leatherback hatchlings drift out of the gyre, get caught up in the North Atlantic current and drift northwards during the so-called hatchling/juvenile ‘lost years’, finding areas where foraging is high-quality17. Having returned to nesting grounds, they are able to find these sites again as adults, using some form of navigation system15. Whatever the process they use, as long as we have our existing ocean currents, and large jellyfish blooms, we are lucky enough to provide an important feeding area for these migratory turtles, the most widely distributed reptile species in the world.
Published work referred to in this article:
1 Botterell ZLR, Penrose R, Witt MJ, Godley BJ (2020) Longterm insights into marine turtle sightings, strandings and captures around the UK and Ireland (1910-2018). Journal of the Marine Biological Association of the United Kingdom 100:869-877
2 Putman NF, Seney EE, Verley P, Shaver DJ, López-Castro MC, Cook M, Guzmán V and others (2020) Predicted distributions and abundances of the sea turtle ‘lost years’ in the western North Atlantic Ocean. Ecography 43:506-517
3 Monzo´n-Argu¨ello C., Dell’Amico F,. Morinie`re P,. Marco, A Lo´pez-Jurado L. F.,. Hays G C, Scott R, Marsh R and Lee P, (2012) Lost at sea: genetic, oceanographic and meteorological evidence for storm-forced dispersal Journal of the Royal Society Interface 9, 1725–1732
4 Lewison RL,. Freeman S A and. Crowder L B (2004) Quantifying the effects of fisheries on threatened species: the impact of pelagic longlines on loggerhead and leatherback sea turtles Ecology Letters, 7: 221–231
5 Bellido J, Castillo J, Pinto F, Marti’n J, Mons J, Ba’ez J and Real R (2010) Differential geographical trends for loggerhead turtles stranding dead or alive along the Andalusian coast, southern Spain Journal of the Marine Biological Association of the United Kingdom, 90(2), 225–231.
6 Kettemer L, Biastoch A, Wagner P,. Coombs E J, Penrose R, and Scott R, (2022) Oceanic drivers of juvenile sea turtle strandings in the UK Endangered Species Research 48: 15–29
7 Darmon, G.; Schulz, M.; Matiddi, M.; Liria Loza, A.; Tom, J.; Camedda, A.; Chaieb, O.; El Hili, H.A.; Bradai, M.N.; Bray, L.; et al. (2022) Drivers of litter ingestion by sea turtles: Three decades of empirical data collected in Atlantic Europe and the Mediterranean. Marine Pollution Bulletin , 185, 114364
8 Sala B, Balasch, A, Eljarrat E, Cardona L (2021) First study on the presence of plastic additives in loggerhead sea turtles (Caretta caretta) from the Mediterranean Sea Environmental Pollution 283 117108
9 Nicolau, L., Ferreira, M., Santos, J. et al. (2016). Sea turtle strandings along the Portuguese mainland coast: spatio-temporal occurrence and main threats. Marine Biology 163, 21 https://doi.org/10.1007/s00227-015-2783-9
10 Garofalo L., Mastrogiacomo A., Casale P., Carlini R., Eleni C., Freggi D., Gelli D. Et al. (2013). Genetic characterization of central Mediterranean stocks of the loggerhead turtle (Caretta caretta) using mitochondrial and nuclear markers, and conservation implications. Aquatic Conservation: Marine and Freshwater Ecosystems, 23: 868–884.
11 Chambault P, Gaspar P and Dell’Amico F (2021) Ecological Trap or Favorable Habitat? First Evidence That Immature Sea Turtles May Survive at Their Range-Limits in the North-East Atlantic. Frontiers in Marine Science. 8:736604. doi: 10.3389/fmars.2021.736604
12 Avens L and Amico FD (2018) Evaluating viability of sea turtle foraging populations at high latitudes: age and growth of juveniles along the French Atlantic coast. Endangered Species Research 37, 25–36
13 Clusa, M., Carreras, C., Pascual, M. et al. (2014). Fine-scale distribution of juvenile Atlantic and Mediterranean loggerhead turtles (Caretta caretta) in the Mediterranean Sea. Marine Biology 161, 509–519 https://doi.org/10.1007/s00227-013-2353-y
14 Beebee, T.J.C. & Griffiths, R.A. (2000) Amphibians and Reptiles. A Natural History of the British Herpetofauna. Harper Collins Publishers, London, UK
15 Dodge KL, Galuardi B, Lutcavage ME. (2015) Orientation behaviour of leatherback sea turtles within the North Atlantic subtropical gyre. Proceedings of the Royal Society B 282: 20143129. http://dx.doi.org/10.1098/rspb.2014.3129
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