Leatherback turtles – Why are they dying on our coastline?

By Rod Braby, Namibian Coast Conservation and Management (NACOMA)

The most abundant sea turtle in Nambian waters in recent times has been the leatherback, the largest sea turtle, which can weigh up to 700 kilograms. Its carapace can measure up to 180 centimetres, making it over two metres from head to tail. In the past three years, hundreds of adult leatherbacks have washed up dead on our coast. These mortalities are cause for concern and need urgent investigation.

Sea Turtles or Chelonians, as they are sometimes known, are reptiles closely related to land tortoises or fresh-water terrapins evolving from the swamps of the Cretaceous geological period some 200 million years ago. Sea turtles are represented by eight species in two families and are different from their cousins on land by not being able to retract their head or limbs. All sea turtles are dependent on land for reproduction and must brave the dangers as they haul themselves ashore to lay their eggs. Clumsy and almost helpless on land, the breeding females are easy to kill. Globally their numbers have been decimated and all species are now endangered. The leatherback is critically endangered.

Five species are known to occur on the Namibian coastline. They are the leatherback (Dermochelys coriacea), loggerhead (Caretta caretta), green (Chelonia mydas), hawksbill (Eretmochelys imbricata) and olive ridley (Lepidochelys olivacea) sea turtles. None have been recorded breeding on our coastline, as temperatures for successful breeding need to be well over 20 degrees Celsius, and the Benguela Current is generally considerably colder. One unsuccessful breeding attempt by an olive ridley was recorded in Swakopmund in 2008 when the sea surface temperature reached an all-time high of 26 degrees Celsius for a period of over three weeks.

The Convention on the Conservation of Migratory Species of Wild Animals (also known as the Bonn Convention) is aimed at conserving migratory species throughout their range. It is one of a small number of intergovernmental treaties concerned on a global scale with the conservation of wild animals and the habitats on which they depend. More recently in Abidjan a memorandum of understanding concerning conservation measures for sea turtles of the Atlantic coast of Africa was signed by a re-presentative of the Ministry of Fisheries and Marine Resources on behalf of the Government of the Republic of Namibia together with 18 countries on the west African coast. These agreements bind Namibia to certain conservation actions to ensure safe passage of these endangered animals.

The main reason leatherbacks have become more abundant on our coast is the ascendancy of one of their most popular food sources – large jellyfish. Jellyfish have replaced fish as the most significant biomass of the Benguela since the collapse of the sardine and anchovy populations in Namibian waters mainly through overfishing prior to Namibian independence.

It is believed that Namibia, in particular the central coastal area, is becoming a global foraging hotspot for leatherbacks in the south Atlantic, much like Monterrey Bay in California is for leatherbacks in the northern Pacific. The leatherbacks on the Namibian coast come mainly from nurseries in Gabon and Brazil to forage in Namibian waters.  This information is derived from tag returns obtained from dead animals found stranded on the Namibian coast. Much effort is being made to protect the nesting grounds in Gabon and the Congo and also the east coast of South America. However, a lot more work needs to be done to first establish and then protect the development areas, foraging areas and migration routes.

The main reasons for death in Namibian waters appear to be accidental by-catch in fisheries including trawl, purse seine and long line; entanglement and subsequent drowning in lobster and mariculture structures; ship strikes; and predation by killer whales and sharks. As most of the Namibian coastline is not monitored, the authorities have only limited knowledge of the incidence and cause of sea-turtle mortality. Another issue that also impacts the leatherbacks negatively is the fact that the two bays with concentrations of jellyfish in Namibia, namely Lüderitz and Walvis Bay, are now rapidly expanding harbours, and have increasing risks of pollution and ship strikes.

Apart from establishing the cause of death and implementing mitigation measures, an active stranding and monitoring network needs to be formally established with proper support from Government, which could also be incorporated into the existing cetacean stranding network currently run by the Ministry of Fisheries and Marine Resources with the support of the Ministry of Environment and Tourism, Local Authorities and the private sector.

This article appeared in the 2009/10 edition of Conservation and the Environment in Namibia.