Field Notes: Hot sands and endangered species

Beaches are an ecological wonder, home to a wide variety of species that live within the beach or migrate to the beach as part of their lifecycle, yet beaches are facing increasing changes and environmental impacts from human uses, erosion, land development and climate change. These impacts are not only on the habitat but also the myriad of land, coastal and marine animals that play a vital role in beach ecosystems and for those animals beaches maybe getting too hot!

So how are beaches getting too hot? The short answer is climate change, the elephant in the room in too many circumstances! Oceans are one of the most important carbon sinks, however increasing carbon dioxide in the atmosphere from global warming are making sea temperatures rise even further and these rising temperatures maybe changing beach sand which is critical to many animals.

Marine turtles are among many species that rely on beaches for part of their lifecycle and ongoing survival of the species. Female turtles migrate back to the beaches within the area that they themselves were born in, sometime travelling hundreds of kilometres to reach it. Here in Tonga, our nesting species are primarily Green turtles (Chelonia mydas) and Hawksbill turtle (Eretmochelys imbricata) whom migrate through Samoa and Fiji as part of their lifecycle.

When a female turtle has reached sexual maturity (Greens 20-50 years, Hawkbill 3-5years) and mated with a male, the migration to the beach begins and the need to dig a nest in the beach sands and/or shoreline vegetation above the high tide seas. Green turtles are immensely strong giving that an adult female can weigh between 68-190kg1 and drag (crawl) their heavy body up often over high sand banks that change with the waves and current movements. If the location chosen is not suitable, the female turtle will try another crawl until nesting is successful and the eggs are laid deep within the sands.

Wouldn’t turtle eggs want warm sand to incubate? Yes, however warm sands are a natural occurrence in the tropical islands and the turtle eggs rely on specific temperatures for the hatching gender of baby turtles. For the eggs to hatch as male, the sand temperature is optimally <27.7℃/81.6℉ and for female turtles optimally >31℃/87.8℉. Temperatures in between these two optimal numbers will produce a mix of male and female babies2. If the sands get too hot then turtle populations may be dominated by females and threaten the long term sustainability of an already threatened and endangered species.

So how do we know what the temperatures are? By placing small temperature sensors into the sand on known turtle nesting beaches, that will automatically record the temperature every 4-6 hours as programmed. These sensors will remain in place for 12 months and data will be collected every 4-6 months. The air temperatures are collected through local weather information such as at Lupepau’u Airport tower and local weather stations.

The turtle beach and nesting field activities are supported under the By-catch and Integrated Ecosystem Management (BIEM) program through the Secretariat of the Regional Environmental Programme (SPREP) as part of the regional Pacific European Union Marine Programme (PEUMP) funded by the European Union and Sweden.

Activities undertaken in Tonga are in partnership with the Department of Environment (MEIDECC) and Ministry of Fisheries. More information on the initiative can be read here.

References:

1 Body weight of Green turtles https://www.worldwildlife.org/species/green-turtle

2 Nesting temperature for marine turtles https://oceanservice.noaa.gov/facts/temperature-dependent.html

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