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Facts about sicklefin lemon sharks

By Clare Murphy 7 months ago

True or false? Sicklefin lemon sharks rigorously and persistently defend themselves when provoked.

Together, we’ll deep dive into the world of this threatened species. While submerged, we’ll learn interesting facts about them and their habitats.  

Are sicklefin lemon sharks dangerous?

Sicklefin lemon sharks are known to be cautious when approached and often retreat. However, as with any wild animal, they can be dangerous when provoked.

They have a reputation for defending themselves persistently and vigorously when they feel they are attacked, for example, if they are touched, poked or speared by divers or spearfishers. Otherwise, they are very shy.

Where are they found around the world?

Sicklefin lemon sharks are found in Australia, Fiji, Madagascar, Myanmar, Papua New Guinea, Solomon Islands, South Africa, Seychelles and Vanuatu.

Historically they were found in India and Thailand. Now they are locally extinct in those places, because they were over-harvested. They used to be recorded frequently in Indonesia. That is no longer the case.  

 

sicklefin lemon shark

Original photo: Julien Bidet

How endangered are sicklefin lemon sharks?

In 2003, the International Union for the Conservation of Nature (IUCN) assessed the global population of sicklefin lemon sharks, and listed it as decreasing. The global population is classified as vulnerable on  IUCN’s red list of threatened species.

Sixteen years have passed since this assessment. Known threats to these sharks have been increasing, meaning they could be more at risk now. The IUCN has identified the need to update the global assessment.  

The sicklefin lemon shark is also an understudied species. This hampers conservation efforts. To know how to best conserve a species, you need to know why numbers are decreasing. Is it habitat loss? Do they take a long time to reach sexual maturity? Or, are they over-harvested, for example?

Scientific data from projects like our lemon shark and turtle conservation project will be critical when this species is assessed by IUCN again. It will help answer these types of questions, so the next assessment is better informed.

Why are sicklefin lemon shark numbers decreasing globally?

The reason for the decrease is multi-faceted:  from the threats they face to the species’ reproductive traits.

 

shark facts

GVI staff and participants attach a transmitter to a sicklefin lemon shark juvenile to collect data on the understudied species

They are susceptible to overharvesting and local depletion

Sicklefin lemon sharks are “home bodies”.  A study in 1984, in Seychelles, showed adult sicklefin lemon sharks roam about 1.3 kilometres from their “home”. This is different from great white sharks, for example, that undertake transoceanic migrations.

Home for adult sicklefin lemon sharks is coral reefs, and the sandy plateaus nearby. Newborns, known as pups, and juveniles are found in coral atoll lagoons, and in mangrove estuaries.

Their small, coastal habitat range, and limited movements, means sicklefin lemon sharks are susceptible to overharvesting at a local level.

 

shark conservation

Original photo: MissMegido

Populations once depleted, can’t recover quickly

Sicklefin lemon sharks have a low reproductive capacity. Their gestation period is ten to 11 months and they have between one and 12 pups per litter. They have slow growth rates, and yet, they don’t reach sexual maturity until they are 2.2 metres long.

In the area surrounding Curieuse Island, they grow just 5.4 centimetres per year. We know this from the data our wildlife conservation volunteers have collected in partnership with Seychelles National Parks Authority staff.

Take this growth rate, and their required size for sexual maturity into consideration. Since pups measure 60 centimetres when born, it could take up to 29 years before sicklefin lemon sharks are ready to breed. However, this needs to be confirmed via further research.

So, what does all of this mean? Once sicklefin lemon shark populations have depleted, they have limited capacity to recover.

 

marine conservation

GVI participants humanely catch and release juvenile sharks to collect valuable data on their population numbers and growth rates.

Sicklefin lemon shark habitat is being destroyed

The IUCN ranks coral reefs as one of the most threatened ecosystems in the world. Between 2014 and 2017, we saw mass bleaching events affecting the world’s coral reefs. When this happens, the coral often dies, meaning this habitat type is lost.

This was a result of increased global surface temperatures due to climate change.  Over a third of the world’s coral reef species are now threatened with extinction.

Sicklefin lemon sharks’ other key habitat, mangroves, is one of the fastest disappearing habitats worldwide (2–8% per year). Globally, 20%–35% of mangrove area has been lost since approximately 1980. This is due to a combination of coastal development, climate change, agriculture and logging.

GVI is running marine conservation programs around the globe, some of which focus on coral reef research and mangrove monitoring.

 

marine conservation

How is the GVI project on Curieuse Island making a difference?

To effectively conserve and manage a species, we need to know more than where they live. We need to understand how a species uses the different habitats in which it is found, throughout its various life stages.

We are investigating how juvenile sicklefin lemon sharks make use of their habitat in our project on Curieuse Island. Participants catch juveniles. Once caught, they are fitted with acoustic transmitters. They are released, and their movement patterns are tracked using a hydrophone.

Gathering this data will help make a lasting impact for sicklefin lemon sharks by contributing to the body of scientific knowledge about them.

 

lemon shark conservation

 

Government agencies like our partner, the Seychelles Ministry of Environment, Energy and Climate Change, need and use the scientific data we collect. This is so they can make well-informed decisions. For example, creating harvesting strategies that take the harvested species’ biology into account.  

Our participants also plant mangrove seedlings on Curieuse Island and other Seychelles locations. This is to help restore degraded mangrove forests in partnership with the Terrestrial Restoration Action Society of Seychelles.

If you can’t volunteer on our lemon shark and turtle conservation project, you can still make a difference for lemon sharks. When you travel, make proactive, informed choices. Avoid shark-fin soup and purchasing shark-tooth souvenirs.

You can also donate to the GVI Trust or IUCN’s Shark Specialist Group.