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The lemon shark is a relatively timid shark. Currently, two species of lemon sharks are known in the world, including Negaprion acutidens and N. brevirostris. Negaprion acutidens is distributed in the coastal waters of Africa, Australia and southern Taiwan, including Dongsha Island, while N. brevirostris is mainly distributed in the east Atlantic waters from New Jersey, Mexico, the coastal areas of the Caribbean, and all the way to the southern coast of Brazil. Given that the lemon shark is a fishing target, its population is decreasing drastically due to overfishing. Therefore, IUCN has listed the lemon shark as a vulnerable threatened species and is also listed as an endangered species in the seas of Southeast Asia. Since the founding of Dongsha Atoll National Park, researchers have frequently visited the island and discovered this kind of small shark migrating around the island. Recent research indicated it is extremely remarkable that the population of young sharks can reach as many as 300 around Dongsha Island, an island with merely 179 hectares. Especially nowadays when shark conservation is a major global issue, the conservation progress of chondrichthyans (sharks and rays) at Dongsha Atoll National Park really stands out.

Negaprion acutidens is a viviparous fish. The gestation of adult female sharks is about 11 months about they will migrate to their birthplace to give birth by the end of their gestation (“natal philopatry”). According to the observations and researches conducted at the surrounding waters of Dongsha Island between 2013 and 2017, pregnant female Negaprion acutidens will migrate to the surrounding waters of Dongsha Island by the end of March every year (when the ocean temperature gradually rises) and they will enter the shallow lagoon waters of western Dongsha Island annually from late March to early May following the high water of spring tide. Schools of young sharks can be seen swimming around the coastal waters of Dongsha Island by mid April. After the female Negaprion acutidens have delivered their infant in early May, the population of lemons sharks at the surrounding ocean of Dongsha Island will reach its maximum between June and September. After the arrival of northeastern monsoon in November, causing the ocean temperature to drop below 20℃, some young sharks will enter the lagoon or deep waters of Dongsha Island to get through the low temperature, while others will migrate to distant sublittoral zones and deep waters where the ocean temperature in relatively stable. They will then leave Dongsha Island and enter the deep waters after they have grown to a certain size. At the same time, net capture records and sound calibration researches in recent years indicated the population of young sharks at the lagoon shallow water area of Dongsha Island way exceeds those at other water areas of the island. After further analyzing their migration route, researchers discovered that the migration of Negaprion acutidens in the surrounding waters of Dongsha Island can be categorized into three patterns. The first pattern is migrating between the lagoon and the estuary of the lagoon, the second pattern is migrating between the northern coast of Dongsha Island and the estuary of the lagoon, and the third pattern is migrating at the western regions of Dongsha Island. Therefore, researchers predicted that by late March of spring, female Negaprion acutidens will migrate to the surrounding waters of Dongsha Island and deliver their infants at the shallow waters of lagoon estuary between early April and early May. With the lagoon estuary of Dongsha Island as their starting point, infants will then distribute themselves into the lagoon, northwards and southwards, thus creating three schools of young sharks staying inside lagoon and the estuary or between the northern coast of the island and the lagoon estuary.

Young Negaprion acutidens born at Dongsha Island have three migration routes.
Young Negaprion acutidens born at Dongsha Island have three migration routes.

 

Moreover, by analyzing the connection between tides and young shark migration also explained that the shallow waters of Dongsha Island lagoon and its southern and northern waters provided a relatively safe habitat and nursery ground for young Negaprion acutidens in feeding themselves, adopting to the environment and evading predators. According to the current discovery on young shark hunting behaviors, for young sharks that mainly feed of fish, the water depth of 45 to 60 cm. is the most appropriate depth for them to hunt. Waters areas that are too deep or too shallow will reduce their hunting success rate. Therefore, when the water level is appropriate, visitors can observe the waters young sharks splash when hunting their prey at the large lagoon estuary shallow waters at western Dongsha Island.

Water depth has great affect on the hunting success rate of young Negaprion acutidens. According to current observations, the sea level of 45 to 60 cm. is the best hunting depth for them.
Water depth has great affect on the hunting success rate of young Negaprion acutidens. According to current observations, the sea level of 45 to 60 cm. is the best hunting depth for them.

The research on the population biology and behavior ecology of Negaprion acutidens of Dongsha has just started and currently, there are still many issues we would like to understand. For instance, what are the behavioral habits of the young sharks, what is the connection between shark migration, tides and water temperature, where will mature sharks migrate to, what is the reproductive and delivery behaviors of adult sharks, are there any lemons sharks staying at the atoll terrace, what is the overall population and distribution of Negaprion acutidens in the waters of Dongsha Island, and etc. Long-term monitoring and investigations are needed to accumulate more basic information, so that the most appropriate conservation strategy can be organized in the future.

Negaprion acutidens investigation at Dongsha Island. Young sharks that were recently born were captured, measured and calibrated before being released.

Picture 1. Negaprion acutidens investigation at Dongsha Island. Young sharks that were recently born were captured, measured and calibrated before being released.

Negaprion acutidens is a relatively timid shark. Researchers will release them after they are labeled and have restored their strength.
Picture 2. Negaprion acutidens is a relatively timid shark. Researchers will release them after they are labeled and have restored their strength.
The vast sea grass bed surrounding Dongsha Island not only is the habitat for infant teleosts, but also the best nursery ground for young Negaprion acutidens.

Picture 3. The vast sea grass bed surrounding Dongsha Island not only is the habitat for infant teleosts, but also the best nursery ground for young Negaprion acutidens.

The pupils of Negaprion acutidens will enlarge at night.
Picture 4. The pupils of Negaprion acutidens will enlarge at night.

Note: The chondrichthyans that have been recorded in the waters of Dongsha Island include the eight species of Carcharhinus limbatus, Carcharhinus longimanus, Carcharhinus melanopterus, Negaprion acutidens, Galeocerdo cuvier, Taeniura meyeni, Himantura fai, and Aetobatus narinari.

Pictures and Article by Chen Yu-yun, Aletheia University

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