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Secrets of Whitetip Reef Shark

Photo by Mika d’Eau

A small shark that does not usually exceed 1.6 m (5.2 ft) in length, this species is easily recognizable by its slender body and short but broad head, as well as tubular skin flaps beside the nostrils, oval eyes with vertical pupils, and white-tipped dorsal and caudal fins, hence the name. It is one of the most common sharks found on the Maldivian reefs:

Habitat

Associated almost exclusively with coral reef habitats, whitetip reef sharks are most often encountered around coral heads and ledges with high vertical relief, and additionally over sandy flats, in lagoons, and near drop-offs to deeper water.[8] They prefer very clear water, and rarely stray far from the bottom during the day.[5] The species is most common at a depth of 8–40 m (26–131 ft).[2] On occasion, they may enter water less than 1 m deep when foraging.

Behaviour

Whitetip reef sharks are rarely aggressive towards humans, though they may investigate swimmers closely.

During the day, they spend much of their time resting inside caves. Unlike other requiem sharks, which rely on ram ventilation and must constantly swim to breathe, this shark can pump water over its gills and lie still on the bottom. At night, whitetip reef sharks emerge to hunt bony fishes, crustaceans, and octopus in groups, their elongate bodies allowing them to force their way into crevices and holes to extract hidden prey. Individuals may stay within a particular area of the reef for months or years, frequently returning to the same shelter.

Whitetip reef sharks hunt primarily at night, when many fishes are asleep and easily taken. After dusk, groups of sharks methodically scour the reef, often breaking off pieces of coral in their vigorous pursuit of prey. Multiple sharks may target the same prey item, covering every exit route from a particular coral head. Each shark hunts for itself and in competition with the others in its group. A whitetip reef shark can survive for six weeks without food.

Photo by Mika d’Eau

Reproduction

This species is viviparous, in which the developing embryos are sustained by a placental connection to their mother. Mating is initiated when up to five males follow closely behind a female and bite at her fins and body, possibly cued by pheromones indicating the female’s readiness. Each male attempts to seize the female by engulfing one of her pectoral fins; at times two males might grasp a female on both sides simultaneously. The male has a limited time in which to achieve copulation, as while he is holding the female’s pectoral fin in his mouth he is being deprived of oxygen.
After a gestation period of 10–13 months, females give birth to litters of 1–6 (usually 2–3) pups. Each female produces an estimated average of 12 pups over her entire lifetime. Females give birth while swimming, making violent twists and turns of their bodies; each pup takes under an hour to fully emerge.[21] The newborns measure 52–60 cm (20–24 in) long and have relatively longer caudal fins than adults. This shark develops slowly compared to other sharks; newborns grow at a rate of 16 cm (6.3 in) per year while adults grow as a rate of 2–4 cm (0.79–1.57 in) per year. Sexual maturity is reached at a length of around 1.1 m (3.6 ft) and an age of 8–9 years. Males can live to 14 years and females to 19 years; the maximum lifespan of this shark may be upwards of 25 years

Threats

Important natural predators of the whitetip reef shark include tiger sharks, possibly also silvertip sharks, though they usually occur at depths greater than those favored by whitetip reef sharks. An 80 cm (31 in) long whitetip reef shark has also been found in the stomach of a giant grouper, though these groupers are unlikely to be significant predators of this species due to their rarity. Baby whitetip reef shark sometimes also finish in the stomach of great barracudas and herons.

The whitetip reef shark is taken by fisheries operating off Pakistan, India, Sri Lanka, Madagascar, and likely elsewhere, using longlines, gillnets, and trawls. The meat and liver are eaten, though sharks from certain areas present a substantial risk of ciguatera poisoning (especially the liver, which contains a much higher concentration of the toxin than the meat). The International Union for Conservation of Nature (IUCN) has assessed this species as Vulnerable, as its numbers have dropped in recent decades due to increasing, and thus far unregulated, fishing pressure in the tropics. Its restricted habitat, low dispersal, and slow reproduction are factors that limit this shark’s capacity for recovering from overfishing. Furthermore, populations in no-take zones, where boats are allowed but fishing prohibited, exhibit levels of depletion comparable to fishing zones due to poaching. Demographic models indicate that these depleted populations will continue to decline by 6.6–8.3% per year without additional conservation measures.

DIVE WITH WHITETIP SHARKS

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