colors of the south pacific

To begin 2020 we have chosen the Black-tip Reef Shark as our Organism of the Month for January.  Carcharhinus melanopterus or Mauri in Tahitian is a fish to be appreciated for its many unique traits, as well as for the crucial role it plays in our lagoons.

Among the five reef sharks worldwide, the Black-tip Reef Shark is the most common and is especially prominent in the waters surrounding Tetiaroa. This shark is easy to spot by its black-tipped dorsal fin and its habitat preference of shallow reef waters near land.  These sharks are considered small by shark standards, averaging 5 feet long (1.5 meters), but, due to their dense cartilaginous bodies they weigh in heavily for their size at 280 pounds (130 Kg).  They are extremely shy and easily spooked, and are not known for aggressive behavior towards people.  When provoked, they will move their bodies in an S shape and roll side to side, much like a cat puffing up to scare off predators.

These shy reef sharks are homebodies, sticking to a home range of about ¼ of a square mile (0.6 square Km) for most of the year, leaving their territory only to mate and give birth.  They are viviparous, meaning they give birth to live babies, as opposed to the more popular marine strategy of spawning.  Mother sharks give birth to an average littler of 2-4 pups (yep, they are called pups!) after a gestational period of approximately 300 days, which varies by location, as opposed to individual.

pups

Shark pups are tiny when born, averaging around 1.5 feet (45cm) but are completely independent within days.  Mother sharks will often birth their young in mangroves or very shallow, protected areas as baby sharks are quite a treat to larger predators.  In the absence of a male, female reef sharks can produce asexually, by fertilizing their own egg with a nearby cell called a polar body.  Asexual reproduction is used a last resort as it leads to extremely inbred pups. 

Shark pups take approximately 5 years to mature, and with such a low birth rate per year, these animals are quite vulnerable as a population.

Reef sharks prefer to hunt smaller lagoon creatures such as fish, crabs, shrimp, and occasionally a smaller shark. They are extremely energetic hunters and often hunt in small groups, corralling schools of small fish into the shallows for easy capture.

Along with their super sensitive sense of smell, reef sharks use electromagnetic pulses to locate prey, which is especially helpful in dark, or murky waters.  Along their heads and eyes, sharks have small, jelly filled pores called “ampullae of Lorenzini” that pick up minute electric pulses from surrounding organisms.  The jelly substance is highly conductive and sends minute voltage changes in the surrounding area across the ampullae membrane, activating nerve cells that then send the information to the shark’s brain.  This phenomenon is known as electroreception and, although it is observed in sharks, rays, and a handful of more ancient fish, how it evolved remains a mystery.  The sensitivity of these structures allows sharks to sense the extremely small electric fields created by muscle contractions.  When an organism is struggling, or even slightly moving nearby, the shark’s senses pick up the pulses and can hone in on the location of that organism, even those hidden beneath sand or in caves.  Further, Ampullae of Lorenzini are able to pick up on the earth’s magnetic field and help sharks with navigation and homing.  In applying this discovery to conservation techniques, research is being done in which fishing nets are outfitted with small magnets.  The thought was that the magnets would discourage sharks from the area and thereby avoid getting caught in the nets.  So far, the results have been pretty impressive.  It appears that the magnets work by confusing the shark senses and deter them from getting too close to the nets. 

Black tip reef sharks are considered a Keystone species and are critical for the balance of our lagoon ecosystem.  Because they prey on a wide variety of lagoon species, they are able to keep populations in check, allowing for the animals lower on the food chain to thrive without overpopulating.  A large part of the black tip reef shark’s diet consists of herbivorous fish who eat primarily algae.

If there are too many algae eaters, they strip the coral and leave smaller creatures without a food source.  If there are not enough algae eaters, the algae can grow too densely on the coral, causing it to die off.

Striated surgeonfish

Striated surgeonfish | maito

unicornfish

unicornfish | ume tarei

dusky damselfish

dusky damselfish | atoti

scribbled rabbitfish

scribbled rabbitfish | paauara

convict tang eating algae

convict tang | manini

parrotfish

parrotfish | paati

Reef sharks are so important, in fact, they have been named a Priority Species by the World Wildlife Federation, sharing the title with favorites such as the giant panda, great apes, and elephants, meaning they are responsible for ecological health mitigation in their environments and/or are cultural icons.  They are both.

shark icon

 

blacktip sharks in French Polynesia

Photo: Stephen Frink

baby sharks

Black-tip Reef Sharks in particular are known to warn people of personal danger by insistently approaching, and often ramming into them.

Owing to their powerful presence and ecological importance, sharks are important in Polynesian culture across the Pacific. Because of this Polynesians have not historically killed sharks for food. On the contrary island groups have legendary sharks that protect the island, and tribes and families have sharks as taura or totems that are ancestors and protectors. Black-tip Reef Sharks in particular are known to warn people of personal danger by insistently approaching, and often ramming into them. There are many stories of people who thought it just weird and annoying than a small shark kept hitting them in the legs only to come to harm from a freak accident. This is of course a long way from how western culture thinks of sharks, and unfortunately this attitude has arrived in these islands in modern times.

Since the spring of 2006 French Polynesia has banned the hunting and killing of sharks within 5 million square kilometers (2 million square miles) of the French Polynesian economic zone.  This law came about in response to an upswing in the mutilation of sharks in order to harvest their fins for soups and delicacies.  The practice quickly got out of control and shark populations began to decline.  Ocean enthusiasts set about collecting signatures to speak out against the inhumane treatment of these animals and created a petition that received so much attention, it led to the ban on shark killing.  Since then, the waters surrounding French Polynesia have supported a healthy shark population which has contributed to a healthy marine ecosystem.

protected area for sharks
360 shark observation

Shark observation in the Tetiaroa lagoon - 360 degree view

Following on 4 years of studies on shark ecology and population dynamics in Tetiaroa, a group of researchers from University of Washington and Florida International University will be starting a new program this year to track sharks in Tetiaroa’s lagoon. The team will use acoustic receivers placed in a grid across the lagoon to track individual Black-tip Reef Sharks who have been fitted with acoustic tags that emit a signal that is picked up by the receivers. In this way, scientists can track sharks 24 hours a day to see if and how they stake out territories, use common hunting grounds, have fixed routes, and generally use the three-dimensional lagoon space. Stay tuned for some great information and graphics from this project.

So next time you are snorkeling through our lovely lagoon, and you see a Black- tip Reef Shark, while marveling at their beauty, also remember the important role they play in the balance of their underwater community.

Photo: S Girardot