Diving with Bull Sharks

Resting on the sandy bottom, digging my hands into it to anchor myself against the current, I could see them slowly circling around us. I was trying to make out the silhouette of one cut against the deep blue of the sea, the only sound that of the bubbles coming from our regulators. A fellow diver pointed to my right, and I turned. Swimming straight towards me, was a massive bull shark.

It was the stuff of nightmares, only this was, in fact, the purpose of our dive. Staring at the shark as it swam smoothly in my direction, totally undisturbed by the current, I couldn’t help but think what an easy meal I would make, if the shark meant to eat me. It came so close, I could see every detail of its muscular body, it’s small dark eyes, down to the pores around its snout with which it can sense electric fields. Then, before it reached me, it effortlessly modified its course, gracefully gliding by me instead.

It was December 2016 and we were diving off the coast of Playa del Carmen, in the southern tip of Mexico. Every year, between the months of November and March, bull sharks congregate in these waters. They are a magnet for shark divers, and a source of fear for many people who go in the water. It is a season of struggle between those who would like to see them protected and those who would like to see them gone. But what are the characteristics that set these sharks apart from others?

Characteristics of Bull Sharks

Bull sharks, Carcharhinus leucas, are strong, stocky sharks with a gray coloration on top that turns to white underneath. They have a characteristic blunt snout, and a long caudal fin. Bull sharks usually grow to be 2.1-3.4 meters long and 90-230 kg, with females being larger and heavier than males. They have an impressive bite force, which has been estimated to be higher than that of the great white shark (1).

Widely described as having an aggressive temperament, bull sharks are one of the shark species that are considered most dangerous to humans. This is due to several factors, including their size, habitat and preferred hunting grounds. These sharks inhabit the warm coastal waters of all the world’s oceans. They swim in shallow water near the coast and at the mouth of estuaries, which humans also frequent. They like to hunt in murky water with low visibility, which could lead to mistaking humans for their natural prey. 

Distribution map for Carcharhinus leucas. By Chris_huh/ CC BY-SA 3.0

Additionally, unlike other shark species, bulls have adapted to thrive in a variety of water salinities. Their presence has been well documented in rivers throughout their range, like the Zambezi River in Africa, the Brisbane River in Australia, and the Mississippi in the United States. Bull sharks have even been found in lakes. In Central America, their presence in lake Nicaragua has earned them the name of Nicaragua shark. They were originally thought to be a distinct shark species from the Pacific Ocean that became trapped there when the lake was formed. However, it was later discovered that the sharks travel to the lake from the Atlantic Ocean, swimming up the San Juan River, and navigating the rapids (2).

Bull sharks are able to survive in fresh water thanks to their kidneys, liver, gills and rectal glands, which work together to balance the salt and water concentration in their bodies. They use fresh water bodies as nursery grounds, and it is thought that this gives the young sharks a safe space to grow without the threat of bigger sharks preying on them.

Most of the sharks that gather in Playa are female sharks, and many of them are pregnant. It is thought that they come to these shores to give birth in the nearby mangroves, but there is still much to learn about them. In 2010, an association called Saving our Sharks (SOS) was founded, with the purpose of leading conservation and research efforts. Studying sharks in their natural environment is a complicated endeavor, but research efforts are slowly beginning to yield answers. How are they doing this?

Research Efforts

One way to study the movement of animals in nature is through the use of tags. At Playa del Carmen, researchers are using both external and internal tags. External tags are placed on the base of the dorsal fin using a fishing harpoon and dart. Internal tags, on the other hand, are placed in the peritoneal cavity. To place them, the shark must first be caught with line and hook, and stabilized alongside the boat. The tag is then surgically implanted. Both kinds of tags carry a code that is specific for that shark, and is passed to receivers placed along the coast every time the shark comes near.

Scientists can also obtain information about animal populations by analyzing small tissue samples, or biopsies. At Playa, small cylindrical probes are used to take tissue samples from the sharks, from the area at the base of the dorsal fin. The tissue samples are then used for different analyses. For example, DNA extracted from the skin has been used to determine the relationship between these sharks and other bull shark populations in the Gulf of Mexico.

The analysis of biopsies can also reveal information about the sharks’ diet. Traditionally, diet has been researched by looking at the stomach content of different specimens. This, however, has several disadvantages, including the need to dissect a large number of sharks, and the possibility of finding empty stomachs. An alternative way to do it is through stable isotope analysis.

Isotopes are atoms of the same element that have different number of neutrons. Most elements found in nature exist as stable isotopes. The specific ratio of isotopes found in the tissues of an organism reflects the isotope composition of its diet. By measuring the isotope composition of animal tissues, scientists are able to study food webs and predator-prey relationships within an ecosystem.

Need for Shark Conservation Efforts

The first shark I ever saw underwater was a bull shark. Contrary to what I had anticipated, it was a peaceful experience. The shark was even a bit shy, barely noticing us before disappearing into the blue. Ever since then, I’ve come to realize all the misconceptions and misinformation that abound about sharks, even among otherwise educated people. “Why didn’t they bite you?” and similar questions are often brought up, even though you have a far greater chance of drowning at the beach, than being bitten by a shark.

Sharks play a vital role in maintaining the balance in their ecosystems. They eat the weak and the sick, keeping fish populations healthy and under control. The elimination of sharks can destabilize entire food chains, and bring about the degradation of the ecosystem. Sadly, millions of sharks are killed each year for their fins and meat. In particular, the conservation status of the bull sharks is NT, Near Threatened.

Fortunately, studies are showing that sharks are worth more alive than dead. In a study published in Oryx: The International Journal of Conservation in 2013 it was estimated that shark fisheries generate around $630 million dollars per year globally, but this amount has been declining over the past decade. On the contrary, the shark tourism industry generates around $314 million dollars per year, and is expected to grow to over $700 million dollars within 20 years. In Palau, a study by the Australian Institute of Marine Science and the University of Western Australia showed that a single reef shark can generate $1.9 million dollars over the course of its lifetime, compared to $108 if it were fished and sold. In Playa del Carmen, SOS estimates a single live shark contributes $220 thousand dollars per season. On the other hand, a dead shark can make the fisherman $300 to $400 dollars, but only once.

The more we learn about the sharks and their behavior, the better we can understand how to protect them. Clearly, however, changing the public’s opinion is key to their survival. After all, a feared shark is most probably a dead shark. This is best done through education and outreach efforts to dispel the myths surrounding sharks and change the negative public portrayal of these animals.

As for me, after having spent countless hours researching sharks and shark attacks, when I finally saw that first shark underwater…a sense of wonder filled me. I marveled at this powerful creature that was for sure capable of inflicting serious harm on me, and yet did not. Time seemed to stand still as I stared at it, mesmerized by its beauty. I already loved sharks but, after this experience, my perspective on them and their relationship with humans changed even more.

Bull shark dive from M Granados on Vimeo.

References:

(1) Habegger, M.L., et al. Feeding biomechanics and theoretical calculations of bite force in bull sharks (Carcharhinus leucas) during ontogeny. 2012. Zoology, 115(6): 354-364.

(2) Thorson, T.B. Movement of Bull Sharks, Carcharhinus Leucas, Between Caribbean Sea and Lake Nicaragua Demonstrated by Tagging. 1976. Investigations of the Ichthyofauna of Nicaraguan Lakes. Paper 38.