Climate Change Is Threatening Baby Bull Shark Food And Habitats

A small, gray dorsal fin slices through the green-blue water of the saltmarsh. These coastal wetlands, flooded and drained by salt water brought in by the tides, occur worldwide and are home to hundreds of animals.

That includes this little bull shark (Carcharhinus leucas) pup.

Today, it is a pudgy little thing with tiny eyes and a blunt snout that is known for head-butting its prey before devouring it. Not picky about what it eats, it thrives along shorelines all over the world. It will grow up to be considered to be the most dangerous shark to humans because of their ability to migrate up rivers and penchant for frequenting waters that people like to frequent as well.

This little bull shark is in the Clarence River, the largest river on the east coast of New South Wales measuring over 249 miles long (400 kilometers). Rising in the Macpherson Ranges on the Queensland and New South Wales border, it is classed as a ‘barrier river estuary’ and has a permanently open entrance; the estuary reaches over 62 miles (100 km) inland. Many threatened animals, such as the Australian painted snipe, rely on this special ecosystem.

Like many other coastal sharks, juvenile bull sharks remain in sheltered waters until they are big enough to fend for themselves. It was thought that they relied on the mangroves here for their nutritional needs via the crustaceans and fish that feed off mangrove. Both mangroves and salt marshes are essential for healthy fisheries, coastlines, and communities— an integral part of the economy and culture. They provide essential food, refuge, or nursery habitat for more than 75% of fisheries species.

However, thanks to a first-of-its-kind study, ecologists have uncovered the key nutrients baby bull sharks rely on to grow and survive… and it comes from the saltmarsh! Dr. Vincent Raoult from the University of Newcastle and co-author of the study said the findings were surprising. “We found that once these juvenile bull sharks reach around four years of age, they move towards the mouth of the estuary and prey on fish and other species that rely mostly on threatened salt marsh habitats,” Dr. Raoult said. “Bull sharks are predators and do not feed directly on any of the primary producers sampled in this study, but on the herbivores such as fish that consume saltmarsh. Surprisingly, mangrove habitats had limited contribution to the diet of juvenile bull sharks.”

By taking tissue samples from 55 juvenile bull shark pups that had previously been landed by commercial fishers, the research team carried stable isotope analysis which is based on the principle “you are what you eat.” Stable isotope ratios vary within and among food webs and are incorporated into an animal’s tissue through its diet. The researchers then took those shark samples and compared them with primary producer samples and organic matter taken from various areas in the Clarence River.

It was a fascinating, but concerning discovery, to have concrete evidence that saltmarshes are more nutritionally important to young bull sharks than mangroves here. Which is why lead author Dr. Yuri Niella from the School of Natural Sciences at Macquarie University urges the protection of this unique ecosystem: “In the Clarence River, these habitats are not only important for bull sharks but also to a number of fish and crustacean species. This means that the restoration of saltmarsh habitats there could not only benefit bull sharks, but is also likely to cause an increase in economic profit from an improvement in fishery catches.”

Unfortunately, thanks to climate change, saltmarshes are under threat. Not just in the oceanic continent of Australia but globally, disappearing due to major threats like climate change, pollution, land use change, and invasive species. Since the 18th century, people have ‘reclaimed’ acres of saltmarsh and tidal creek habitats by filling them in to create dry uplands for agriculture, houses, and other forms of coastal development. “It’s estimated we’ve already lost 50 per cent of our salt marshes across the world,” explains Raoult. “Saltmarsh is what we call a ‘coastal primary producer’ – it supports a range of organisms, such as crustaceans, elasmobranchs, and teleost fish, which are then preyed on by juvenile bull sharks.”

While numerous governing bodies have developed and implemented legislation to protect the remaining coastal wetlands, some believe these regulated activities may be ‘too little, too late.’ Which is why this kind of research is important to inform adaptative management strategies aimed at reducing the impacts of climate change.

“Understanding the importance of estuarine habitats for the animals living in these areas can help inform targeted management decisions to achieve the best outcomes for both people and animals,” says Niella. It is the hope that through permits and innovative solutions (like installing larger culverts under roads and railways to restore tidal exchange), many local, state, and federal groups can help restore former healthy saltmarshes.