Great question, Tully! Animals on land breathe air, which is made up of different gasses. Oxygen is one of these gases, and is made by plants (hug a plant today and say thanks). All animals need to breathe in oxygen to survive.
When the air goes into our lungs, oxygen goes into our blood and is delivered all around the body. Air is light, so it’s easy to move around. This makes it pretty easy to breathe air back and forth — a bit like blowing up balloons and letting them deflate.
Things are different for fish. Fishes also need oxygen, but rather than getting it from air, they have to get it from water.
But there is less oxygen available in water than air. And to make matters worse for the poor fishes, water is thicker than air, so it takes much more work to move it around. This makes the problem of getting that oxygen in the fishes’ body even harder.
This is why fish need gills
Rather than breathing in and out through the mouth, fish use a one-way system, passing water in one direction over their gills.
Water goes in the mouth, across the gills and out through the opercula (the bony covering protecting their gills).
But gills and lungs are more similar than you might think. Both have really big surface areas which increases the amount of water or air that touches the gill or lung tissue, and so increases the amount of oxygen available.
What’s more, the walls of the lungs and gills are very thin and loaded with tiny tubes that transport blood (called “capillaries”).
This means the capillaries come into close contact with the air or water outside, letting oxygen pass across the thin walls and into the blood. At the same time, carbon dioxide, which is a waste product from our bodies, passes out.
Gills are also important for controlling how much salt is in the body, but let’s leave that story for another day.
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Climate change adds another overarching stressor to the mix. So how will sharks cope as the ocean heats up?
Our new research looked at Port Jackson sharks to find out. We found individual sharks adapt in different ways, depending where they came from.
Port Jackson sharks from cooler waters in the Great Australian Bight found it harder to cope with rising temperatures than those living in the warmer water from Jervis Bay in New South Wales.
This is important because it goes against the general assumption that species in warmer, tropical waters are at the greatest risk of climate change. It also illustrates that we shouldn’t assume all populations in one species respond to climate change in the same way, as it can lead to over- or underestimating their sensitivity.
But before we explore this further, let’s look at what exactly sharks will be exposed to in the coming years.
The southeast of Australia is a global change hotspot, with water temperatures rising at three to four times the global average. In addition to rising water temperatures, oceans are becoming more acidic and the amount of oxygen is declining.
Any one of these factors is cause for concern, but all three may also be acting together.
Rapid climate change represents an existential threat to all life on Earth and sharks can’t evolve fast enough to keep up because they tend to be long-lived with low reproductive output (they don’t have many pups). The time between generations is just too long to respond via natural selection.
Dealing with rising temperatures
When it comes to dealing with rising water temperature, sharks have two options: they can change their physiology to adapt, or move towards the poles to cooler waters.
Moving to cooler waters is one of the more obvious responses to climate change, while subtle impacts on physiology, as we studied, have largely been ignored to date. However, they can have big impacts on individual, and ultimately species, distributions and survival.
We collected Port Jackson sharks from cold water around Adelaide and warm water in Jervis Bay. After increasing temperatures by 3℃, we studied their thermal limits (how much heat the sharks could take before losing equilibrium), swimming activity and their resting metabolic rate.
While all populations could adjust their thermal limits, their metabolic rate and swimming activity depended on where the sharks were originally collected from.
With a rise in water temperature of just 3℃, the energy required to survive is more than twice that of current day temperatures for the Port Jackson sharks in Adelaide.
The massive shift in energy demand we observed in the Adelaide sharks means they have to prioritise survival (coping mechanisms) over other processes, such as growth and reproduction. This is consistent with several other shark species that have slower growth when exposed to warmer waters, including epaulette sharks and bonnethead sharks.
On the other hand, a 3℃ temperature rise hardly affected the energy demands of the Port Jackson sharks from Jervis Bay at all.
Threatening the whole ecosystem
Discovering what drives responses to heat is important for identifying broader patterns. For example, the decreased sensitivity of the Jervis Bay sharks likely reflects the thermal history of the region.
Australia’s southeastern coastline is warmed by the East Australian Current, which varies in strength both throughout the year and from year to year. With each generation exposed to these naturally variable conditions, populations along this coastline have likely become more tolerant to heat.
Populations in the Great Australian Bight, in contrast, don’t experience such variability, which may make them more susceptible to climate change.
So why is this important? When sharks change their behaviour it affects the whole ecosystem.
The implications range from shifts in fish stocks to conservation management, such as where marine reserves are assigned.
Port Jackson sharks, for example, are predators of urchins, and urchins feed on kelp forests — a rich habitat for hundreds of marine species. If the number of sharks decline in a region and the number of urchins increase, then it could lead to the loss of kelp forests.
There’s little research dedicated to understanding how individuals from different populations within species respond to climate change.
We need more of this kind of research, because it can help identify hidden resilience within species, and also highlight populations at greatest risk. We have seen this in action in coral bleaching events in different parts of Australia, for example.
We also need a better handle on how a wide range of species will respond to a changing climate. This will help us understand how communities and ecosystems might fragment, as each ecosystem component responds to warming in different ways and at different speeds.
Steps need to be taken to address these holes in our knowledge base if we’re to prepare for what follows.
A recent global assessment of shark populations at 371 coral reefs in 58 countries found no sharks at almost 20% of reefs and alarmingly low numbers at many others.
The study, which involved over 100 scientists under the Global FinPrint project, gave New Zealand a good score card. But because it focused on coral reefs, it included only one region — Rangitāhua (Kermadec Islands), a pristine subtropical archipelago surrounded by New Zealand’s largest marine reserve.
It is a different story around the main islands of New Zealand. Many coastal shark species may be in decline, and less than half a percent of territorial waters is protected by marine reserves.
In New Zealand, there are more than a hundred species of sharks, rays and chimaeras. They belong to a group of fishes called chondrichthyans, which have skeletons of cartilage instead of bone.
Some 55% of New Zealand’s chondrichthyan species are listed as “not threatened” by the International Union for Conservation of Nature (IUCN). Not so encouraging is the 32% of species listed as “data deficient”, meaning we don’t know the status of their populations. Most species (77%) live in waters deeper than 200 metres.
Seven species are fully protected under the Wildlife Act 1953. They are mostly large, migratory species such as the giant manta ray. Some are threatened with extinction according to the IUCN, including great white sharks, basking sharks, whale sharks and oceanic white tip sharks.
Historically, basking sharks were caught as bycatch in New Zealand fisheries, and seen in their hundreds in some inshore areas. Sightings of these giant plankton-feeders suddenly dried up over a decade ago. We don’t know why.
Commercial shark fisheries
Eleven chondrichthyan species are fished commercially in New Zealand under the quota management system. Commercial fisheries for school shark, rig and elephant fish took off from the 1970s and now catch around 8,000 tonnes per year in total.
Not surprisingly, the global assessment found a ban on shark fishing to be the most effective intervention to protect sharks. Several countries have recently established large shark sanctuaries, sometimes covering entire exclusive economic zones.
These countries tend to have ecotourism industries that provide economic incentives for protection — live sharks can be more valuable than dead ones.
Other effective interventions are restrictions on fishing gear, such as longlines and set nets.
Waters within 12 nautical miles of the Kermadec Islands have been protected by a marine reserve since 1990. In 2015, the Kermadec Ocean Sanctuary was announced but progress has stalled. The sanctuary would extend the boundaries to the exclusive economic zone, some 200 nautical miles offshore, and increase the protected area 83-fold.
A large population of Galapagos sharks, which prefer isolated islands surrounded by deep ocean, thrive around the Kermadec Islands but are found nowhere else in New Zealand. Great white sharks also visit en route to the tropics. Many other species are found only at the Kermadecs, including three sharks and a sex-changing giant limpet as big as a saucer.
What makes the Global FinPrint project so valuable is that it uses a standard survey method, allowing data to be compared across the globe. The method uses a video camera pointed at a canister of bait. This contraption is put on the seafloor for an hour, then we watch the videos and count the sharks.
Baited cameras have been used in a few places in New Zealand but there are no systematic surveys at a national scale. We lack fundamental knowledge about the distribution and abundance of sharks in our coastal waters, and how they compare to the rest of the world.
Sharks are generally more vulnerable to exploitation than other fishes. While a young bony fish can release tens of millions of eggs in a day, mature sharks lay a few eggs or give birth to a few live young. Females take many years to reach sexual maturity and, in some species, only reproduce once every two or three years.
These biological characteristics mean their populations are quick to collapse and slow to rebuild. They need careful management informed by science. It’s time New Zealand put more resources into understanding our oldest and most vulnerable fishes, and the far-flung subtropical waters in which they rule.
Shark skin might look perfectly smooth, but inspect it under a microscope and you’ll notice something strange. The entire outer surface of a shark’s body is actually covered in sharp, little scales known as denticles. More remarkable still, these denticles are incredibly similar to human teeth, as they’re also comprised of dentine and enamel-like materials.
Your dentist will no doubt have warned you that acidic drinks like fizzy cola damage your teeth. This is because acid can dissolve the calcium and phosphate in the enamel tooth covering. For the first time, scientists have discovered a similar process acting on the tooth-like scales of sharks in the ocean.
The carbon dioxide (CO₂) that humans release into the atmosphere doesn’t just heat the planet. As more of it dissolves in the ocean, it’s gradually increasing the acidity of seawater. In the past 200 years, the ocean has absorbed 525 billion tonnes of CO₂ and become 30% more acidic as a result. Now scientists worry that the lower pH is affecting one of the ocean’s top predators.
An unwelcome sea change
Over hundreds of millions of years, the denticles that make up shark skin have evolved to allow sharks to thrive in different environments. Different species have distinct denticle shapes and patterns that enable a range of remarkable functions. I’ve spent the last four years attempting to understand how the development of these scales is genetically controlled in shark embryos, and how their intricate details give each species an edge.
Denticles have highly specialised ridges which help reduce drag by up to 10%, allowing sharks to swim further and faster while using less energy. This works in a similar fashion to the ridges in the hulls of speed boats, which help the vessel move more efficiently through the water. In fact, these scales are so effective at reducing drag that scientists and engineers have long tried to create shark skin-inspired materials for boats and aircraft that can help them travel further on less fuel.
The patterning of denticles also works as a defensive armour, which protects sharks from their environment and from other predators. Some female sharks – such as the small-spotted catshark – have even developed a region of enlarged denticles which provide protection from a male shark’s bites during mating.
The changing chemistry of the ocean has been linked to coral bleaching, but its effect on other marine animals is less clear. To address this, researchers exposed puffadder shysharks – a species found off the coast of South Africa – to different levels of acidity in aquariums, and used a high-resolution imaging technique to examine the effect of acid exposure on their skin. After just nine weeks, they found that increased water acidity had weakened the surfaces of their denticles.
Corrosion and weakening of the denticle surface could degrade the highly specialised drag-reducing ridges, affecting the ability of these sharks to swim and hunt. Many shark species are top-level predators, so if they’re not able to hunt as effectively, this might have an unpredictable impact on the population size of their prey and other animals in the complex marine environment. Some species of shark need to swim constantly to keep oxygen-rich water flowing over their gills and to expel CO₂ – another process which might be hindered by increased drag.
Sharks belong to an ancient group of vertebrates known as the cartilaginous fishes, which split from the bony fishes – a lineage that later gave rise to humans – roughly 450 million years ago. Sharks, and other cartilaginous fish like rays, arose long before the dinosaurs, and have outlived multiple mass extinction events. But the rate of change in the marine environment over the last few centuries is an unprecedented challenge. These ancient predators may struggle to adapt to the fastest known change in ocean chemistry in the last 50 million years.
As Australians look forward to the summer beach season, the prospect of shark encounters may cross their minds. Shark control has been the subject of furious public debate in recent years and while some governments favour lethal methods, it is the wrong route.
We examined the world’s longest-running lethal shark management program, the New South Wales Shark Meshing (Bather Protection) Program, introduced in 1937. We argue it is time to move on from shark nets and invest further in lifeguard patrol and emergency response.
Managing shark bite
In NSW, 51 beaches between Newcastle and Wollongong are netted. The nets don’t provide an enclosure for swimmers. They are 150 metres long and suspended 500 metres offshore. In the process of catching targeted sharks they also catch other animals including turtles, rays, dolphins, and harmless sharks and fish.
Catching and killing sharks might seem a commonsense solution to the potential risk of shark bite to humans. But the story is not so simple.
Most research and public debate focuses on human safety or marine conservation. Our research sought to bring the two into conversation. We considered a range of factors that contribute to safety and conservation outcomes. This included catch of target and non-target species in nets, damage to marine ecosystems, global pressures on oceans, changing beach culture, human population growth and changes in lifeguarding and emergency response. Here’s what we found.
Fewer sharks, fewer bites
As the graph below shows, shark catch in the NSW netting program has fallen since the 1950s. This includes total shark numbers and numbers of three key target species: white shark (also known as great white or white pointer), tiger shark and bull shark.
But if we take a longer term view, we see that shark bite incidence relative to population is substantially lower from the mid-20th century than during the decades before.
The decline in shark bite incidence is great news. But key points are frequently overlooked when society tries to make sense of the figures.
Lifeguard patrol and emergency response are key
In NSW, lifeguard beach patrol grew over the same time period as the shark meshing program. More people swam and surfed in the ocean from the early 20th century as public bathing became legal. The surf lifesaving and professional lifeguard movements grew rapidly in response.
Today, 50 of the 51 beaches netted through the shark meshing program are also patrolled by lifeguards or lifesavers. Yet improved safety is generally attributed to the mesh program. The role of beach patrol is largely overlooked.
So, claims that shark bite has declined at netted beaches might instead be interpreted as decline at patrolled beaches. In other words, reduced shark interactions may be the result of beach patrol.
More good news is that since the mid-20th century the proportion of shark bites leading to fatality has plummeted. This is most likely the result of enormous improvements in beach patrol, emergency and medical response.
It’s time to move on from shark nets
Debate over shark management is often polarised, pitting human safety against marine conservation. We have brought together expertise from the social sciences, biological sciences and fisheries, to move beyond a “people vs sharks” debate.
There is no reliable evidence that lethal shark management strategies are effective. Many people oppose them, institutions are moving away from them, and threatened species are put at risk.
The NSW Department of Primary Industries, manager of the shark meshing program, is investing strongly in new non-lethal strategies, including shark tagging, drone and helicopter patrol, personal deterrents, social and biophysical research and community engagement. Our study provides further evidence to support this move.
Investing in lifeguard patrol and emergency response makes good sense. The measures have none of the negative impacts of lethal strategies, and are likely responsible for the improved safety we enjoy today at the beach.
Most of the 24 million annual visitors to Queensland don’t notice the series of seemingly innocuous yellow buoys at many popular beaches. Beneath the waves lies a series of baited drumlines and mesh nets that aim to make Queensland beaches safe from the ominous threat of sharks.
Earlier this week the Queensland government lost a legal challenge in the Federal Court to continue its shark culling program in protected areas of the Great Barrier Reef, and Fisheries Minister Mark Furner has written to the federal government to request legal changes to keep the program operating.
Through a series of baited drumlines and mesh nets, shark control programs aim to reduce local populations of large sharks, thereby reducing the number of times humans and shark meet along our coastline.
Scientists believe that resident sharks may learn that nets and drumlines placed in their local areas represent an obstacle and actively avoid them. This in itself deters and reduces the local population of large sharks in that particular area.
Second, there’s no clear evidence that sharks avoid drumlines. In fact, baited drumlines and nets actively attract, not deter, large sharks. Similar programs in Hawaii were stopped after an expert review concluded their effectiveness had been overstated.
While a reduction in fatalities is often attributed to the success of the shark control program, it could also be that reduced response times and better medical interventions are more successful at saving lives in recent decades.
Culls, nets and baited drumlines are a blunt tool, unable to completely remove the threat of people and sharks meeting on our beaches. Advances in technology and improved education of swimmers may be a more effective way to create safer beaches in Queensland with less ecological cost.
Modern technology allows us to help people avoid sharks, by modifying our behaviour at beaches. Shark-detecting drones are being trialled on New South Wales beaches as part of that state’s A$16 million shark management strategy, allowing for real-time monitoring of popular coastal areas.
Underwater “clever buoys” installed at NSW beaches in place of baited drumlines allow for real-time detection of sharks using sonar technology, instantly notifying lifeguards of the location, size and direction of sharks. Solar-powered, beach-based shark warning systems operate on remote beaches in Western Australia, cutting the response time between shark sightings and authorities alerting beachgoers from nearly an hour to a matter of minutes.
Education about shark behaviour can also help. Sharks are more active in certain places, like river mouths, and at certain times, such as at dawn and dusk.
In fact, the Queensland government is prioritising research into shark and human behaviours. This research could support education that mitigates the risk of shark interactions, without causing ecological harm.
Earlier this year the Queensland government committed to a A$1 million annual funding boost towards trialling alternative technologies. Adoption of modern innovations and better education for the general public would improve beach safety while avoiding the expensive and ineffective methods of culls, baited drumlines, and nets.
The cost of shark control programs
While we will never have an exact idea of how many sharks used to roam the eastern coastline, historical estimates from shark control programs suggest that the number of large sharks has declined by 72-97% in Queensland and by as much as 82% in NSW since the middle of the 20th century.
NSW and Queensland shark controlprograms combined have removed more than 1,445 white sharks from the eastern Australian coastline since the middle of the 20th century. To put this in context, current estimates indicate that the eastern population of white sharks sits at around 5,460 individuals in total.
The idea that sharks numbers have boomed in recent years represents a classic example of shifting baseline syndrome. The number of sharks on our beaches may seem to have grown since the late 1990s, but it is a fraction compared with a 1960s baseline, and long-term trends indicate that declines are ongoing.
The number-one priority at our beaches is keeping swimmers safe. At the same time, we have a responsibility to protect threatened and endangered species. There are smarter ways to manage both humans and sharks that will make our beaches safer and help protect sharks.
Unlike the many species which stalk the shallow, coastal waters that fisheries exploit all year round, pelagic sharks roam the vast open oceans. These are the long-distance travellers of the submarine world and include the world’s largest fish, the whale shark, and also one of the fastest fish in the sea, the shortfin mako shark, capable of swimming at 40mph.
Because these species range far from shore, you might expect them to escape most of the lines and nets that fishing vessels cast. But over the last 50 years, industrial scale fisheries have extended their reach across the world’s oceans and tens of millions of pelagic sharks are now caught every year for their valuable fins and meat.
On average, large pelagic sharks account for over half of all shark species identified in catches worldwide. The toll this has taken on species such as the shortfin mako has prompted calls to introduce catch limits in the high seas – areas of the ocean beyond national jurisdiction where there is little or no management for the majority of shark species.
We wanted to know where the ocean’s shark hotspots are – the places where lots of different species gather – and how much these places are worked by fishing boats. We took up the challenge of finding out where pelagic sharks hang out by satellite tracking their movements with electronic tags. This approach by our international team of over 150 scientists from 26 countries has an important advantage over fishery catch records. Rather than showing where a fishing boat found them, it can precisely map all of the places sharks visit.
Nowhere to hide
For a new study published in Nature we tracked nearly 2,000 sharks from 23 different species, including great whites, blue sharks, shortfin mako and tiger sharks. We were able to map their positions in unprecedented detail and discern the most visited hotspots where sharks feed, breed and rest.
Hotspots were often located in frontal zones – boundaries in the sea between different water masses that can have the best conditions of temperature and nutrients for phytoplankton to bloom, which attracts masses of zooplankton, as well as the fish and squid that sharks eat.
Then we calculated how much these hotspots overlapped with global fleets of large, longline fishing vessels, which we also tracked by satellite. This type of fishing gear is used very widely on the high seas and catches more pelagic sharks than trawls and other gear. Each longline vessel is capable of deploying a 100km long line bearing over 1,000 baited hooks.
We found that even the most remote parts of the ocean that are many miles from land offer pelagic sharks little refuge from industrial-scale fishing fleets. One in four of the places sharks visited each month overlapped with the areas longline fishing vessels operated in.
Sharks such as the North Atlantic blue and the shortfin mako – which fishers also target for their fins and meat – were much more likely to encounter these vessels, with as much as 76% of the places these species visited most in each month overlapping with where longline vessels were fishing. Even internationally protected species such as great whites and porbeagle sharks encountered longline vessels in half of their tracked range.
It’s now clear that much of the world’s fishing activity on the high seas is centred on shark hotspots, which longlines rake for much of the year. Many large sharks, which are already endangered, face a future without refuge from industrial fishing in the places they gather.
High seas marine protected areas
The maps of shark hotspots and longline fishing activity that we created can at least provide a blueprint for where large-scale marine protected areas aimed at conserving sharks could be set. Outside of these, strict quotas could reduce catches.
The United Nations is creating a high seas treaty for protecting ocean biodiversity – negotiations are due to continue in August 2019 in New York. They’ll consider large-scale marine protected areas for the high seas and we’ll suggest where these could be located to best protect pelagic sharks.
Satellite monitoring could give real-time signals of where sharks and other threatened creatures such as turtles and whales are gathering. Tracking where these species roam and where fishers interact with them will help patrol vessels monitor these high-risk zones more efficiently.
Such management action is overdue for many shark populations in the high seas. Take North Atlantic shortfin makos – not only are they overfished
and endangered, but now we know they have no respite from longline fishing during many months of the year in the places they gather most often. Some of these shark hotspots may not exist in the near future if action isn’t taken now to conserve these species and the habitats they depend on.
Our beaches are our summer playgrounds, yet beach litter and marine debris injures one-fifth of beach users, particularly children and older people.
Our research, published in the journal Science of the Total Environment, found more than 7,800 injuries on New Zealand beaches each year – in 2016, some 595 of them were related to beach litter. The most common injuries caused by litter were punctures and cuts, but they also included fractured limbs, burns, head trauma, and even blindness.
Children under 14 suffered 31% of all beach litter injuries, and were injured by beach litter at twice the rate compared with other locations in New Zealand. Beach litter injury claims exceeded NZ$325,000 in 2016, representing a growing proportion of all beach injury claims. Beach injury claims changed from 1.2% of the total in 2007 to 2.9% in 2016.
Our study relied on reported injury insurance claims in New Zealand, and thus probably underestimates the true injury rate, particularly for minor wounds. Our 2016 survey of beachgoers in Tasmania found that 21.6% of them had been injured by beach litter at any time previously – even on the island state’s most picturesque beaches.
Alarmingly, most beach users in the Tasmanian survey did not consider beach litter an injury risk, despite the high rate of self-reported injuries.
Awash with danger
As more debris washes ashore and our recreational use of our coasts increases, it is more likely than ever before that we will encounter beach litter, even on remote and “pristine” beaches.
Global studies have found up to 15 items of debris per square metre of beach, even in remote locations. On Henderson Island – a supposedly pristine South Pacific outpost miles from anywhere – some 3,570 new pieces of litter arrive every day on one beach alone.
Beach litter typically includes a huge range of items, such as:
sharp and rusted metal such as car bodies, food cans, fish hooks, and barbed wire
flammable or toxic materials such as cigarette lighters, flares, ammunition and explosives, and vessels containing chemicals or rotten food
sanitary and medical waste such as used syringes, dirty nappies, condoms, tampons and sanitary pads
bagged and unbagged dog faeces and dead domestic animals.
The health hazards posed by beach litter include choking or ingesting poisons (particularly for young children), exposure to toxic chemicals, tripping, punctures and cuts, burns, explosions, and exposure to disease.
Degrading plastic can also produce toxins that contaminate seafood, potentially entering human or ecological food chains.
Despite the potential severity of these hazards our understanding and study of human health impacts from beach litter is poor. We know more about the impacts of beach litter and marine debris on wildlife than on humans.
Two of our previous studies in Australia and New Zealand have found beach litter that can cause punctures and cuts at densities 227 items per 100 square metres of beach, and choking hazards at densities of 153 items per 100 square metres of beach. These exposures to beach litter hazards in Australia and New Zealand may be 50% higher than global averages (based on preliminary data).
Even “clean” beaches can be hazardous, and may even increase the likelihood of injury. Visitors to a recently cleaned or supposedly “pristine” beach may be less vigilant for hazards. What’s more, Europeanstudies have found that actively cleaned beaches can still have hazardous debris items.
The risk of injury will continue to increase without concerted efforts to prevent addition of new debris and the active removal of existing rubbish. Besides watching where we tread when at the beach and participating in beach cleanups, we also need to make sure we deal with rubbish thoughtfully, so litter doesn’t end up there in the first place.
Catch numbers of large apex sharks (hammerheads, tigers and white sharks) declined by 74-92%, and the chance of catching no sharks at any given beach per year has increased by as much as seven-fold.
Coinciding with ongoing declines in numbers of sharks in nets and drum lines, the probability of recording mature male and females has declined over the past two decades.
Our discovery is at odds with recent media reports of “booming” shark numbers reaching “plague” along our coastlines. The problem with those claims is that we previously had little idea of what the “natural” historical shark population would have been.
Why is the decline of sharks on the Queensland coastline a cause for concern? Large apex sharks have unique roles in coastal ecosystems, preying on weak and injured turtles, dolphins and dugongs, actively scavenging on dead whale carcasses, and connecting coral reefs, seagrass beds and coastal ecosystems.
Australia’s mixed view on sharks
As a nation, Australia has a long history with sharks. Some of the oldest stories in the world were written by the indigenous Yanyuwa people in the Northern Territory some 40,000 years ago, describing how the landscape of their coastal homeland was created by tiger sharks.
European settlers in the late 18th and early 19th centuries further described Australian coastlines as being “chock-full of sharks”, and upon visiting Sydney in 1895, the US author Mark Twain remarked:
The government pays a bounty of the shark; to get the bounty the fishermen bait the hook or the seine with agreeable mutton; the news spreads and the sharks come from all over the Pacific Ocean to get the free board. In time the shark culture will be one of the most successful things in the colony.
With the rise of Australian beach and surf culture, and the growing population density in coastal communities in the mid-20th century, increasing numbers of unprovoked fatal encounters with sharks occurred along the Queensland and New South Wales coastlines.
Yet despite this long history of shark exploitation, the historical baseline populations of sharks off Australia’s east coast were largely unknown.
Through mesh nets and baited drumlines, the Queensland Shark Control Program targets large sharks, with the aim of reducing local populations and minimise encounters between sharks and humans. Records of shark catches dating back as far as the 1960s provide a unique window into the past on Queensland beaches.
While we will never know exactly how many sharks roamed these waters more than half a century ago, the data points to radical changes in our coastal ecosystems since the 1960s.
The exact causes of declining shark numbers are difficult to pinpoint, largely because of a lack of detailed records from commercial or recreational fisheries before the 2000s. The Queensland government also acknowledges that the program itself has a direct impact on shark populations by selectively removing large, reproductively mature sharks from the population.
The data indicates that two hammerhead species – the scalloped and great hammerheads, both of which are listed as globally endangered – have declined by as much as 92% in Queensland over the past half century.
Similarly, the once-abundant white sharks have also shown no sign of recovery, despite a complete ban on commercial and recreational fishing in Queensland, implemented more than two decades ago.
The idea that shark populations are reaching “plague” proportions in recent years may represent a classic case of shifting baseline syndrome. Using shark numbers from recent history as a baseline may give a false perception that populations are “exploding”, whereas records from fifty years ago indicate that present day numbers are a fraction of what they once were.
Our results indicate that large shark species are becoming increasingly rare along Australia’s coastline. We should not be concerned about a “plague” of sharks, but rather the opposite: the fact that previously abundant apex shark species are increasingly at risk.