Can even a little fishing be too much?

Grey reef shark numbers have dropped by 75% in the remote reaches of the Pacific Ocean, even though only a small fishery is active there and shark fishing and trade bans have recently been implemented. This revelation is significant with regard to the poorly understood but vitally important small-scale fisheries and their impacts on sharks globally.

Teraina is remote. The kind of remote that means you pass Australia and Papua New Guinea to find the palm-fringed islands of Samoa and New Caledonia. And then you zoom in a bit more on a world map to finally pinpoint Kiribati, lost in the Pacific Ocean between Hawaii and French Polynesia. Even then, the green fleck in Kiribati’s Northern Line Islands isn’t visible until there’s almost nothing left on your world map but blank blue ocean and one tiny coral atoll.

The only way to access Teraina is by boat.

And it’s typically here, in the wildest reaches of our planet, far removed from cities and bustling coastlines, that we have believed that sharks – and nature – are still safe and thriving.

But a study published at the end of 2024 in the journal Marine Ecology Progress Series shatters this illusion. The paper was written by Maurice Goodman from Stanford University, with co-authors that include Professor Mahmood Shivji, the director of the Save Our Seas Foundation Shark Research Center (SOSF-SRC) and Guy Harvey Research Institute, Nova Southeastern University.

Goodman and his colleagues have shown that even out here, where we think that people are outnumbered by seabirds and turtles and fringing corals – and are seemingly out of reach of frantic trade routes – numbers of grey reef sharks have dropped by as much as 75% in 15 years.

Overfishing is driving sharks to extinction globally

One-third of all sharks and rays on earth are heading to extinction. And it’s easy enough to pinpoint the single greatest issue driving the declines: overfishing.

In fact, all threatened sharks are impacted by overfishing, and for two-thirds of these species, the sole threat is fisheries (the remaining one-third are also impacted by climate change, habitat loss and pollution). Coastal sharks in the tropics and subtropics are bearing the current brunt of these declines.

And it’s not only shark and ray diversity that is at stake; the latest research shows that shark and ray abundance has halved in as little as 50 years. As we move into an increasingly challenging period on earth, we risk losing both the breadth of life that performs a range of ecological functions in our oceans and the sheer number of sharks in the sea that are available for food and livelihood security.

One-third of global reef shark catches come from small-scale fisheries

There is enormous pressure on our oceans, and much of the blame lies squarely at the feet of large-scale fisheries for their unsustainable harvest of sharks. As much as 37% of shark species are in decline due to industrial fishing.

But the issue gets more complicated: 96% of all threatened sharks are taken by industrial fisheries in combination with other fisheries. Most sharks are caught unintentionally, as bycatch, and are generally retained to be consumed, marketed or traded. In its 2024 report on the global status of sharks, rays and chimaeras, the International Union for Conservation of Nature (IUCN) Shark Specialist Group highlights that only 26% of shark species are actually targeted by fisheries. By far the most overwhelming issue is the incidentally caught sharks that are then retained or are in no condition to survive if they are released.

The impact of industrial-scale fishing on shark populations is compounded by the shark bycatch in small-scale fisheries; for reef sharks specifically, one-third of global catches emanate from small-scale fisheries.

Despite what their name implies, small-scale fisheries are central to the ocean economy. According to estimates from reviews in 2005 and 2011, they generate nearly a quarter of the world’s consumable fish and directly employ 44% of people involved in fishing. Lydia Teh and Rashid Sumaila estimated in their study in 2013 that about 22 million of the 260 million people involved in global marine fisheries are small-scale fishers.

While the statistics illustrate how integral small-scale operations are to food and job security, we are also increasingly understanding the importance of what non-monetary value these fisheries bring to people: their identity and links between kin, households and communities. These are often fisheries comprising vulnerable people at the coal face of the biodiversity and climate crises, whose fishing activities are – according to Teh and Sumaila’s characterisation – generally the kind of effort geared towards household consumption or local sale, conducted at low economic activity levels with minimal mechanisation, and mostly in inshore coastal areas.

Sharks are, in general, slow-growing animals that reach sexual maturity later in life and reproduce slowly, which is why they are so susceptible to fishing pressure. Reef sharks are a particularly worrying group. They include species that are generally associated with coral reefs around the world. In most cases, these are coastal habitats that invariably bring sharks and humans into closer contact. And it’s this group of sharks that sits right at the confluence of most of our recent discoveries from global conservation assessments: extinction risk is highest in tropical coastal waters, and especially where people are disproportionately reliant on fisheries for food security. Reef sharks are often highly resident, are typically found in places where fisheries are poorly understood and managed, and are caught in fisheries that, more often than not, are unreported in global FAO fisheries statistics.

What are reef sharks?

Reef sharks live primarily on shallow tropical coral reefs around the world. In their 2015 paper detailing recent advances in our understanding of reef sharks, scientists Geoffrey Osgood and Julia Baum listed 29 species as reef sharks. They noted at the time that nine of these species were listed as Data Deficient on the IUCN Red List of Threatened Species.
And herein lies something of a problem. Although coral reefs are some of the world’s most popular dive spots and are highly utilised regions where fisheries, tourism and a host of other activities collide, we know surprisingly little about the sharks that live there. As Osgood and Baum point out, even ecologist Peter Sale’s seminal books on coral reefs and their ecology make no mention of these large, charismatic – and surely, important – reef-dwellers in either the original 1991 classic edition or the 2006 follow-up. For all their visibility, reef sharks seem to have coasted under the research radar for longer than we’d like to admit.
This is a trend that is changing; there’s been a sharp rise in reef shark research in the past 30 years. Osgood and Baum identified 1,101 scientific studies in their review in 2015. They identify three main orders: the bullhead sharks, carpet sharks and ground sharks. Of these, perhaps those most familiar to us are the sharks of the order Carcharhiniformes, which are all teetering on the edge of uncertainty, with worrying trends in their IUCN Red List statuses: silvertip shark (Vulnerable), grey reef shark (Endangered), blacktip reef shark (Vulnerable), Caribbean reef shark (Endangered), Galápagos shark (Least Concern) and whitetip reef shark (Vulnerable).
By definition, reef sharks tend to be restricted to their preferred habitat – a trait scientists dub ‘high site fidelity’. This increases their vulnerability to overfishing. Beyond the general liking for complex coral-covered reef habitats, the preferences of reef shark species for different types of coral-covered habitats vary as widely as the group itself. The tawny nurse shark, epaulette shark, zebra shark and blacktip reef shark, for instance, all seem to prefer shallow lagoon, reef flat and reef ledge habitats. This contrasts with the preference for deeper reef habitats shown by the Galápagos shark, Caribbean reef shark and grey reef shark.
The type of coral cover, the depth of habitat and the water temperature are all environmental factors that drive shark habitat selection. However, other biological factors like competition between species also play an important and complex role. Overall, numerous tracking studies have confirmed that many reef sharks maintain limited home ranges, in addition to showing residency and high site fidelity.

Coral reefs need reef sharks

Coral reefs are complex submarine citadels, filled with tens of thousands of different marine species. Our scientific lens has only really been trained on these ecosystems for the past 50 years but, as many scientists point out, coral reefs have been used by human beings for thousands of years. Even at low levels, this focused and sustained exploitation may have reduced the number of big predators on these reefs over time. And so reef sharks were technically already ‘in the red’ long before modern coral reef scientific surveys started.

In the western Pacific specifically, fishing on coral reefs started at least 35,000 years ago. The ecological impact of this is thought to have been limited, but we know that coral reefs have been critical habitats for different reef shark species, many of which remain resident on reefs throughout their entire life cycle.

There are also sharks that maintain large home ranges and migrate across different ecosystem types; tiger sharks and great hammerhead sharks, for instance, move across coral reef habitats as part of their wider use of other coastal and pelagic (open ocean) ecosystems. These more transient reef-users include bull sharks, silky sharks, sandbar sharks and dusky sharks, all of which would be considered top-tier apex predators that sit high in the ocean food web. And it is thought that, even though their prey includes sharks, mammals and turtles and they are not resident on coral reefs, their nomadic presence may still exert some kind of influence on the nature of coral reef communities.

On the other hand, the more reef-associated Galápagos sharks, whitetip reef sharks, blacktip reef sharks and grey reef sharks are mid-level predators. These species often have home ranges smaller in extent than 50 square kilometres (19.3 square miles) and might be highly specific to individual reefs in particular. The jury is out on the strength of evidence for strong trophic cascades linked to the presence or absence of reef sharks on coral reefs, but certainly their mid-trophic web position places most of these sharks alongside a diversity of reef fish. There is so much more work to be done exploring what ecological – not merely what trophic – role these predators play in the systems they inhabit. As George Roff and his co-authors wrote in their 2016 study ‘The ecological role of sharks on coral reefs’: ‘Restoring populations of sharks is important and can yet deliver [an] ecological surprise.’

Reef sharks are disappearing from coral reefs around the world

In 2020, Aaron MacNeil published an estimate of global reef shark populations from 15,000 baited underwater video system (BRUVs) deployments. The survey gathered data from 371 reefs in 58 nations, and the results were shocking: absolutely no sharks were observed on almost 20% of the reefs surveyed. Reef sharks had all but completely disappeared from reefs in several nations; in eight nations they were considered functionally extinct.

The larger the human population near the coast, MacNeil found, the fewer the reef shark observations. Similarly, it appeared that the closer a local market, the fewer the sightings of reef sharks. This is surprising, because shark fins are considered non-perishable and traders will often travel to remote communities – and fishers will travel great distances – to trade in shark fins. So this result seems to indicate that the rise in demand for exported shark fins had catalysed growth in the local market for shark meat.

This fits with what Nick Dulvy pointed out in 2021: even where sharks are not the intended catch, retention of sharks is high. It’s a finding that still rings true today; the IUCN’s report on the global status of sharks, rays and chimaeras revealed that often markets for shark meat have found local footing with the increase in catches to supply the global fin trade.

The human impact on reef shark populations in the Pacific worries scientists

On Teraina, sharks were traditionally harvested only for cultural reasons – and then only occasionally. The low-lying coral atoll was uninhabited until the 1930s and its population was low right through to the 1990s. By 2010, Teraina was recorded as the least populated of the Northern Line Islands, but more densely populated than its nearest inhabited neighbour (Tabuaeran Atoll) and Kiritimati (the largest atoll in Kiribati): its population sat at 1,690 residents.

Around the year 2000, subsistence fishing was rapidly transitioning to targeted fishing for shark fin sales to foreign vessels. By 2013, 17 fishers were identified on the atoll, and the main reason given for targeting sharks was to sell fins. When Maurice Goodman and his colleagues interviewed Teraina’s fishers in 2013, only 4% of families owned motorised boats. Paddle and sail canoes were also available, but fishing offshore was limited without a motor. The combination of interviews conducted in 2013 and again in 2019 points to a decline in reef shark catches that parallels the rise in shark fin value, fishing rates and a shift from cultural and subsistence-based shark fishing to an export industry.

At the same time, the researchers combined their interviews with fishers with data collected from tagged grey reef sharks at Palmyra Atoll nearby to model their population. And findings from ecological surveys corroborated both the fishers’ testimonies and the statistical modelling results: using a variety of different survey techniques, no grey reef sharks were recorded in either the Line Islands or the neighbouring Gilbert Islands – and only very low numbers of other species like blacktip and whitetip reef sharks were observed.

This is as surprising as it is concerning, most especially since the declines are the result of only a small number of fishers and motorised boats. We often speak of sharks being highly susceptible to overfishing, mostly as a consequence of their vulnerable life histories. Nevertheless, it is astonishing that some sharks are so vulnerable that even modest, small-scale fishing effort can decimate populations. And today, even very geographically isolated communities are connected to global wildlife trade networks.

The future of small-scale fisheries matters

At first glance, this is a surprising study that speaks to how we shape, and reshape, our cultural relationships with resources. Dig a bit deeper, and we understand that in our globalised world there are few places the tendrils of global trade markets no longer reach. But it is critical to understand the impact of small-scale fisheries worldwide, not least because this study shows that remarkably low fishing pressure, with little to no technology, can still drive vulnerable shark populations to collapse.

Moreover, the importance of how we prioritise understanding and cooperating with small-scale fisheries has two sides: we must understand the impact of small-scale fisheries if we are to respond to biodiversity conservation and management appropriately. We must also understand that there is always an impact on small-scale fishers, their livelihoods and the cultural fabric of which they form part.

In 2016 Kiribati made the bold decision to ban commercial shark fishing across its 3.4 million square kilometres (1.3 million square miles), as well as the sale and trade of shark products. The impact that has had on shark harvests and the recovery of shark populations in the region is not yet known, but it’s likely that fishing has declined and that sharks could recover. However, the researchers point out that shark population abundances will remain low for decades yet.

What happens to shark populations targeted by small-scale fisheries matters because the nature of many (if not most) of these fisheries is that they are highly vulnerable to changes and declines, and they underpin food and job security across coastal communities. The IUCN’s report on the global status of sharks, rays and chimaeras highlighted that in some low-income nations more than 80% of reported fishing income comes from sharks, rays, skates and chimaeras.

In Teraina, the researchers point out that urgent attention needs to be paid to economic opportunities for small-scale fisheries, whose activities can result in catastrophic declines in the biodiversity on which they depend. It is essential, they say, to co-develop ‘adaptive, resilient and equitable management strategies with fishers and their communities, set long-term participatory goals to prioritise actions, build the learning capacity, knowledge access and agency of communities to manage their marine resources, and implement international trade regulations for shark products’.

Where spatial conservation strategies like marine protected areas are implemented (especially in the race to meet 30×30 ocean protection targets outlined by international agreements), there can be unintended consequences. The burden of exploitation of species may just shift elsewhere, but for fishers without the means to move elsewhere or offshore – or to pivot to alternative livelihoods – the impact can also be catastrophic.

More recent surveys of the communities of the Northern Line Islands suggest that people here were not overly reliant on sharks or shark products for their livelihoods, and that the economic implications from the 2016 ban have been minimal. But the factors involved in how this plays out are complex and highly localised, and because of this, what worked for Kiribati and the Northern Line Islands might not work for small-scale fisheries elsewhere.

The researchers showed that if Kiribati had not implemented the shark fishing ban in 2016, grey reef shark populations near Teraina would have been at or near total collapse. In the most optimistic scenario, their statistical modelling suggests that it will take more than a decade for these sharks to recover to some degree.

Small-scale fishing might not be the most important cause of shark mortality, and there are always dozens of factors to take into account (like illegal or unreported fishing activities), but the results of this study strongly suggest that the impact of small-scale fisheries on vulnerable shark species should not be discounted, and that understanding their dynamics will prove critical in our rapidly changing world.

References

Dulvy NK, Pacoureau N, Matsushiba JH, Yan HF, VanderWright WJ, Rigby CL, Finucci B, Sherman CS, Jabado RW, Carlson JK, Pollom RA, Charvet P, Pollock CM, Hilton-Taylor C, Simpfendorfer CA. 2024. Ecological erosion and expanding extinction risk of sharks and rays. Science 386: eadn1477.

Dulvy NK, Pacoureau N, Rigby CL, Pollom RA, Jabado RW, Ebert DA, Finucci B, Pollock CM, Cheok J, Derrick DH, Herman KB. 2021. Overfishing drives over one-third of all sharks and rays toward a global extinction crisis. Current Biology 31(21): 4773–4787.

Dulvy NK, Fowler SL, Musick JA, Cavanagh RD, Kyne PM, Harrison LR, Carlson JK, Davidson LN, Fordham SV, Francis MP, Pollock CM. 2014. Extinction risk and conservation of the world’s sharks and rays. eLife 3: p.e00590.

Goodman MC, White TD, Kazdan JL, Bradley D, Shivji M, Casagrandi R, Mari L, Gatto M, Eurich JG, McCauley DJ, Connor RJ. 2024. Reef shark population declines on remote Pacific reefs: inferences from multiple methods in a data-limited fishery. Marine Ecology Progress Series 751: 97–114.

MacNeil MA, Chapman DD, Heupel M, Simpfendorfer CA, Heithaus M, Meekan M, Harvey E, Goetze J, Kiszka J, Bond ME, Currey-Randall LM. 2020. Global status and conservation potential of reef sharks. Nature 583(7818): 801–806.

Nadon MO, Baum JK, Williams ID, Mcpherson JM, Zgliczynski BJ, Richards BL, Schroeder RE, Brainard RE. 2012. Re-creating missing population baselines for Pacific reef sharks. Conservation Biology 26(3): 493–503.

Osgood GJ, Baum JK. 2015. Reef sharks: recent advances in ecological understanding to inform conservation. Journal of Fish Biology 87(6): 1489–1523.

Roff G, Doropoulos C, Rogers A, Bozec YM, Krueck NC, Aurellado E, Priest M, Birrell C, Mumby PJ. 2016. The ecological role of sharks on coral reefs. Trends in Ecology & Evolution 31(5): 395–407.