You might imagine that sharks are indestructible; those intimidating teeth, the incredible speed, their ability to survive in the cold, dark, high pressure environment of the deepest oceans. However, in reality, sharks are extremely vulnerable to many anthropogenic threats and are declining globally to a shocking degree. So what are the threats that sharks face? Is finning the only issue? And is there any hope that we can turn it around?
Sharks are one of the Most Threatened Groups on the Planet
Sharks have been around for over 400 million years, but over the past decades, their populations have declined globally by as much as 70%. Today at least a quarter of all sharks and a third of all chondrichthyan fishes are at risk of extinction in the wild. Continuing assessment means that new species are added to the IUCN Red List of Threatened Species every year (Dulvy et al, 2021).
To give you some examples, such iconic species as the hammerheads (Sphyrna lewini and Sphyrna mokarran) are now considered Critically Endangered, the beautiful whale (Rhinocodon typus) and basking sharks (Cetorhinus maximus) are both classified as Endangered, and even the magnificent great white (Carcharodon carcharias) is classified as Vulnerable (Bland et al, 2017; IUCN, 2020).
Many people imagine that finning is the major driver of shark declines around the world. But whilst it is true that finning is a massive problem, it is far from the only threat sharks face. In fact, there is a myriad of different other fisheries issues and various types of habitat degradation that are a risk to sharks (IUCN, 2020).
Sharks are Extracted Unsustainably in Fisheries
By far the biggest threat to all sharks is fisheries. In total, it is estimated that 830,000 tonnes of sharks and rays are landed by fisheries every year (Lucifora et al, 2011) and this overfishing is a factor threatening many endangered species. Sharks have been targeted since the 1960s for many products, including their meat, skin, teeth, jaws and liver oil, and also for their fins, which are sold for shark fin soup on the lucrative Asian market (Stevens et al, 2000).
Sharks are also commonly caught as “bycatch”, meaning they are accidentally killed by fishing gear which is being used to fish other species. Many species of sharks which are victims of bycatch are not even particularly commercially valuable. Sometimes sharks caught as bycatch are thrown back in the hopes they survive, but they are also often landed anyway, to be sold as cheap meat. To learn more about this issue, check out If You Love Me Let Me Go and A Fish Out of Water.
As if this wasn't bad enough, many sharks are also killed every year by "ghost nets" - discarded fishing gear left to roam the ocean unsupervised (to learn more, see Ghosts in the Ocean) and in beach nets, which are implemented to protected ocean users at bathing beaches.
Sharks' Habitats are Being Seriously Degraded
In regards to habitat degradation, sharks can be impacted by pollution, ocean acidification and deoxygenation, prey depletion, habitat loss and urbanisation. The habitat can also be degraded by the loss of prey species that sharks depend on, which can be caused by overfishing, pollution or habitat loss. These processes are a particular problem for coastal shark species, which are especially exposed by their close proximity to human activity. Yet offshore sharks are also affected; especially by activities like shipping. To learn more, check out We're Gonna Need Fewer Boats.
There are many forms of pollution which can be detrimental to sharks. Chemical pollution from industries or oil spills are a serious problem. These accidents can be instantly lethal, but can also cause damage to the health of sharks when they are exposed throughout their lives. To learn more, see Not so Slick. A more insidious, potentially huge problem comes in the form of littering and plastic pollution, which can cause death by entanglement; ruining a shark's health or even causing a slow death by suffocation or starvation. To learn more, read The Worst... Beware of the Plastics!.
Habitats can also be damaged by human activities, such as shipping or dredging. Areas of critical habitat can be entirely lost due to urbanisation and development; with construction work damaging vital underwater structures or reclaiming areas for human use. Even the noise from these activities can also be seriously disruptive to sharks. To learn more about "noise pollution" check out Keep the Noise Down.
Habitat degredation also includes extended, ongoing damage from climate change. For example, climate change-associated ocean acidification, can have physiological effects, which damage the shark's health. Whatsmore, acidification can also erode eggs in “oviparous” (egg-laying) species, meaning populations cannot be bolstered by reproduction. To lean more, you can read Pickled Eggs. Similarly, ocean deoxygenation caused by climate change can change the distribution of certain species of sharks, alter the structure of their skin and affect their ability to find prey (Dziergwa et al, 2019).
Sharks are Especially Vulnerable to Population Declines
Confounding these issues, is the “life history strategy” employed by sharks. Sharks are described as having a “K-selected life-history strategy” (as opposed to “R-selected”), meaning they have very slow growth and late maturity, reach a large size over a long life-span, and require significant investments for reproduction. This means that, when overexploited to the point that populations are severely depleted, recovery can be incredibly slow, or even impossible.
However, there may be some hope… some light at the end of the tunnel… With growing public awareness and increased pressure generated by public opinion, conservation initiatives designed to protect sharks can gain momentum! It is up to each and every one of us who cares about sharks, to ensure our voices are heard, by supporting conservation and management measures, and demanding sustainability.
References
Bland L, Keith D, Miller R, Murray N & Rodríguez J. (2017). Guidelines for the application of IUCN Red List of Ecosystems Categories and Criteria, version 1.1. 10.2305/IUCN.CH.2016.RLE.3.en. Access online.
Dulvy NK, Pacoureau N, Rigby CL, Pollom RA, Jabado RW, Ebert DA., ... & Simpfendorfer CA (2021). Overfishing drives over one-third of all sharks and rays toward a global extinction crisis. Current Biology, 31:21, 4773-4787. Access online.
Dziergwa J, Singh S, Bridges CR, Kerwath SE, Enax J & Auerswald L (2019). Acid-base adjustments and first evidence of denticle corrosion caused by ocean acidification conditions in a demersal shark species. Scientific Reports, 9:1, DOI: 10.1038/s41598-019-54795-7. Access online.
IUCN (2020). The Red List of Threatened Species. Access online.
Lucifora, L.O., García, V.B. & Worm, B. (2011). Global diversity hotspots and conservation priorities for sharks. PLoS One, 6:5, e19356. Access online.
Stevens JD, Bonfil R, Dulvy NK & Walker PA (2000). The effects of fishing on sharks, rays and chimaeras (chondrichthyans), and the implications for marine ecosystems. ICES Journal of Marine Science, 57, 476-494. Access online.
Womersley F, Humphries NE, Queiroz N & Vedor M (2022). Global collision-risk hotspots of marine traffic and the world’s largest fish, the whale shark. Proceedings of the National Academy of Sciences, 119:20. Access online.
By Sophie A. Maycock for SharkSpeak
