Why are sharks threatened?PhD
Sharks are some of the most threatened animals in the world. Here’s why.
Sharks are fascinating, awe-inspiring, ecologically important and misunderstood. Unfortunately, they’re also threatened. According to the latest count by the IUCN Red List, 36% of shark species and their relatives are considered threatened with extinction – only amphibians are worse off! How is it possible that a group of animals that have been on this planet longer than trees and have survived multiple mass extinction events can face such a major conservation challenge?
Sharks have relatively few pups relatively infrequently – their very life history makes them inherently vulnerable
Many fish reproduce by spawning, releasing huge clouds of sperm and eggs into the water. The eggs will be fertilised externally, outside the female, which takes much, much less energy. It means that these fish can produce a lot more offspring at a time, even though many of them don’t survive. Shark eggs, in contrast, are fertilised internally. Some shark species lay eggs, some give live birth just like mammals and some have a strange mix of these methods. In every case, compared with the typical bony fish, sharks have relatively few offspring relatively infrequently, which means that their populations just can’t grow quickly. It also means that shark populations can’t bounce back very quickly from a population decline or collapse.
The greatest threat to shark populations is unsustainable fishing
Sharks are fished around the world to supply global markets for shark meat and fins. Incidentally, if you’ve only ever heard that sharks are fished for their fins, that’s a common and problematic misunderstanding. Sharks’ strategy of having relatively few young relatively infrequently means that they are extremely vulnerable to overfishing.
It is important to note that this doesn’t mean that there’s no such thing as a sustainable shark fishery – sustainable shark fisheries do exist. The life history of sharks means that they cannot recover from population declines as quickly as bony fish do, so shark fisheries have to be managed very carefully, generally with a much lower total allowable catch relative to their populations than bony fish. The idea of sustainable shark fishing is widely supported by experts and the best available data.
There are, however, many unsustainable shark fisheries and they lead to overfishing and population declines. Unsustainable commercial fishing practices are the greatest threat to shark populations by far.
Some (but not very many) shark species are threatened by habitat destruction
Sharks are a diverse group of animals found all over the world and they therefore face a variety of threats. Many people typically think that habitat destruction (i.e., physical damage to a habitat resulting in it no longer being a suitable place to live, as distinct from environmental changes like oxygen dead zones or climate change) only affects animals that live in rainforests, but the ocean is also composed of distinct habitats, and many sharks live in shallow coastal waters (or even rivers) for some or all of their life. For example, the destruction of coastal mangrove habitat in Bimini caused various forms of harm to local lemon shark populations. Some especially destructive fishing practices like bottom trawling (dragging a large and heavy net across the sea floor) can, if not properly managed, destroy habitat-building organisms like sponges and corals, which generally affects other marine life more than it affects sharks. Overall, nine species of threatened sharks have habitat destruction included in their list of threats.
Some already threatened shark species are further endangered by recreational fishing
Recreational fishing – fishing for fun rather than to get enough fish to sell – is not often thought of as a conservation issue. However, recent research shows that for populations of fish that are already depleted by other threats, recreational fishing can be a big problem, including for river sharks in Australia and hammerhead sharks in Florida. Generally speaking, environmentally friendly recreational fishing practices should include not targeting threatened species of fish!
Climate change and plastic pollution are big problems for many ocean animals, but not really for sharks
While climate change is causing many ecological disruptions around the world, it isn’t known to affect very many species of sharks very much. When waters get too hot for a shark species, the sharks can often just move somewhere else. Ocean acidification, sometimes called climate change’s ‘evil twin’, can cause birth defects and difficulties with the sense of smell in some shark species, but compared to the existential threat posed by overfishing this is generally not considered to be a major issue. Similarly, plastic pollution can be devastating to many types of marine animals, but to the best of our knowledge, it is not a population-level threat to any shark species. Put another way, if we totally solve overfishing but don’t solve climate change or plastic pollution, sharks will be in great shape. If we totally solve climate change and plastic pollution but don’t solve overfishing at all, sharks are still in very deep trouble. This certainly doesn’t mean that we shouldn’t fix climate change and plastic pollution – just that sharks aren’t a reason to prioritise this.
There is a lot of misinformation about threats to sharks and how to mitigate them
If any of the information in this article surprised you, please consider where you’re learning about sharks and their conservation. There is a staggering amount of misinformation about why sharks are in trouble and what we can do to help out there. Sometimes it’s shared by well-intentioned but uninformed shark enthusiasts, but often by people trying to take advantage of you. It’s great that you want to help to save sharks, but please get your information from qualified and experienced experts with credentials rather than from the loudest voices on social media!
Why are sharks threatened?
How does slow reproduction threaten sharks? Sharks have few pups quite infrequently, which means that their populations just can’t grow quickly. It also means that shark populations can’t bounce back very quickly.
How does overfishing threaten sharks? Slow reproduction combined with the massive demand for shark meat, liver oils and fins around the world has made sharks extremely vulnerable to unsustainable fishing practices.
NB: when the demand for a certain product becomes higher than the rate of production, the practice becomes unsustainable – in this case the product is sharks and the rate of production is how quickly they reproduce.
Recreational fishing Recent research shows that for populations of fish that are already depleted by other threats, recreational fishing can be a big problem.
How does habitat destruction threaten sharks? Critical habitats like mangroves, coral reefs and rivers are commonly affected by numerous destructive actions by humans, such as bottom trawling, encroachment, pollution and dredging, to name a few.
How does climate change threaten sharks? Climate change and ocean acidification do affect sharks, but perhaps not as drastically as they affect other marine species. If we totally solve overfishing but don’t solve climate change, sharks will be in great shape. If we totally solve climate change but don’t solve overfishing, sharks are still in very deep trouble.
How can I help save sharks? Learn from the best, not the loudest. If you want to help save sharks, make sure you get your information from qualified and experienced experts with credentials rather than from the loudest voices on social media.
David S. Shiffman, et al., 2020, Inaccurate and Biased Global Media Coverage Underlies Public Misunderstanding of Shark Conservation Threats and Solutions, Science Direct.
David S. Shiffman, et al., 2015, Preferred conservation policies of shark researchers, Society for Conservation Biology.
Colin A. Simpfendorfer, et al., 2017, Bright spots of sustainable shark fishing, Science Direct.
Kristine L. Stump, 2013, The effects of nursery habitat loss on juvenile lemon sharks, Negaprion brevirostris, ProQuest.
Nicholas Dulvy, et al., 2014, Extinction risk and conservation of the world’s sharks and rays, eLife.
Peter M. Kyne, Pierre Feutry, 2017, Recreational fishing impacts on threatened river sharks: A potential conservation issue, Wiley Online Library.
David S. Shiffman, et al., 2017, Fishing practices and representations of shark conservation issues among users of a land-based shark angling online forum, Science Direct.
Andrew Chin, et al., 2010, An integrated risk assessment for climate change: analysing the vulnerability of sharks and rays on Australia's Great Barrier Reef, Wiley Online Library.
Charles W. Bangley, et al., 2018, Increased Abundance and Nursery Habitat Use of the Bull Shark (Carcharhinus leucas) in Response to a Changing Environment in a Warm-Temperate Estuary, Nature.
Adam T. Ford, et al., 2021, Understanding and avoiding misplaced efforts in conservation, Facets Journal.