Why Sharks are Important for Marine Ecosystems

Written by Benjamin Taubman

Research and Writing Intern Learn More About Benjamin

November 13, 2022

This past summer, a myriad of shark sightings on Cape Cod and Long Island prompted local officials to enact temporary closures on several beaches[1]Schiller, “Several Shark Sightings in Cape Cod and Long Island Prompt Beach Closures.”. In response to these closures, dozens of news outlets blew up the situation and published a seemingly endless stream of shark-smear stories, perpetuating the tarnishing of the species’ reputation[2]“Long Island Sees Fifth Shark Attack in Two Weeks.”.

Although blockbuster movies, such as “Jaws” and “The Meg,” and sensational tabloids may drum up irrational fear of sharks, the alarm is unreasonable and statistically unjustified. In fact, the odds of a shark-related fatality in the U.S. are about 1 in 4,332,817 – roughly 54 times less likely than death by a lightning strike[3]“Risk of Death.”. Panic induced by the media takes away from the fact that sharks are vital to countless ecosystems and are an incredibly endangered species. 

Sharks long predate human existence and are one of the oldest living species on the entire planet. Around 1,000 species of genetically-linked sharks, skates, rays, and chimaeras have been swimming in the Earth’s deep blue for over 1,000 years. A shocking statistic, nearly two-thirds of shark species are threatened with extinction[4]Cardeñosa et al., “Two Thirds of Species in a Global Shark Fin Trade Hub Are Threatened with Extinction.”. Let’s dive into why these ancient, majestic predators are in danger.

The Loss and Pollution of Shark Habitats

Habitat loss has had a devastating impact on shark populations. Human impacts such as climate change, urban development, mining, and aquaculture have destroyed boundless swathes of seagrass plains, coral reefs, and mangroves[5]Lotze et al., “Depletion, Degradation, and Recovery Potential of Estuaries and Coastal Seas.”[6]Carugati et al., “Impact of Mangrove Forests Degradation on Biodiversity and Ecosystem Functioning.”. Mangroves are especially important because they act as a nursery habitat for sharks. Just as a baby nursery provides a safe space for human infants to live while they are especially susceptible to injury and danger, mangroves provide a protected area for young sharks to spawn, survive, feed, and grow to maturity[7]Heithaus, “Nursery Areas as Essential Shark Habitats.”. In a 2010 study analyzing the effect of mangrove destruction on juvenile lemon sharks (Negaprion brevirostris) in the Bahamas, researchers found that the nursery loss had disastrous effects on the species[8]“The Effects of Nursery Habitat Loss on Juvenile Lemon Sharks, Negaprion Brevirostris – ProQuest.”

In Bimini, Bahamas, a large resort was constructed on a mangrove forest, destroying approximately 39% of the habitat and significantly degrading the ecosystem which remained. The destruction caused the survival rates of the juvenile sharks to plunge to 25.5%, where a 39% survival rate is needed to maintain population stability. For the sharks that do survive, their annual growth rate shrunk by 1 cm, from 6.5 cm per year to 5.5 cm[9]“The Effects of Nursery Habitat Loss on Juvenile Lemon Sharks, Negaprion Brevirostris – ProQuest.”. Although this is just one case study, the implications of the loss of mangroves and other nursery habitats on the fate of sharks extend to species and populations all across the ocean.

In addition to the direct destruction and degradation of shark habitats, human pollution of said habitats is another adversity shark populations face. It is no secret that the ocean has a plastic waste problem. Every year, between 4.8 and 12.7 million tonnes of plastic waste enter the ocean, posing a threat to species at every level of the ecosystem[10]Jambeck et al., “Plastic Waste Inputs from Land into the Ocean.”. Nonetheless, one of the primary threats that plastic poses to sharks is entanglement.

Entanglement poses a serious threat to sharks because it significantly degrades their chance of survival and leads to starvation, suffocation, immobilization, and death. A 2019 study reviewing shark entanglement found that ghost fishing gear – fishing gear that has been dumped, abandoned, or lost in the ocean – was responsible for two-thirds of all entanglement cases. The most common and likely pieces of ghost fishing gear to entangle sharks are large, nylon fishing nets that can persist in the ocean indefinitely[11]Parton, Galloway, and Godley, “Global Review of Shark and Ray Entanglement in Anthropogenic Marine Debris.”.

Furthermore, far less visible than plastic waste, chemical pollutants also pose a significant threat to sharks. Over the past century, it is estimated that 50 to 97 million tons of toxic industrial waste were dumped into the ocean by the United States alone[12] Kivenson et al., “Ocean Dumping of Containerized DDT Waste Was a Sloppy Process.”. Moreover, sea-based activities such as maritime shipping, accidental oil and chemical spills, and other industrial emissions release a plethora of dangerous chemical pollutants like mercury, cadmium, and – say it with me – Dichlorodiphenyltrichloroethane (DDT) into the ocean every day[13]Tornero and Hanke, “Chemical Contaminants Entering the Marine Environment from Sea-Based Sources.”.

Chemicals such as these pose a significant threat to the health and development of shark embryos and can cause birth deformities and defects in shark pups after they spawn. Additionally, chemicals ingested before birth can remain in the sharks for their whole lives, bleeding into future generations. Studies have also found a correlation between the increased concentration of toxic chemicals and the development of neurological disorders, structural damage to organs and gills, and cancer[14]“Are Concentrations of Pollutants in Sharks, Rays and Skates (Elasmobranchii) a Cause for Concern?”

The Shark Fishing Industry

Human overfishing of sharks – joined with the species’ relatively low reproductive rates and slow growth – poses the most significant threat to the sharks of the world[15]Worm et al., “Global Catches, Exploitation Rates, and Rebuilding Options for Sharks.”. Currently, it is estimated that over 100 million sharks are killed by the fishing industry each year[16]Worm et al., “Global Catches, Exploitation Rates, and Rebuilding Options for Sharks.”. Putting this figure into perspective, that is about 274,000 sharks per day, or 200 each minute. This grand shark culling is the result of a blossoming $1 billion dollar shark fishing industry for commodities including fins, meat, and oil. Of these products, shark fins have the highest value per pound and largest market share, with an annual import volume of 16,815 tons – worth over $377 million a year[17]Dent and Clarke, “State of the Global Market for Shark Products.”. Unfortunately, the process of ‘shark finning’ – fishing sharks and harvesting their fins – is a highly wasteful practice. Since the fishermen are only seeking the fins, and wish to save valuable space on their boats, they cleave the fins from the shark and then throw the rest of its body back into the ocean, squandering the plentiful remains.

Alarmingly, in a study examining the species composition of sharks traded in a Hong Kong fish market, researchers found that over 70% of the sharks were observed to be threatened coastal species[18]Cardeñosa et al., “Two Thirds of Species in a Global Shark Fin Trade Hub Are Threatened with Extinction.”. As the name suggests, coastal sharks live in coastal habitats such as estuaries, coral reefs, and seagrass plains. Considering that they mature even more slowly and only reproduce a few kin over their lifetime, coastal sharks are especially susceptible to overfishing[19]Smith, Sim, and Flajnik, Immunobiology of the SHARK..

Although shark fishing has existed for centuries, recreational shark fishing is a relatively new phenomenon. After getting a significant boost in popularity after the release of the 1975 blockbuster “Jaws,” millions have taken to the seas to recreationally angle the top predators. Many shark fishing clubs emerged and, by 1979, recreational fishers captured over 1.2 million sharks in the southeast United States alone. In addition to the United States, recreational shark fishing is the most popular in Australia, New Zealand, the United Kingdom, and Canada[20]Gallagher et al., “Shark Recreational Fisheries.”.

Today, recreational shark fishing is less popular yet sharks still represent 6% of all fish caught recreationally[21] Lynch, Sutton, and Simpfendorfer, “Implications of Recreational Fishing for Elasmobranch Conservation in the Great Barrier Reef Marine Park.”. Nonetheless, one notable difference is what happens to the sharks after they are caught. In the past, fishermen would often just kill their sharks after catching them. Now, after realizing the current threat to shark populations and receiving backlash due to ethical concerns, most sharks caught recreationally are released. However, even though the majority of fishers used best practice guidelines and 86% placed an emphasis on releasing them back into the water in good condition, sharks caught then released still had a mortality rate of up to 90%[22]Gallagher et al., “Shark Recreational Fisheries.”[23] Lynch, Sutton, and Simpfendorfer, “Implications of Recreational Fishing for Elasmobranch Conservation in the Great Barrier Reef Marine Park.”.

Bycatch, the marine organisms caught unintentionally during commercial fishing, is yet another fishing problem that hurts shark populations. It is estimated that bycatch threatens over 70% of shark species and that it can have a significant effect on the decline of specific populations[24]Gallagher et al., “Vulnerability of Oceanic Sharks as Pelagic Longline Bycatch.”. From 1998 to 2005, in the fisheries off the coast of southern Africa, it is estimated that 73,500 sharks were caught as bycatch[25]Petersen et al., “Pelagic Shark Bycatch in the Tuna- and Swordfish-Directed Longline Fishery off Southern Africa.”. Additionally, between 1988-89 and 1997-98, nearly 540,000 sharks were caught as bycatch in New Zealand fisheries. Most of these sharks were processed on board the ships, yet, only their fins were kept[26]Francis, Griggs, and Baird, “Pelagic Shark Bycatch in the New Zealand Tuna Longline Fishery.”.

Sharks: Ecosystem Regulators

The health of entire coastal ecosystems relies on coastal great shark species. This is because these great sharks are a keystone species – organisms that are disproportionately important and necessary to the functioning of an ecosystem. Preserving keystone species is paramount; when a keystone species’ population vanishes or significantly dwindles, there are cascading consequences and the entire ecosystem is threatened with collapse[27]Jordán, “Keystone Species and Food Webs.”. Consequences of keystone species removal were demonstrated from 1970 to 2003 when the population of several great shark species off the coast of North Carolina’s Chesapeake bay plunged by almost 90% due to overfishing[28]Myers et al., “Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean.”.

In the estuaries of Chesapeake Bay (a highly diverse ecosystem where freshwater from rivers and streams meets with saltwater from the ocean) great shark species feed on smaller shark species such as the cownose ray (Rhinoptera bonasus) and little skate (Leucoraja erinacea), which, in turn, feed on scallops and clams, two species critical to the local economy of Virginia and Maryland[29]Myers et al., “Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean.”

When the great sharks in the Bay plummeted due to overfishing, the number of skates and rays rose to unmanageable levels, and the populations of scallops and clams were decimated. In normal harvest seasons, local fishermen would typically net over 840,000 metric tons of the scallops and clams, however, in 2003, the fishermen only net 300 metric tons, a 99% decrease[30]Myers et al., “Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean.”. This relationship between a top predator and its effects on the surrounding ecosystem can be best described using an ecological phenomenon known as a trophic cascade – when the loss of a predator leads to indirect effects on the populations of other organisms[31] Ripple et al., “What Is a Trophic Cascade?”. If humans do not do more to conserve shark species and they keep disappearing from the world’s oceans, this will lead to a multitude of trophic cascades which would dramatically shift ecosystems and potentially lead to the destruction of countless other marine species.

The Future of Sharks

Although the staggering decline in shark species might seem like a hopeless outcome, there are still reasons to believe that these predators can make a comeback. Two good reasons are the global shark ecotourism industry and the addition of marine protected areas (MPAs).

Ecotourism is a form of conservation tourism where tourists observe threatened natural environments and ecosystems in a sustainable manner. The practice creates economic-linked incentives to create long-term conservation efforts for ecosystems and directly supports local communities[32]“What Is Ecotourism.”. When ecotourism has been safely and sustainably implemented, the practice has effectively protected terrestrial species such as elephants[33]Flower, Burns, and Jones, “How Tourist Preference and Satisfaction Can Contribute to Improved Welfare Standards at Elephant Tourism Venues in Thailand.” and conserved the growth and health of marine ecosystems such as coral reefs[34]Moritz et al., “The ‘Resort Effect.’”.

As of 2013, the global shark ecotourism industry is booming and employs more than 10,000 people, services 590,000 shark observers a year, and generates over $314 million, about 33% of the total revenue from the shark fishing industry. Given that individuals are economically incentivized to protect shark species, it is estimated that the shark ecotourism industry could generate over $780 million by 2030[35]Cisneros-Montemayor et al., “Global Economic Value of Shark Ecotourism.”.

Another conservation initiative, MPAs are clearly defined zones that restrict activities that impact marine life. For MPAs that protect shark species, this includes banning the fishing and retention of sharks, closing other spatial fisheries, and placing restrictions on specific fishing gear like the large nylon nets mentioned earlier in the article. Although most shark MPAs have only been in place since 2009, as of 2018, there are 38 MPAs that cover an area of 21 million square kilometers, or about 6% of the ocean’s surface. Additionally, a number of MPAs that were originally designated for other species have ended up helping shark species as well[36] Rigby et al., “A PRACTICAL GUIDE TO THE REPORT – EFFECTIVE DESIGN AND 2019.”.

As an individual, you can directly support shark sustainability efforts through a couple of methods. One is being aware of consumer goods that contain shark products. For example, one study discovered that a highly endangered shark species, the shortfin mako (Isurus oxyrinchus) was an ingredient in 44% of 87 pet food products tested. Additionally, the study tested 24 cosmetic products and found that three (12.5%) contained endangered shark materials. However, the issue does not simply stop there. Due to insufficient labeling laws, none of the products tested were even labeled as using shark products, making it difficult, and somewhat impossible, for consumers to even choose alternative products[37]Cardeñosa, “Genetic Identification of Threatened Shark Species in Pet Food and Beauty Care Products.”. However, by raising awareness of this issue, holding corporations and the government accountable, and pushing for better labeling laws, it can be solved. 

Some other ways of helping include: participating in coastal cleanups, donating to shark conservation charities such as Shark Trust and the Atlantic White Shark Conservatory, and supporting other non-profits and research organizations, like the Water Research Foundation and Alliance for Climate Education, which research and help solve other ocean conservation-related issues. By working towards a better future for the species, we can all guarantee sharks bite fish, not the dust. 

Benjamin Taubman
Benjamin Taubman

Research and Writing Intern

Learn More About Benjamin

References

References
1 Schiller, “Several Shark Sightings in Cape Cod and Long Island Prompt Beach Closures.”
2 “Long Island Sees Fifth Shark Attack in Two Weeks.”
3 “Risk of Death.”
4, 18 Cardeñosa et al., “Two Thirds of Species in a Global Shark Fin Trade Hub Are Threatened with Extinction.”
5 Lotze et al., “Depletion, Degradation, and Recovery Potential of Estuaries and Coastal Seas.”
6 Carugati et al., “Impact of Mangrove Forests Degradation on Biodiversity and Ecosystem Functioning.”
7 Heithaus, “Nursery Areas as Essential Shark Habitats.”
8, 9 “The Effects of Nursery Habitat Loss on Juvenile Lemon Sharks, Negaprion Brevirostris – ProQuest.”
10 Jambeck et al., “Plastic Waste Inputs from Land into the Ocean.”
11 Parton, Galloway, and Godley, “Global Review of Shark and Ray Entanglement in Anthropogenic Marine Debris.”
12  Kivenson et al., “Ocean Dumping of Containerized DDT Waste Was a Sloppy Process.”
13 Tornero and Hanke, “Chemical Contaminants Entering the Marine Environment from Sea-Based Sources.”
14 “Are Concentrations of Pollutants in Sharks, Rays and Skates (Elasmobranchii) a Cause for Concern?”
15, 16 Worm et al., “Global Catches, Exploitation Rates, and Rebuilding Options for Sharks.”
17 Dent and Clarke, “State of the Global Market for Shark Products.”
19 Smith, Sim, and Flajnik, Immunobiology of the SHARK.
20 Gallagher et al., “Shark Recreational Fisheries.”
21, 23  Lynch, Sutton, and Simpfendorfer, “Implications of Recreational Fishing for Elasmobranch Conservation in the Great Barrier Reef Marine Park.”
22 Gallagher et al., “Shark Recreational Fisheries.”
24 Gallagher et al., “Vulnerability of Oceanic Sharks as Pelagic Longline Bycatch.”
25 Petersen et al., “Pelagic Shark Bycatch in the Tuna- and Swordfish-Directed Longline Fishery off Southern Africa.”
26 Francis, Griggs, and Baird, “Pelagic Shark Bycatch in the New Zealand Tuna Longline Fishery.”
27 Jordán, “Keystone Species and Food Webs.”
28, 29, 30 Myers et al., “Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean.”
31  Ripple et al., “What Is a Trophic Cascade?”
32 “What Is Ecotourism.”
33 Flower, Burns, and Jones, “How Tourist Preference and Satisfaction Can Contribute to Improved Welfare Standards at Elephant Tourism Venues in Thailand.”
34 Moritz et al., “The ‘Resort Effect.’”
35 Cisneros-Montemayor et al., “Global Economic Value of Shark Ecotourism.”
36  Rigby et al., “A PRACTICAL GUIDE TO THE REPORT – EFFECTIVE DESIGN AND 2019.”
37 Cardeñosa, “Genetic Identification of Threatened Shark Species in Pet Food and Beauty Care Products.”

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