Sharks Are in Trouble … Without Them, Whole Ecosystems May Disappear
(Part 1 of 2) by Alexandra Smith, Blue Parks Intern
Featured Picture: Carcharhinus longimanus © IUCN Photo Library ©Jeremy Stafford-Deitsch
Imagine an ocean without sharks-it’s not too far-fetched. A recent study found that in the last 50 years, populations of 18 shark and ray species (called elasmobranchs) have declined by a staggering 70%, and half of all open ocean shark species (16 out of 31) are endangered or critically endangered. [1] The oceanic whitetip shark population has declined by 98% in the last 60 years, spotting a reef shark in its natural reef habitat is now a rare occurrence, and in the Galapagos alone 90% of shark species are listed on the IUCN Red List. [2, 3, 4]
Overfishing is the principal threat to oceanic sharks. [1] Sharks and rays are slow-growing animals and don’t reproduce until they are quite old, making it hard for them to recover from overexploitation. [1] While many fishers don’t target sharks, they frequently catch them accidentally. This is known as bycatch, and the sharks often don’t survive. Some fishers do target sharks, usually after depleting their original target species. In both instances, the fishers may sell these sharks for their meat, gills, fins, and liver. [5] The shark-finning trade continues to grow and accounts for an astounding 17 million shark deaths every year-almost 47,000 sharks every day of the year, almost 2,000 each hour. [6] In particular, because so many shark species are slow-growing, the survival of juvenile sharks heavily influences population growth. This means that high juvenile mortality can put populations at risk. For example, bycatch is the leading cause of death for juvenile whitetip sharks in the Northeast Pacific, which are critically endangered globally. [7, 2]
Open-ocean fisheries are on the rise, using fishing gear that deploys hundreds of hooks simultaneously and massive purse seine nets that often scoop up sharks as bycatch. The number of sharks incidentally caught in fishing gear like this has about tripled in the last 50 years. [5] Closer to shore, coastal fishing is ubiquitous worldwide and can have serious consequences for coastal and reef shark species. Many coastal fisheries operate in fish and shark nursery areas, depleting sharks’ food sources, increasing bycatch probability, and leaving sharks no safe place to birth and raise their young. [8]The Atlantic States Marine Fisheries Commission explains that recreational shark fishing is also now a “popular sport at all social and economic levels”. [9] Along with nearshore fishing, the decline of coastal habitats (such as mangroves and coral reefs) due to climate change and development is an added threat to sharks who rely on these places for nursery habitat, food, and shelter. [10]
Sharks are apex predators and keystone species, meaning they play a significant role in their ecosystem, where they keep other species (mainly smaller predators) in balance. A well-balanced marine food web is essential for species abundance and diversity, both of which make for healthier ecosystems. The overfishing of large shark species (such as bull, great white, dusky, and hammerhead) has cascading negative effects that all fishers and communities should be concerned about. Where sharks are in decline, we are seeing big changes in marine food webs. Medium-sized predators (called mesopredators) like various finned fish and small-bodied shark species are increasing in abundance, causing an unnatural shift, in the same way that animals like coyotes and foxes surged in population and devasted smaller prey species when top predators like wolves were eliminated from terrestrial ecosystems. [1] In the ocean, the increase in mesopredators means more predation on herbivores, such as parrotfish, which can result in algae overgrowth of coral reefs which weakens and kills them. Without healthy populations of herbivores, whole ecosystems can be lost. We’ve seen this happen in shifts from a coral reef to an algae-dominated landscape in the Caribbean, leaving reef-dependent organisms without habitat. [11]
The decline of large shark species can also have implications for the shellfish industry. Along the coast of North Carolina, for example, when top predatory sharks declined rapidly in the early 2000s, their prey (rays, skates, and smaller sharks) exploded in abundance. Specifically, cownose rays, which eat large amounts of bay scallops, oysters, and clams, were coming in direct competition with fishers and threatening scallop populations. While this example is localized and the cascading impacts may vary between ecosystems and species, it is a prime example of why we need a holistic ecosystem-based approach to conservation and management. [6]
Sharks are incredibly important ocean creatures, and in the face of current overexploitation, they need our help. There’s hope if we take action now. In our next blog – “Sharks Are in Trouble … Here’s What To Do About It (Part 2 of 2)” – we will explore the tangible ways we can all help turn the tide for declining shark species, as well as the crucial role marine protected areas play in their recovery. Read Part 2 Today!
Sources:
1. Pacoureau, N. et al. (2021) Half a century of global decline in oceanic sharks and rays. Nature 589: 567-571.
3. MacNeil, M.A. et al. (2020)Global status and conservation potential of reef sharks. Nature 583: 801-806.
4. Jacquet, J. et al. (2008) In hot soup: sharks captured in Ecuador’s waters. Environmental Sciences, 5(4): 269-283.
7. Benson, J.F. et al. (2018) Juvenile survival, competing risks, and spatial variation in mortality risk of a marine apex predator. J Appl Ecol. 55: 2888- 2897.
8. Bradai, M.N. et al. (2018) Overview on Mediterranean Shark’s Fisheries: Impact on the Biodiversity. Chapter 10 in “Marine Ecology – Biotic and Abiotic Interactions.” IntechOpen.
9. Atlantic States Marine Fisheries Commission (2021) Coastal Sharks. Commercial and Recreational Fisheries. Retrieved March 2021 from http://www.asmfc.org/species/coastal-sharks