Sharks are among the largest and most wide-ranging predators in the ocean. They inhabit a diverse array of different habitats; from estuaries to the open ocean, at great depths and in coastal shallows, in reefs, mangroves and even in Arctic waters! Therefore, their declining populations could have implications in many ecosystems and impact upon countless other species in the communities they inhabit... but why are sharks so significant? And what could happen if they disappeared?
It's Going Down
Shark populations are declining globally. This is because sharks are extracted in fisheries, both when they are targeted for their fins, meat or oil, or when caught as "bycatch" (accidental capture in fisheries targeting other commercial species). Sharks also die due to "ghost fishing" - when fishing gear is abandoned/lost and continues to entangle marine life when drifting feely in the ocean.
There are also several indirect effects of fishing which affect sharks. Bottom trawling can damage structures which are critical habitats for sharks and overfishing commercial fish species can limit prey availability for sharks. Sharks are also killed purposefully by nets and drum-lines installed to protect recreational beaches, and fall victim to culling programs in response to attacks upon humans (Ferretti et al, 2010; MacNeil et al, 2020).
Sharks can also suffer from habitat degradation; such as pollution from industrial activities or sedimentation caused by urbanisation. This is particularly problematic for coastal shark species, which have a close proximity to human activity (Ferretti et al, 2010; MacNeil et al, 2020).
Life in the Slow Lane
Sharks are especially vulnerable to overexploitation because they have K-selected life history strategies. This means that they have long-life spans, during which they grow slowly to a large size. They do not mature rapidly, have low "fecundity" (aka fertility) and must invest significant resources in reproduction. This is comparable to animals with R-selected life history strategies, such as rabbits, which breed like... well... rabbits! After population declines, the recovery of shark stocks can be very slow, or even impossible (Ferretti et al, 2010).
In general, sharks have twice the risk of extinction from fisheries compared to R-selected fishes, but the threat of extinction varies between shark species. For instance, after stock depletion, smaller, coastal shark species are able to rebound more rapidly than large pelagic species. Deep-water sharks are thought to be the most vulnerable, with population growth rates as much as 63% lower than coastal species (Ferretti et al, 2010; MacNeil et al, 2020).
Top Down
But what effect could shark declines have on the ecosystem as a whole? Sharks play a unique and fundamental role in the health of marine habitats because they exert "top-down" control. The direct pressure (from predation on other species) and indirect pressure (threat of predation causing behavioural changes in other species) that shark exert are critical for structuring and balancing the ecosystem (Ferretti et al, 2010; Casey et al, 2017).
Several studies have shown that the removal of sharks could be catastrophic. For example, modelling has shown that the removal of multiple reef shark species in the Caribbean induces significant "trophic cascades". The removal of the top predator caused an explosion in the population of fish consumers, which depressed herbivore activity and this is thought to be responsible for a recent switch from coral- to seaweed-dominated reefs. Not only did this change the ecosystem significantly, but also means the community is more vulnerable to further disturbances (Ferretti et al, 2010).
Another study in Prince William Sound, Alaska found that the presence of sleeper sharks (Somniosus pacificus) significantly influenced the behaviour of harbour seals. The removal of the sharks released the seals from predation pressure, which changed how they dove and swam, so they used their habitat in a different manner. This impacted upon the fish species the seals hunting (Ferretti et al, 2010).
Similarly, modelling has shown that the loss of sharks in the Galapagos would release toothed whales and sea lions from predation pressure, which would cause significant declines of commercial fish species. This would directly affect the local communities; people may lose a vital component of their diet if their fisheries stocks collapsed and would also feel serious economic effects if the fisheries were closed (Ferretti et al, 2010).
Stop Right There!
If we are to halt these trophic cascades in their tracks, it will be vital to protect sharks and to encourage their population growth. Scientists have found that shark populations will recover more rapidly if juvenile animals survive to adulthood. As many sharks utilise "nursery habitats" near the coast during their first few months of life, it will be critical to ensure we protect these areas from fishing and human disturbance (Ferretti et al, 2010).
But most importantly, it is critical that shark fisheries are made sustainable immediately! We must ensure that we are not extracting too many sharks from our oceans and that endangered species are able to begin to recover.
If you want to help to protect sharks from declines you can:
Choose to only buy fish which have logos on their packaging to show they were sourced from sustainable fisheries.
Avoid accidentally buying shark meat by memorising the common names given to shark- shark meat is commonly labelled as "Huss", "Flake", "Dogfish", "Catfish", "Rock Salmon", "Grayfish", "Steakfish", "Whitefish", "Lemon Fish", "Moki", "Cape Steak", "Sea Ham" and "Gummy".
Choose not to eat shark fin soup.
Support shark conservation groups such as Shark Stewards and Shark Angels.
You can even write to your government to express your concern. Google who your representative is for environmental affairs or fisheries and write them a sternly worded letter!
References
Casey JM, Baird AH, Brandl SJ, Hoogenboom MO, Rizzari JR, Frisch AJ, Mirbach CE & Connolly SR (2017). A test of trophic cascade theory: fish and benthic assemblages across a predator density gradient on coral reefs. Oecologia 183. Access online.
Ferretti F, Worm M, Britten G, Heithaus M & Lotze H (2010). Patterns and ecosystem consequences of shark declines in the ocean. Ecology Letters, 13, 1055–1071. Access online.
MacNeil MA, Chapman DD, Heupel M, Simpfendorfer CA, Heithaus M, Meekan M, Harvey E, Goetze J, Kiszka J, Bond ME, Currey-Randall LM, Speed CW, Sherman CS, Rees MJ .. Cinner JE (2020). Global status and conservation potential of reef sharks. Nature, 583. Access online.
By Sophie A. Maycock for SharkSpeak
