Shark Fin Hunting Ecological Effects
The Great White Shark, the starring antagonist in the infamous movie Jaws, struck fear into the public’s hearts making many see sharks as ferocious, uncaring predators. But today, I would like to show another side of sharks, one where they are facing the threat of potential extinction and their ecological importance in maintaining the health of the food web. This blog was written using various scientific studies of the consequences if our top shark predators were to be removed from an ecosystem.
Background
Before I present the study which will be going over the effects that shark hunting has on an ecosystem, I would like to first talk about our marine food webs. Our apex predators, or our top predators in an ecosystem, help keep the ecosystem healthy by preventing consumers lower in the food chain such as mesopredators, which are one level below apex predators, from overpredating smaller consumers and resource species. These resource species include small fish, invertebrates, and seagrass. But we cannot forget that we also are part of that food web as well via fishing and netting. Because of overfishing and shark hunting, we are seeing changes in the interactions between organisms in various ecosystems in which under normal circumstances, would not occur.
Top-Down Control
Every ecosystem is under the rules of top-down control. And to better understand the cause and effect of the loss of Top-Down Control, let’s say for example that the apex predator population -a shark- was removed from a healthy and well balanced ecosystem. The change will cause the mesopredators -which are now the new top predator- to experience a higher survival and reproductive rate because of the lack of predators in the area.
Consequences of the Loss of Top-Down Control
What happens when there is a loss of Top-Down Control you might ask? What it will result in is the destabilization of various prey populations. Because mesopredators are not necessarily fully equipped with all the tools that the previous apex predator had to serve as a keystone species for that ecosystem, the mesopredators -which tend to be smaller in size- may refrain from preying on larger, tougher animals such as sea lions for food that the original shark would prey on regularly. This can be especially true for mesopredators that are K-selected (species that have a few offspring but invest a high amount of parental care) who cannot undergo evolution as quickly than if they were not K-selected and therefore result in the “neglecting” of certain prey species over others meaning that some could experience a huge drop in population while others could experience a huge increase in population over an extended period.
Will Not Always will be 100% Terrible…
However, when a top predator like a shark is removed from an ecosystem, it’s not always bound to be terrible. The severity of the effects do depend on a few variables which I listed. These include the type of mesopredator present, the type of shark that was originally present in the ecosystem, and how easily the mesopredator can adapt to its new role as the top predator that the shark had. In other words, the more similarities that the mesopredator has to its top predator counterpart, the less of a negative impact the ecosystem will experience during this situation.
Great White Shark Facts
Before we dive into the study, I would like to give some information on both the Great White Shark (Carcharodon carcharias) and the Sevengill Shark (Notorynchus cepedianus). First we have the Great White Shark with a length of at least 6.4 meters or 21 feet and a weight ranging from 680-1,800 kilograms or 1,500-4,000 pounds. This shark can be found in highly productive offshore waters (which are waters that have a high quantity of organic matter) that have a temperature of about 0 degrees celsius to 22 degrees celsius or 32 degrees fahrenheit to 72 degrees fahrenheit on average. These animals are also largely solitary and sometimes do swim together from one point to another. Their diet consists mainly of other sharks, sea turtles, seals, sea lions, porpoises, dolphins, and small whales which they obtain either from ambush hunting or scavenging.
Sevengill Shark Facts
Now we have the Sevengill shark and like its name implies, it does have seven gills. It is about half the size of a Great White Shark at a length of at least 3 meters or 10 feet and a weight of around 17% of a Great White’s at about 107 kilograms or 236 pounds. Sevengill sharks are not picky about where they live as they are sighted in bays, estuaries, and are considered to be circumglobal in tropical to temperate waters. They too, like the Great White are also somewhat social but in a different way. When Sevengill sharks are being social, they hunt in packs. Their diet is different to that of Great Whites. They consume smaller organisms such as smaller sharks, crustaceans, rays, chimaeras, hagfish, dolphins, porpoises, seals, shark egg cases, and sea snails just to name a few. Similarly to Great Whites, they are also opportunistic scavengers but in contrast, they are pursuit hunters meaning that they stalk and then charge at their prey.
Data Analyses (Part I)
The study included two graphs: the population of Great Whites and the comparison of Great White Shark population and Sevengill Shark population over time. First let’s look at the population of Great Whites on the graph. Even though the graph has information on Great White Shark predation rates on seals and not the direct population of Great Whites, this is a way to measure the population of Great Whites. This is because if there was a higher predation rate, we can then infer from the data that there could be more Great Whites in the area and the same conclusion can be made vice versa. We can also see that in the year 2014, there is a change-point in the “population” of Great Whites due to increased human activity at Seal Island such as fishing, netting, and shark hunting. Now let’s talk about the graph that shows Great White and Sevengill Shark populations by the number of sightings per hour.
Data Analyses (Part II)
From the graph we can infer that originally there was a stable population of Great Whites and there were no Sevengills present at the time from the year 2000 to 2016. However, the change point occurred in the Great White population in the year 2015, close to the changing point in the previous graph which was 2014. And as we continue looking further right of the graph, we can see in 2017, the Sevengill sharks began to drastically increase their numbers as the population of Great Whites continued to decrease. What this reveals is that there is a loss of Top-Down Control in the marine ecosystem of Seal Island and therefore, we can infer that there will probably be some destabilization of various prey populations with certain groups being preyed on much more than others than the norm since the Sevengill shark is much smaller in length and lighter in weight than that of the Great White, limiting its prey options.
Did you find anything interesting in today’s blog? I hope you learned as much as I did when writing this and are ready to spread awareness of the situation to your family and friends. :)
Citations:
[1] Aaron W., Alejandro F., Boris W., Michael H. “Predicting Ecological Consequences of Marine Top Predator Decline.” 4 Mar. 2008, citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.577.6967&rep=rep1&type=pdf.
[2] Boris W., Francesco F., Gregory B., Heike L., & Michael H. (2010). Patterns and Ecosystem Consequences of Shark Declines in the Ocean, onlinelibrary.wiley.com/doi/epdf/10.1111/j.1461-0248.2010.01489.x.
[3] “Carcharodon Carcharias.” Florida Museum, 18 Oct. 2018, www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharodon-carcharias/.
[4] Hammerschlag, Neil, et al. “Disappearance of White Sharks Leads to the Novel Emergence of an Allopatric Apex Predator, the Sevengill Shark.” Nature News, Nature Publishing Group, 13 Feb. 2019, www.nature.com/articles/s41598-018-37576-6
[5] “Sevengill Sharks ~ Marinebio Conservation Society.” MarineBio Conservation Society, 22 Dec. 2020, www.marinebio.org/species/sevengill-sharks/notorynchus-cepedianus/#:~:text=Sevengill%20sharks%2C%20Notorynchus%20cepedianus%20(P%C3%A9ron,and%20cow%20sharks%20order%2C%20Hexanchiformes.
[6] “Sevengill Sharks ~ Marinebio Conservation Society.” MarineBio Conservation Society, 22 Dec. 2020, www.marinebio.org/species/sevengill-sharks/notorynchus-cepedianus/.
[7] “White Shark.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., www.britannica.com/animal/white-shark.