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What are the most and least abundant shark species in the world?

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Once thought to be extinct, the megamouth shark is one of the world’s most endangered animals. Credit: NOAA.

By Blaise Jones

Determining which shark species are the most abundant is no easy tasks. You simply cannot go out to the ocean and count every shark you see. The closest scientists can do to this is aerial surveys, and even these struggle to be accurate.

Most of our numbers when it comes to shark populations are estimates. The most common way of estimating shark populations is via “mark-recapture”. This is where an animal is marked by researcher, usually by some sort of tag in this case, and then released. As the study goes on and more and more animals are caught, tagged, and released, the scientists keep track of the percentage of tagged animals they recapture and untagged animals they catch. Using these two numbers they are able to estimate the overall population.

There are a few problems with this method though, most notably that the best answer is always going to be an educated guess. Since it is currently impossible to simply count all the sharks out there, scientists can never get an exact number. There are other factors as well that are ignored for the sake of the survey that could affect the numbers. Scientists have to assume that mortality is the same for tagged and untagged animals, tagged animals do not lose their tags, tagged animals are caught at the same rate as untagged animals, the overall population isn’t suffering any significant ingresses or egresses, that the tagged animals mix evenly with the untagged animals, and that each capture session is capturing from areas with an even distribution of size, age, and sex. On top of all this there is the worry that other parties (i.e. fishermen who catch tagged animals) won’t report the catch or might even kill the animal.

As you can see, determining shark populations is a bit of a guessing game.

Spiny dogfish, the species of shark that is likely the most abundant in the world. Credit: NOAA.

Best Guess: Most Populous Shark

With all that taken into consideration, there are some solid estimates on shark populations that most researchers will agree upon. While all shark species are facing dangers to their populations, some are certainly doing better. The ground sharks are a group that are doing better than every other group, especially the dogfish. On the IUCN (International Union for the Conservation of Nature) red list, a list of all the endangered and threatened animals on the planet, there are eight shark species listed as least concerned, and of those eight six are ground sharks.

These eight species include the blackmouth catshark, cookiecutter shark, goblin shark, largetooth cookiecutter shark, longnose velvet dogfish, silky shark, smallspotted catshark, and the starry smoothhound.

While these species are the least impacted by fishing, none of them are the estimated most abundant species. That goes to the spiny dogfish. Commercial landings of this shark in North America alone get as high as 50,000 metric tons a year! Estimate put the worldwide population of the spiny dogfish at around 2 billion sharks. However, not all ground sharks are so lucky.

Concerned Shark Citizens

Over the past 20-30 years of population research scientists have seen downward trends in the population numbers of every single shark species, with 56 percent of all sharks considered to be Threatened or Near Threatened.

The most threatened shark species are those targeted by the finning industry. There are more than 140 shark species that are threatened, according to IUCN, but 14 of them are due almost exclusively to the finning industry. Those sharks include: the blue shark, great hammerhead shark, scalloped hammerhead shark, smooth hammerhead shark, silky shark, oceanic whitetip shark, common thresher shark, pelagic thresher shark, bigeye thresher shark, sandbar shark, shortfin mako shark, bull shark, dusky shark, and tiger shark.

And these are just the major known species. There are species of shark out there with very little known about their populations. The megamouth shark was only discovered in the 1970s, with only 54 known individuals. The Irrawaddy river shark is known by only one specimen, and its close relative the Northern River shark is thought to have only 250 living specimens. The Pondicherry shark is only known by 20 specimens and hasn’t been seen since the 1970s.

SOURCES:

“Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess

“The Encyclopedia of Sharks” by Steve Parker

http://www.sharksavers.org/files/8013/3702/5512/IUCN_Status_of_Shark_Species_Shark_Savers.pdf

https://projects.ncsu.edu/cals/course/fw353/Estimate.htm

http://www.sharktrust.org/en/iucn_least_concern

http://www.sharksavers.org/en/education/biology/different-types-of-sharks/ground-sharks/

 

-TSF-

#sharks #endangered #populations #TSF

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