Sharks Counting: Population Size Is Lower Than Estimated

A team of scientists investigated the size of an unfished community of reef sharks which will help in getting the clear picture of the shark population. Many shark populations were declined over the past several decades.

As written in Science Daily, researchers from UC Santa Barbara and colleagues conducted an eight-year study of a healthy shark population on Palmyra, uninhabited atoll in the central Pacific Ocean. It is a part of a marine refuge that extends 50 nautical miles from its shores.

Fishing is not allowed within these borders to protect various species that includes grey reef sharks. The investigators were surprised to find much fewer sharks than expected.

Lead author Darcy Bradley said that they estimated grey reef sharks population size between 6,000 and 8,000 which is a density of about 20 sharks per square kilometer. Darcy Bradley is a postdoctoral researcher in UCSB's Sustainable Fisheries Group.

She added that previous research that used underwater visual survey methods estimated a density between 200 and 1,000 sharks per square kilometer. So, she is thinking that density would end up a lot bigger than their estimate.

As written in the official site of UCSB, the research team captured reef sharks across Palmyra and fitted them with numbered ID tags from 2006 to 2016. They also tracked the movement of them using acoustic telemetry tags that emit a sound. It is then recorded by acoustic receivers located underwater.

Research team tagged 1,300 reef sharks out of which 350 individuals were recaptured. They recorded information on the sex and size with the location of animal's capture. The investigators then plugged all the data into an algorithm to estimate the total population size.

Some may think that smaller shark population is a bad news but Bradley doesn't think that. She thinks that a healthy shark population smaller than assumed means that other shark populations are more precarious than previously suggested.

It also means that the recovery goal for shark population is lower so that recovering shark populations is somewhat easier. "Given that the way we manage fisheries and ecosystem health depends on having decent estimates of abundance, we need to continue to improve the way we count things in the ocean," she said.