Last updated March 8, 2018 at 9:10 am
Just because somewhere is remote, does not mean it’s pristine.
Grey Reef Sharks. Credit: Kirsty Richards
If we want to help save and preserve sharks, scientists need to be able to figure out how many of them there are.
Marine scientists have lots of different ways of doing this, from diving surveys to remote underwater vehicles. But when you estimate a current population, what number do you compare it to? What are the baseline numbers of animals that existed in pristine conditions?
Figuring out this baseline number is a difficult problem when it comes to marine animals, as we’ve only been scientifically monitoring the ocean for a fraction of the time that we’ve been exploiting it.
Sharks have been neglected for a long time
Sharks especially have been neglected by research for centuries, with much of what we know about their ecology and conservation coming from the last 20 years.
For a long time remote coral reef ecosystems were considered pristine, in part because of the large numbers of sharks found there.
But it’s not clear whether the abundance of sharks found at these sites is natural or an effect of change caused by fishing elsewhere.
The Chagos archipelago is one of the most remote coral reef systems on earth, and is situated in one of the largest marine protected areas in the world. But even in this remote and protected area, sharks are still vulnerable to illegal fishing.
Silvertip and grey sharks. Credit Andradi Brown
Francesco Ferretti and colleagues report an innovative method for calculating the baseline levels of sharks in this area in a new study in Science Advances.
They combined long-term statistics from regional fisheries (including statistics from the British colonial government, as well as interviewing one of the original divers from the scuba diving surveys carried out in 1975), with scientific monitoring data from between 1976 and 2012, to reconstruct baseline population levels for sharks.
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They found that even at the start of scientific surveying in the 1970s, observed shark numbers were already much lower than would be predicted from ecological theory or from historical fishing records dating back to 1948. By 2012, Grey reef sharks were at 79% of baseline, and Silvertip sharks at 7%.
The Chagos archipelago had been exploited by early French explorers since 1744, over 200 years before the first scientific surveys. By the time the first scientists began looking at sharks, they were already in decline.
This research shows that we need to think carefully about labelling sites as pristine, and that better modelling of baseline numbers can help in managing conservation for these species.
Silvertip sharks. Credit Dan Bayley
