BERN (SWITZERLAND)- Sharks are more caught by fishermen in cooler ocean regions than in the warm equatorial areas are where marine life is most biodiverse. Why, remains a mystery.

Research, led by Marius Roesti at the University of Bern in Switzerland, shows that the general idea that that predators should be most active near the equator is not true. The researchers collated data from different fishing commissions around the world between 1960 and 2014. The researchers looked at the number of hooks set in a given place in the ocean at a given time and the number of predatory fish that were caught (not only sharks, but also fish like tuna). In total, the data consisted of more than 900 million caught fish.

Active predators

Even after making allowances for the fact that fishing vessels are spread unevenly across the oceans, Roesti and his colleagues found that predatory fish were most likely to be caught in the mid-latitudes of the ocean, roughly between 30 and 60 degrees north and south of the equator, rather than the warmer tropics. The finding suggests it is here that the predators are most active and interact most with prey species.

The researchers also found that the number of predatory fish caught by fishing vessels has declined over the years, which Roesti thinks is probably due to overfishing. “We think that something as big as the ocean is not vulnerable to overexploitation,” Roesti says to New Scientist. “From our data, it’s clear that overfishing is a major issue.”

Read more on New Scientist.