Sharks are easier to catch in cooler waters, and we have no idea why

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سه شنبه 12 فروردین 1399 ساعت 17:43
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New Scientist
Fishing vessels are more likely to catch apex predators, including sharks, and tuna in cool ocean regions, even though the warm equatorial areas are where marine life is most biodiverse. The finding means biologists need to rethink why the tropics are an ecological hotspot.
More species live near the equator than in temperate or polar regions. For a century, we have suspected that predation helps explain this . The idea is that competition through predation drives evolution among prey species, which in turn encourages evolution among predators. If more predation occurred in the tropical seas for some reason, they could be why they have become more diverse over time than the cooler seas nearer the poles.
If this idea is correct, then predators should be most active near the equator, and so this is where fishing vessels should catch the most of them. But they don’t, according to research led by Marius Roesti at the University of Bern, Switzerland.
Roesti and his colleagues collated data from different fishing commissions around the world. Every commercial fishing vessel must report the number of fish that it catches in the open ocean, and what species those fish belong to.
56 years of fishing
Using data gathered between 1960 and 2016, 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. In total, the data consisted of more than 900 million caught fish.
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.
“We have no idea why this is the case,” says Roesti, though the finding does fit with an equally puzzling recent study that suggests speciation rates for marine fish are actually higher away from the tropics, even though tropical seas contain more species.
Martin Genner at the University of Bristol, UK, says these findings are noteworthy and suggests they could be explained by the different levels of organic matter at different latitudes of the ocean. It is also possible that predator species might be less able to tolerate tropical temperatures.
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,” he says. “From our data, it’s clear that overfishing is a major issue.”
Journal reference: Nature Communications , DOI: https://www.nature.com/articles/s41467-020-15335-4
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Sharks are easier to catch in cooler waters, and we have no idea why

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