Long ago, far from human intervention, sharks and rays swam abundantly in the high seas. Recent research shows that 18 shark and ray species have collapsed by 70% since the 1970s.
Decline of Shark and Ray Populations
Oceanic sharks and rays have long thrived in the high seas. Shortfin mako sharks, the fastest sharks on Earth, chased after their prey with incredible speeds up to 20 miles an hour. Scalloped hammerheads scanned the ocean for food using their wide-set eye and sensory organs.
These animals often traveled widely across the open waters so vast that scientists did not predict that overfishing would ever endanger their populations.
Nicholas Dulvy, co-chair of the International Union for Conservation of Nature's Shark Specialist Group, says, "A decade ago we would have extremely heated debates about listing sharks as threatened."
Today, analysis of historical and current populations have painted a clearer picture. A recent study published in the journal Nature found that 18 shark and ray species populations have declined by 70% since the 1970s, roughly 50 years ago.
At this rate, researchers are concerned that species might entirely disappear in a decade or two.
When scientists analyzed the numbers of the oceanic whitetip sharks, a common species, in 1970, they were shocked at the 98% decline only in the last 60 years. Dulvy explains that the trend has become commonplace in all three oceans.
The species has now been listed as critically endangered by the International Union for Conservation of Nature.
Although fisheries seldom target oceanic sharks, their gill plates, liver oils, and fins are sold when caught.
The research servers troubling news for both the sharks and ocean health since the species is a top predator in the food web that keeps smaller predators in check.
Dulvy explains that sharks' increasing rarity equates to an 18 times increase in an individual shark's likeliness to get caught.
Studying Shark and Ray Populations
For the study, researchers gathered data on 18 species that can be found across the globe.
A growing public awareness of shark conservations prompted fisheries management to collect shark data, which provided researchers with an influx of new information.
Scientists ended up with over 900 datasets spanned from 1905 to 2018, each representing a species population changes in a particular region. With the aid of international experts and computer models, the team analyzed the data to conclude the best estimate of the worldwide change in shark abundance.
The team also took into consideration the development of open-ocean fishing. Long lines with hooks ensnare sharks unsuspectingly, and their use has roughly doubled in the last half of the century. At the same time, the number of sharks caught in the lines has tripled.
Dulvy explains that there is inevitable uncertainty in their analysis where authors may have underestimated species' declines in areas where overfishing has occurred for several decades.
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