How many fish in the sea? Less than we thought

There are plenty of fish in the sea, so the old saying goes.

But that’s not as true as it used to be: Climate change, pollution and overfishing have slashed global fish populations. 

Now a new study suggests the problem may be worse than expected. It finds that global fish populations have been overestimated by an average of 11.5 percent — highlighting “that governments need to take more precautions to protect fish populations,” Manuela Andreoni wrote in The New York Times. 

To reach these results, researchers sifted through nearly four decades of data from 230 of the world’s most important fisheries.

“We’ve worked across all of these stocks and see this signal that tells us we’re basically overestimating, on average, how much is out there,” Amanda Bates, a marine ecology professor at the University of Victoria and one of the authors of the study, told The New York Times. “And we’re making management decisions based on that.”

Scientists’ reactions to the study have been mixed. Some see it as a red flag that points to ongoing challenges in measuring the health of fisheries — a critical source of food and jobs for millions worldwide. Others argue that the study overstates the problem, and many corrective measures are already in place.

Counting fish has always been tricky — “like counting trees, except that they move and you can’t see them,” Andreoni wrote. 

But counting fish accurately is essential for managing healthy fisheries — without reliable numbers, marine protected area managers are not able to make informed decisions on conservation efforts, such as fishing restrictions. 

A recent study from Conservation International and the Smithsonian Institute showed why accurately measuring fish populations is so important. It focused on the age of fishes and found that in many marine protected areas, adult fish populations — a key indicator of a population’s health — were not improving as much as researchers hoped, due to coastal development pressure, ocean temperature spikes and inadequate enforcement of fishing restrictions. 

The findings are a “wake-up call” for marine protected areas, said Steve Canty, the study’s author. In the majority of places studied, the numbers of adult fish within populations were either stagnant or decreasing. That matters because adult fish produce exponentially more — and better — eggs than smaller, younger females.  

“By breaking down the age groups, we’re able to clearly see there are much higher numbers of juvenile fish than adults, showing adult fish are not benefiting from the protections in place,” he said. “This is hindering our efforts to rebuild fish populations.”

On a global scale, understanding what makes marine protected areas effective — and having accurate fish counts on which to base management decisions — is urgent. Governments have their sights set on 2030, when countries have committed to protect 30 percent of the planet’s land and seas. 

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Mary Kate McCoy is a staff writer at Conservation International. Want to read more stories like this? Sign up for email updates. Also, please consider supporting our critical work.