How many fish in the sea

Perhaps you have seen a headline recently: there will be more plastic in the ocean than fish in the sea by the year 2050.

This was a perplexing headline for a few reasons, but the question that distracted me most was “how do you count the fish in the sea?”

As SCUBA divers,  free divers, or fisherman know; counting fish is tricky business.  They move, they dart, they circle back around. They spend time in tight groups, they eat each other, and they lay a great deal of eggs.

So the method of counting fish has not been done based on human observation, but based on the satellite imagery of phytoplankton in the world’s ocean.

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Phytoplankton, visible by satellite image because its abundance actually changes the color of the ocean.  All food chains in the ocean use phytoplankton as the base, and therefore, all fishy numbers come from the amount of fish that phytoplantkon can support. You can guess the tonnage of fish it supports, but you cannot guess the actual number of fish.

According to an article on BBC News Magazine, “Will there be more fish or plastic in the sea in 2050?”….”The new numbers for fish tonnage that the Ellen MacArthur foundation has cited (the foundation that made the plastics claim) are based on a 2008 study led by Simon Jennings from the Centre for Environment, Fisheries and Aquaculture Science”

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“His new study does not distinguish between fish and other marine predators, but it concludes that there may be between 2 billion tonnes and 10.4 billion tonnes of marine creatures in the oceans. ‘We’re not absolutely confident in our methods to determine what proportion of this is fish at the moment,’ says Jennings. ‘It’s a very uncertain number to predict.'”

Other counting methods? Checking certain areas of the ocean, and multiplying based on educated guesses.  But as you can probably see, this number is also uncertain.  What it comes down to is that calculating exact numbers is impossible.

Read the article here: 

You might think this distraction about numbers is pulling attention away from the heart of the matter: that the ocean is filled with plastic, and that is a bad thing.  However, the issue at hand is not that there is debris in the ocean (although it is terrible).  Having the correct numbers and data to back up your claims is essential in making arguments for change. To be imprecise is to open your side to attack and distraction.


How many fish in the sea are there? That number is unknowable.

How much plastic is in the ocean? Too much, that much is known.




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