Removing massive amounts of fish (and their poop) has altered ocean chemistry: study

In this Tuesday, Nov. 26, 2019 photo, Menhaden are stacked in a hold of the Windmill Point menhaden fishing boat at Omega Protein's menhaden processing plant on Cockrell's Creek in Reedville, Va. The Trump Administration is threatening to effectively ban a company that makes fish oil pills from fishing in the Chesapeake Bay over mounting concerns from regulators, governors and environmental groups about overfishing. Earlier this year, the company Omega Protein exceeded harvest limits in the bay by more than 30% on a bony and oily fish called Atlantic menhaden. (AP Photo/Steve Helber)

There are not a lot of fish in the ocean. Well, there are, but not as much as there were.

McGill University Department of Earth and Planetary Sciences professor Eric Galbraith led a study that found that overfishing has altered the ocean's chemistry and may play a part in climate change.

There would now be twice as many fish in the ocean as there were in the 1990s, when fishing was at its peak, said Glabraith, which has changed the chemical composition in the deep ocean.

"The important message here is that this is another benefit to be gained from reducing overfishing," he said. So, if you reduce fishing pressure, this aspect of the ocean will go back fairly quickly within a few decades to what it was naturally."

Galbraith said it's the first time anyone has tried to put a number on the total amount of fish in the ocean using computer models to realistically simulate how fast fish can grow and adding fishing data to see how many are being taken out of the ocean.

As a result of taking five to 10 billion tons of fish out of the ocean, the ocean's circulation system -- how nutrients and carbon naturally move around the ocean -- was affected, Galbraith's study found.

"So these fish were all swimming around, they were eating, they were pooping and poop was sinking down into the deep ocean," he said. "Fish poop contains a lot of carbon, and so it [fishing] would have been removing carbon from the upper part of the ocean and sticking it down in the deep ocean."

Removing that carbon removes CO2 from the ocean's surface, which pulls carbon dioxide out of the atmosphere in the same way trees do.

Though Galbraith admits that researchers do not know the exact number of how much that removed CO2 has changed the atmosphere, it is clear that something happened.

"This paper is really just a realization of: 'Whoa!' The fishing had much more of an impact on how the ocean chemistry function than we realized," he said.

When it comes to climate change, Galbraith said the effect of fishing is of a similar magnitude as the more commonly studied effects of how global warming has altered ocean circulation.

"It's another thing to try to understand better," said Galbraith. "It's an impact that we weren't aware we had had. I think the good part is that it's an impact that we can undo, relatively easily by just fishing less."

Overfishing is an issue that has been studied for centuries, but typically from an economic standpoint.

The Grand Banks of Newfoundland, for example, was overfished in the late 20th century leading to the Canadian Grand Banks fishery closing in 1992 due to the collapse of cod and other fish stocks.

Galbraith's study, however, shows that overfishing's impact extends to the whole system of life in the oceans and the chemistry of the water itself.

"This is the tragedy of overfishing, is that when you fish too hard, you reduce the amount of fish that are there, which makes it more difficult to catch fish which makes fish more expensive, and you end up having less fish to eat. So this is also telling us that another benefit of reducing overfishing would be that we would move the natural circulation system back closer to how it used to be." 

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