Biologist: Dead zones in the ocean are threatening our most important food fish

Eric Prince

Three years ago, I interviewed Eric Prince, a research fisheries biologist at the National Oceanic and Atmospheric Administration's Southeast Fisheries Science Center. He and his colleagues had found that a huge dead zone, an area of the ocean with very little oxygen, had developed in the Atlantic. It spread from the east coast of South America across the ocean to the west coast of Africa; it was the size of North America.

"Ninety-five percent of all the marine organisms in the ocean depend on adequate levels of dissolved oxygen," he said. "They tend to be squeezed by this hypoxia [diminished oxygen] into a very narrow layer at the surface of the ocean. In the process of being compressed near the surface, they become a lot more vulnerable to overexploitation by surface-eaters. This involves some of our most important food fish, yellowfin tuna for example."

He explains what has happened in our oceans in the 3 years since we spoke.

Lynne Rossetto Kasper: Could you explain what you found when we talked to you in 2011?

Eric Prince
Eric Prince

Eric Prince: The paper that I discussed with you last time was published in Fisheries Oceanography in 2010. That paper focused on the Atlantic Ocean, which is much smaller than the Pacific Ocean, which had already-documented oxygen minimum zone results in hypoxia-based habitat compression.

LRK: Meaning zones in the ocean where there was no oxygen in the water?

EP: Right, or very little.

The animals in the surface mixed layer above the thermocline -- there's a tremendously productive population of animals in that zone, but not much below.

That was really the essence of it: Everything we found in our previous paper in the eastern tropical Pacific, we also found very much applied to the eastern tropical Atlantic as well. Only it was a smaller body of water, the hypoxia and the oxygen minimum zone wasn’t as large. It wasn’t quite as hypoxic, meaning it had a little bit of oxygen, but not enough to support the tropical pelagic predators that we work with.

LRK: What’s pelagic?

EP: It’s actually an oceanographic term applied to animals that are where there is not a bottom. They are in the upper waters of the water column, and that’s where they live.

LRK: They live near the surface?

EP: Near the surface in the open water.

LRK: What accounts for this change?

EP: It’s a whole series of things that happen. Basically, it’s a phenomenon that occurs on the west coast of continents where there are meteorological factors including winds that parallel coastlines. These winds eventually turn into what are called an Ekman spirals. They end up pushing the surface water off the coast, allowing deep, nutrient-rich water -- but very hypoxic water with no oxygen -- to come to the surface and mix.

It occurs basically in the equatorial waters of the world. Except right at the equator there are two currents that go against each other that tend to mix a small, little area there -- it demolates the waters below the thermocline with oxygen. But everything else is pretty much squeezed into a very shallow area near the surface of the ocean. That’s what they call hypoxia-based habitat compression.

There is a very thin, very steep temperature gradient of those thermoclines in these areas. There’s also a cold shock to very small animals that don’t have the bulk. For the small sardines and anchovies and for those different families of animals we call the preferred prey of the tropical pelagic predators, those guys are too small to deal with the temperature, no less the dissolved oxygen change that exists below that thermocline. That population of prey is really stuck.

In some cases, certain species of tuna have adapted to low ambient dissolved oxygen. In other cases, animals like swordfish have also adapted to low oxygen. They can live below the thermocline. But they come up to feed on this highly dense prey species, which is like a big magnet where all the prey species are in one location.

LRK: As I recall, because a lot of these fish were being forced up closer to the surface, fisheries, people who were out there doing commercial fishing, were reading this like there were plentiful fish -- but there really weren’t.

EP: What you end up with is overly optimistic data because the animals are squeezed in density in that part of the world, which is higher than in others. What you need to do is correct for that.

LRK: Because it looks as though there are more fish there.

I’m looking at a map of the Atlantic Ocean between the west coast of Africa and the east coast -- the part of South America that juts out. It’s showing that this hypoxia, this dead zone, is huge. This is not just along the coastline; this zone is a very wide band across the entire Atlantic. How do we stop this?

EP: I’m not sure we do. The thing that fuels a lot -- but I’ve never read it in a peer-reviewed paper -- is the fact that dissolved oxygen in the oceans is inversely related to temperature. The higher the temperature, the less dissolved oxygen that can be driven into the ocean.

LRK: So with climate change ...

EP: It’s getting warmer, and the surface water temperatures have been predicted by climate models to increase as much as even 5 degrees Celsius by the end of this century.

I was in a meeting in London called Planet Under Pressure, and a very well-known scientist who’s really a leader in the world on hypoxia, Robert Diaz, gave a talk. I asked the question afterward: “All the projections of the temperature increase by 5 degrees Celsius -- but if it even increases by half that amount, what would you predict in terms of the growth of the oxygen minimum zone? What about our pelagic fisheries?”

I was really shaken by his response. He said, “The oxygen minimum zone in the Atlantic is going to start in North Africa and go all the way down to the tip of South Africa. It’s going to cover every single part of the eastern South American coast. We’re simply not going to have those fisheries.”

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