Nature and Biodiversity

The five biggest threats to our oceans

Greg Stone
Trustee, Pacific Rising
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When I give talks about how we have degraded our oceans to the cusp of catastrophe, people often ask me what they can do. If you want to save the oceans, by all means eat only sustainably sourced fish and stop using plastic shopping bags that become ocean detritus. These things are worth doing. But the problem in our oceans is not something that will be solved through those simple actions alone.

When you get sick, you may recognize the symptoms – coughing, sneezing, perhaps a fever – but despite observing those things, you are not qualified to make a diagnosis. You go to a doctor. We need to be willing to call in the “ocean doctors”, marine experts, and listen to their diagnosis and follow their prescriptive response.

Tools such as the Ocean Health Index have allowed us a better understanding of the determinants of our oceans’ health. There is no one-size-fits-all solution to improving ocean health, but there are challenges which, if addressed with urgency, will allow the oceans to recover and support our growing population far into the future.

1. Overfishing

A 'No Fishing' sign is painted onto the quayside in Scarborough

We have methodically depleted the fish in our oceans. First, we exhausted those we could catch with small boats and rods close to the shore. Then we went further and exhausted the pelagic fish, such as herring and tuna. Then we went deeper, catching species such as the orange roughy, which can live to 150 years and don’t breed until they are 20 years old.

As a rule, it’s unwise to eat things that are older than you are: we should eat food lower down the chain that can reproduce quickly. The oceans are like a deep freezer full of fish, which we’ve now almost emptied.

The good news is that scientists know exactly how to replenish the stocks – by creating marine protected areas, or fish regeneration zones – and how to fish sustainably. This is not a question of knowledge, but of will: we could take action today, if we want to.

2. Coastal pollution

EDF (Environmental Defense Fund) coastal scientist Angelina Freeman takes a sample of oil while surveying the conditions of Bartaria Bay near Venice, Louisiana

Everyone remembers the Deepwater Horizon oil spill, but few people realize that the Gulf of Mexico suffers an even greater insult daily from the chemicals routinely carried into it by the Mississippi. Industrial agriculture is pouring reactive nitrogen and phosphorous into the oceans through every river on Earth, creating what are called “ocean dead zones”.

As with overfishing, we already have the scientific knowledge to rectify coastal pollution quickly by changing our practices on land; it’s about the will to act.

3. Habitat destruction

A Zayapa red crab perches on a rock at Punta Albemarle in Isabela island at Galapagos National Park

While marine habitats deal with the pressure of coastal pollution, most notably coral reefs, there are other ways we are systematically destroying the ecosystems that marine plants and animals need to survive. These include clearing mangrove forests for shrimp production and scraping entire ecosystems off seamounts, or underwater mountain ranges, through deep-sea trawling.

Again, these issues are well understood. We could act now.

4. Warming

A drop of water falls from a melting piece of ice on Argentina's Perito Moreno glacier near the city of El Calafate, in the Patagonian province of Santa Cruz

The rate at which oceans are warming may not sound dramatic – the temperature rise over the past century is estimated at about 0.1 degree Celsius – but that is enough to kill the algae that keep corals alive, move species into new areas, and cause sea levels to rise.

Even if we stopped pumping additional carbon dioxide into the atmosphere, the effects of climate change would continue to play out for a century. That’s not to say that we shouldn’t act on emissions; we must. But we also need to adapt, which will be especially disruptive in relation to rising sea levels. Unfortunately, there is no immediate solution to address global warming, which makes a response similar to…

5. Acidification

A cockler shows his catch on the sands of Morecambe Bay in north-west England

Like warming, acidification is related to carbon dioxide, which dissolves in oceans to form carbonic acid. The greater the acidity, the less able marine-calcifying organisms are to form shells, disrupting their reproductive process.

Addressing acidification is also an issue of knowledge and will. There are some localities where, for reasons we do not yet fully understand, the water’s pH is lowering more slowly. We need to investigate these areas while protecting them as well as we can by tackling the first three items on this list – the ones where we already know what we have to do.


Returning to our patient analogy, if they’re suffering from multiple illnesses, the chances are they won’t make it – but if you can treat some ailments, they may be able to survive the rest. Under the cumulative weight of acidification, warming, habitat destruction, coastal pollution and overfishing, the ocean environment may soon become fit only for jellyfish.

I have seen in the Phoenix Islands, however, in the central Pacific, that oceans can be resilient and recover if warming is all they have to cope with: the Phoenix Islands Protected Area, established in 2008, has kept coral reefs healthy and marine life abundant. If we tackle the first three items on the list, we give the oceans a fighting chance against the others.

Author: Greg Stone is Executive Vice-President and Chief Scientist for Oceans at Conservation International. He is a member of the World Economic Forum’s Global Agenda Council on Oceans.

All images courtesy of REUTERS. Main image: A fishing boat in the waters off British Columbia. REUTERS/Ben Nelms

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