Ocean protection accounts for 10% of fish in the world’s coral reefs – but we could save so much more

Ocean fish populations have fallen dramatically in the past half-century, and climate change is expected to make the problem worse. Governments have designated “marine protected areas”, where where human activity is constrained to protect ocean life. But have these efforts worked?

About 8% of Earth’s oceans are protected, including about 3% where fishing is banned altogether. Our new study of nearly 2,600 tropical coral reefs around the world is the first to examine whether these areas have helped fish populations.

We found about one in ten kilograms of fish on coral reefs is the result of efforts such as marine protected areas and other restrictions on fishing. This is promising news. But our study also reveals great room for improvement.

Getting to grips with marine protection

Maintaining healthy fish populations is important. Many communities depend on fishing for their food and livelihoods. And fish play a vital role in ocean ecosystems.

Marine protected areas are a key policy tool used to increase fish populations. They cover a range of ocean areas including lagoons, coastal waters, deep seabed waters and coral reefs.

The areas go by several names, including marine parks and conservation zones. Some, where fishing is prohibited, are known as no-take zones.

Governments often quote figures on the area of ocean protected when seeking to tout their conservation policies. For example in Australia, we are told the federal, state and territory governments have established marine parks covering 4.3 million square kilometres or 48% of our oceans.

But the extent to which marine protected areas actually conserve marine life varies enormously from place to place. So simply counting up the protected ocean area doesn’t tell you much about what has actually been achieved.

Measuring success

We and our colleagues wanted to assess the extent to which marine protection efforts have increased the amount of fish on coral reefs.

We developed a computer model based on about 2,600 reefs across the global tropics, which includes reefs in the Pacific, Indian, and Atlantic oceans. From that, we estimated the amount of fish currently on each reef – measured in the kilograms of fish per hectare, or “biomass”.

The estimations were based on information such as:

• environmental conditions such as ocean temperature and the type of habitat where the reef is located

• the intensity of fishing activity, known as “fishing pressure”

• how strong the protection is – for example whether it bans fishing, or just restricts it

• the level of compliance with no-take zones.

We then simulated what would happen if we changed the type of protection strategy in each location while keeping everything else the same.

We ran a few scenarios:

• no coral reef conservation existed anywhere and all reefs could be fished without constraint

• sites currently fished without constraint (which amounted to over half of our sites) had restrictions in place

• fishing was prohibited on 30% of all reefs.

And the results?

We found both marine protected areas and other fishing restrictions account for about 10% of the fish “biomass” on reefs. In other words, about one in ten kilograms of fish on coral reefs is due to protection efforts.

No-take zones punch above their weight. Of the fish biomass attributable to protection efforts, about 20% comes from just 3% of sites in no-take zones. This proportion would be even higher if illegal fishing in no-take zones was stamped out.

But we found any type of fishing restriction was useful. If everywhere currently fished without constraint was subject to some level of protection – such as banning nets or spear guns – the biomass of fish globally would be another 10.5% higher, our study found. This essentially matches all conservation efforts to date.

Our modelling also showed fish on coral reefs could be increased by up to 28% globally if the area of no-take zones rose to 30%.

But these reefs must be chosen strategically. That’s because protection strategies can lead to wildly different results, depending on local conditions. For example, sites with lower fishing pressure in the surrounding seascape got a bigger boost from protection than places surrounded by intensive fishing effort.

This may be because at heavily fished locations, algae often overtakes coral as the dominant feature. Algae is less fish-friendly than coral, so fish populations may not bounce back quickly even when fishing pressure is reduced.

Grounds for optimism

Our study tested the mettle of global coral reef conservation. On one hand, we found conservation efforts have made a contribution to the amount of fish on global coral reefs, which provides grounds for cautious optimism.

But on the other hand, this contribution is quite modest. Our study shows much greater gains could be made not only by expanding protected marine areas, but also by improving compliance in existing ones.

Most nations have signed a global agreement to protect 30% of Earth’s land and waters by 2030. That means the amount of ocean in marine protected areas globally will increase nearly fourfold in just six years.

As governments continue this task, we hope our results help identify ocean sites that will benefit most from protection.