A warming ocean could reduce fish species by half

A new study, partly funded by the JRC’s Exploratory Research project EUReefs (Mapping the hidden links between the future of coral reefs and European economy), finds that local fish richness across the globe could be cut in half if corals are lost.

This would lead to more areas with low or intermediate fish species richness and fewer fish-diversity hotspots.

Such changes would impact not only biodiversity, but also the estimated half a billion people who rely almost exclusively on coral reefs for their sustenance, through protein provided by reef fish.

Corals in decline

Corals - which are living organisms officially classed as animals - increasingly bleach and often die when the ocean’s water warms. Disease, pollution and overfishing also cause coral declines. Some alarming studies even suggest that under projected ocean warming, all corals might be lost during the 21st century.

Removing some corals by hand can cause more than half of a reef’s fish species to swim away. Yet what happens to those fish if there are no alternative reefs to swim to? The few fish species that feed on corals will inevitably starve, but the rest might find alternative rocky habitats.

What will happen to fish in a world without corals?

Thus far, it has been hard to do the larger-scale studies that can project what fish will remain in a world without corals.

Now, a new study led by Giovanni Strona (University of Helsinki), with collaboration from the JRC, finds that global projections of fish diversity are as low as small-scale experiments suggest.

The researchers started by mapping tropical fish and coral diversity across the world’s oceans for every square degree latitude and longitude.

These new maps showed what marine biologists have long known, namely that fish and coral diversity vary widely, with many more species in the indo-Pacific “coral triangle” than in the western Atlantic and eastern Pacific.

Coral diversity begets fish diversity

Diversity hotspots have long been explained by the way that latitude, habitat, temperature, and geography affect speciation and extinction rates among corals and fish alike.

After controlling for factors that drive diversity in general, the authors found that areas with diverse corals still tended to have more diverse fish species, suggesting that coral diversity begets fish diversity.

This is not particularly surprising given that corals provide a unique food source for some fish species, as well a three-dimensional habitat that many fish use for shelter.

Food webs will begin to unravel if corals become extinct

After modelling the relationship between coral diversity and fish diversity, the authors did a simple thought experiment. They anticipated global coral extirpation by extrapolating the fish vs corals association down to the point where no coral species were left.

That extrapolation suggested that around 40% of the world’s tropical reef fish species are likely to disappear should corals disappear.

As shown in smaller scale experiments, this is a much bigger loss than those species known to depend directly or even indirectly on coral, suggesting that coral reef food webs will begin to unravel if corals do go extinct.

This unravelling would be more intense some places than others. The Central Pacific, for example, would to lose more than 60% of its reef fish species, while the Western Atlantic would lose 10%.

A cause for concern, and a call to action

For anyone who has ever enjoyed snorkelling on a coral reef, or for the millions of people who depend on reef fish for food, including in European Overseas Territories, this thought experiment should be concerning.

It should also inspire greater efforts to conserve and restore coral reefs. The benefits of doing so will extend far beyond corals, to fish and other organisms that depend directly or indirectly on corals.

Further information

A future ocean that is too warm for corals might have half as many fish species

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A future ocean that is too warm for corals might have half as many fish species