Study Finds Coral Bleaching Permanently Changes Fish Communities

A new study has shown that coral bleaching is changing fishing communities in the Seychelles.
Christopher McFadden

Coral reef fish stocks are being replaced with seaweed-loving ones in the Seychelles according to new research.

This is due to significant changes to the ecosystem as coral reefs are not able to recover.

Coral bleaching is changing fish communities in the Seychelles

A recent study has shown that bleaching coral reefs appear to be changing fish communities forever. Most notably, larger predatory fish and tiny fish, like the Damselfish, are being hit hard by this phenomenon. 

The study, conducted over many years in the Seychelles, indicates that many coral-reef ecosystems are being replaced by seaweed and associated seaweed-loving fish like Rabbitfish.

The study was published in the journal Global Change Biology, and its findings were based on observations since 1998.

It is commonly believed that rising oceanic temperatures are one of the main driving factors behind coral bleaching.


Rising sea temperature and coral bleaching since 1998 are, in their view, leading to changes in biodiversity and permanent shifts in the range of fish species coexisting on coral reefs, which still remain in place today.

coral reef bleaching
Source: Pixabay

If their conclusions are correct, the researchers believe a similar pattern is occurring around the world on other damaged coral reefs. It may be that this is the "new normal" state for post-bleached reefs around the globe.

"The new normal for coral reefs will be reef fish communities which have fewer species and are dominated by herbivores and invertebrate feeding fish. This will alter the way coral reefs function, and the fishery opportunities for coastal communities adjacent to coral reefs," said Professor Nick Graham of Lancaster University.

What were the study's findings?

The Lancaster University-led study observed and tracked reef recovery in the Seychelles for over 16 years. They were also able to gather data from another major coral bleaching event that occurred in 2016. 

Between 1998 and 2016, fish communities did recover but did not return to levels seen prior to the bleaching events. 

Before major bleaching events, fish communities tended to consist of large predatory fish, like snappers and groupers and large amounts of very small fish like Damselfish and Butterflyfish

bleached coral
Coral bleaching in Okinawa, Japan. Source: The Ocean Agency/XL Catlin Seaview Survey

As the coral reefs became bleached, the zones in question became dominated by algal-feeding Parrotfish and Rabbitfish and invertebrate-feeding fish such as emperors and wrasses. The reason for this was pretty self-explanatory - the original habit had effectively collapsed. 

In areas where coral reefs were unable to recover, seaweed quickly rushed in to fill the void, and fish communities responded accordingly. 

But it's not all bad news

Interestingly, the research also indicates that some coral reefs are making a comeback. But most of them, it appeared, have seen seaweed fields replacing coral reefs in places.

Unsurprisingly, the most significant change in fish communities over the areas studied were primarily associated with seaweed-dominated zones. The most dominant fish in these areas were herbivorous fish that love to eat seaweed

Other studies also indicate that the time period between bleaching events is reducing over time. Bleaching events are now typically around every ten years or so. 

bleached coral underwater
Dead coral, Great Barrier Reef. Source: The Ocean Agency/XL Catlin Seaview Survey

However, in the Seychelles, the period was a little longer, at about 18 years. Yet despite this long period of time, fish communities were still not able to fully recover.

According to the study's lead author, Dr. James Robinson: "Although the 18-year period between major mass bleaching events allowed corals to recover on some reefs, we found evidence that fish populations were not able to return to their pre-bleaching levels, and they were substantially altered on the reefs that become dominated by seaweeds. The Seychelles case study suggests under current levels of ocean warming - where the average frequency of bleaching events is less than 10 years - permanent changes to reef fishes are likely on most coral reefs globally."

The original study was published in the journal Global Change Biology.

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