Coral bleaching effects on biodiversity permanent, study says

coral bleaching effects on biodiversity
© iStock/Rainer von Brandis

A new long term study on the wider effects of coral bleaching has found the phenomenon can permanently change surrounding fish populations.

The study of coral bleaching effects in the Seychelles, led by researchers at Lancaster University and published in the Global Change Biology journal, found that the bleaching of coral in 1998 – itself caused by rising ocean temperatures – led to irreversible changes to the surrounding marine habitat and a subsequent shift in the species of fish able to survive in the area. The study tracked changes in marine populations around Seychelles reefs over 16 years between the mass bleaching event in 1998 and the next event in 2016; and showed that fish which had left the area in the immediate aftermath of the first bleaching event did not then return.

Dr James Robinson of Lancaster University, the study’s lead author, said: “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.”

Numbers of damselfish and large snappers fell drastically around reefs affected by coral bleaching, to be replaced by herbivores such as rabbitfish, which feed on the seaweed which replaced the damaged coral. The researchers hypothesise that coral bleaching effects similar to those observed in the Seychelles were likely to have occurred in other reefs affected by bleaching events.

Professor Nick Graham, also of Lancaster University, said: “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.”

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