Scientists studying the effects of tropical fish from the north intruding into reefs off the NSW coast have identified the first victims of ‘tropicalisation’ of temperate waters.

But surprisingly, the scientists also found greater diversity and more abundance of fish overall, indicating there are both winners and losers in this global phenomenon attributed to climate change.

In the study published this month in Global Change Biology, scientists from UNSW, University of Sydney and the NSW Department of Primary Industries looked at the diversity of fish populations on the reefs surrounding the Solitary Islands Marine Park off the coast of Coffs Harbour.

They used data from baited underwater video surveys collected over a period of 17 years – between 2002 and 2018 – to analyse changes in the diversity and abundance of marine fishes on the reefs.

Lead author of the study Shannen Smith is a PhD candidate with UNSW’s School of Biological, Earth and Environmental Sciences. She says that as the sea temperatures have warmed, there has been a steady increase of tropical fish venturing into these waters.

“One of the problems with these fish arriving in temperate waters is that they eat a lot of the seaweed,” she says.

“In fact, by 2009, we observed complete kelp loss in this ecosystem, and we weren’t sure what that would mean for the broader fish community.”

The scientists were interested to know how the loss of kelp in the Solitary Islands ecosystem would affect the diversity of the fish. Would the tropical fish continue to thrive? And what would happen to the local fish populations as the main feature of their habitat disappeared?

“When we looked at these reefs where extensive kelp loss has been documented, we found that the overall abundance and diversity of fish actually increased,” says Ms Smith.

“This really surprised us. We expected to see cold-water fish species that typically live in kelp forests to decline. We expected too that perhaps tropical invader numbers might plateau, but their abundance continues to increase.”

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