Northern tropical fish, pushing south as waters get warmer due to climate change, are destroying kelp forests off NSW near Coffs Harbour, according to the latest research.
The decline of the kelp impacts commercial and recreational fishing.
The study analysed underwater video covering 10 years between 2002 and 2012 during which the water warmed by 0.6 degrees.
“Kelp forests provide vital habitat for hundreds of marine species, including fish, lobster and abalone,” says Dr Adriana Vergés from the University of NSW and Sydney Institute of Marine Science.
“As a result of climate change, warm water fish species are shifting their range and invading temperate areas. Our results show that over-grazing by these fish can have a profound impact, leading to kelp deforestation and barren reefs.”
Watch the warm water species rabbitfish feeding off NSW:
This is the first study demonstrating the effects of warming on kelp forests. Higher temperatures not only have a direct impact on seaweeds, they also have an indirect impact by increasing the appetite of fish.
“Increases in the number of plant-eating fish because of warming poses a significant threat to kelp-dependent ecosystems both in Australia and around the globe,” says Dr Vergés.
The study is published in the journal Proceedings of the National Academy of Sciences.
The team recorded underwater video around August each year at 12 sites along a 25 kilometre stretch of coast next to the Solitary Island Marine Park off northern NSW.
The video shows kelp disappearing.
At the same time the tropical and sub-tropical seaweed-eating fish swimming in these areas more than tripled.
Grazing also intensified, with the proportion of kelp with obvious feeding marks on it increasing by a factor of seven over the decade.
“We also carried out an experiment where we transplanted kelp onto the sea floor,” says Dr Vergés.
“We found that two warm-water species — rabbitfish and drummer fish — were the most voracious, eating fronds within hours at an average rate of 300 bites per hour.
“The number of fish that consumed the smaller algae growing on rock surfaces also increased, and they cleared the algae faster when there was no kelp present. This suggests the fish may help prevent kelp regrowing as well, by removing the tiny new plants.”
In Australia, kelp forests support a range of commercial fisheries, tourism ventures, and recreation activities worth more than $10 billion per year.
“The decline of kelp in temperate areas could have major economic and management impacts,” says Dr Vergés.
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