According to a new study from the Australian Institute of Marine Science, reef fish such as emperors, tropical snappers, and rock cod help keep the quantity of crown-of-thorns starfish on the Great Barrier Reef under check.
The study, which was published in Nature Communications today (December 8, 2021), discovered that the abundance of coral-eating starfish increases in areas where fish species known to consume the starfish are removed. The Great Barrier Reef is the world’s biggest coral reef system, with over 2,900 distinct reefs and 900 islands spanning over 2,300 kilometers (1,400 miles) over a 344,400 square kilometer region (133,000 sq mi).
Coral reefs in the Indo-Pacific are home to crown-of-thorns starfish (Acanthaster spp.). Because they feed on the living tissue of many hard coral species, they are a substantial contributor to coral loss when present in great quantities.
Since the 1960s, four outbreaks have occurred on the Great Barrier Reef, the most recent of which is still ongoing. The Great Barrier Reef is the world’s largest single structure built by living beings and can be viewed from space. Hundreds of billions of microscopic organisms known as coral polyps make up this reef structure.
“More than 50 years ago concern was raised that removal of predators may contribute to starfish outbreaks. However, at the time only one predator of the starfish was known, the giant triton sea snail,” Dr. Frederieke Kroon, AIMS ecologist and lead author said.
According to recent research, approximately a hundred different types of coral reef creatures feed on different stages of the starfish’s life cycle. Eighty of these are fish, including emperors, tropical snappers, and rock cod, which are all popular seafood species.
“Our study is the first to explore how fisheries harvests of these fish species may affect starfish abundance.”
To begin, the researchers examined AIMS’ long-term coral reef fish and starfish abundance data obtained at reefs that were open to fishing versus reefs that were closed to fishing. The biomass of emperors, snappers, and rock codes was 1.4 to 2.1 times higher on reefs closed to fishing, but starfish densities were nearly three times lower.
More than 50 years ago concern was raised that the removal of predators may contribute to starfish outbreaks. However, at the time only one predator of the starfish was known, the giant triton sea snail.
Dr. Frederieke Kroon
“It is well known that no-take marine reserves increase fish biomass and diversity of large fishes. Previous studies have suggested marine reserves could also influence starfish numbers, but our study provides strong evidence there are fewer crown-of-thorns starfish on reefs with more predatory fish,” said Dr Kroon.
The researchers also compared 30 years of Queensland Department of Agriculture and Fisheries reef fish harvest data with crown-of-thorns starfish abundance data from AIMS’ long-term reef monitoring over the same time period.
The link between fishery harvests and starfish counts, according to Dr. Kroon, is apparent.
“We found crown-of-thorns starfish density increased in areas where more reef fish biomass was harvested,” she said.
“This relationship was strong for emperors, particularly red throat and spangled emperors Lethrinus miniatus and L. nebulosus, both of which are well-known predators of crown-of-thorns starfish.”
Tropical snappers and rock cod, as well as coral trout, have a strong association (Plectropomus spp. and Variola spp.).
“Since adult coral trout are not known to eat crown-of-thorns starfish, we are interested in what may explain this relationship. One possibility is that juvenile coral trout may eat small starfish, as part of their invertebrate diet,” Dr. Kroon said.
“Combined, our results suggest that the removal of emperors, tropical snappers, and rock cod contribute to increases in starfish numbers.”
The findings have provided an opportunity to investigate new tools for controlling outbreaks on the Great Barrier Reef and perhaps across the Indo-Pacific, such as targeted fisheries-based management.
“Starfish outbreaks continue to be a major cause of coral loss, but unlike other pressures like climate change, can be managed at local and regional levels,” Dr Kroon said.
“Targeted fisheries-based management, in combination with current crown-of-thorns starfish management interventions such as direct manual control, could assist in further controlling outbreaks.”
The findings, according to Dr. Kroon, contribute significantly to understanding the drivers of starfish outbreaks, such as the starfish’s inherent tendency to reproduce in large numbers and the role of water quality, as they are not mutually exclusive.
“It is very likely not one, but multiple factors which contribute to the outbreaks,” she said. “Large-scale, long-term data such as those used this study, as well as experimental studies are the best scientific tools we have to help understand the complexities of crown-of-thorns starfish outbreaks, and to implement effective and efficient management interventions for their control.”
