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It has been shown that when coral reef fish on the Great Barrier Reef are separated from their fellow fish they tend to lose weight or get stressed.

Photo by Eva McClure

Scholars from the ARC Centre of Excellence for Coral Reef Studies at James Cook University and the University of Glasgow have successfully measured the metabolic rates of individual fish in shoals for the first time to observe why they tend to stay as a group.

Photo by Mark McCormick

The lead author from James Cook University, Lauren Nadler suggested “We have suspected that shoaling fish gain a ‘calming effect’ from living in a group. But up until now we have been unable to measure how widely spread this effect is among individuals and under what conditions it may vary.”  

Her team conducted the work near Lizard Island on the Great Barrier Reef where shoals of the blue-green damselfish, Chromis viridis were then either kept within their shoals or isolated from them.

Nadler continued: “The fish that were isolated lost weight after the first week, which meant they were less healthy than those in groups.”

The metabolic rate is a measure of stress of all the fish that are either in a shoal or isolated. It has been revealed that isolated fish have a 26 per cent decrease in metabolic rate when isolated but were calmer and less stressed when they had their shoal-mates with them.

Mark McCormick, professor from James Cook University also commented: “If these fish were out in the ocean by themselves, in order to stay alive, they would need more food to keep up their energy. Since they don’t have their buddies around to help look out for looming predators, foraging would be risky.

“The extra energy fish gain from shoaling is so important because it allows them to survive and reproduce and to pass on their genes to the next generation of fish.”

Dr Shaun Killen, co-author of the research from University of Glasgow continued: “Many animals live in groups as a way to reduce the risk of being captured by a predator or for finding food more consistently, but these results show that group-living may also have direct benefits for reducing energy use and stress.”

He suggested: “Many important research questions come out of these findings. For example, it would be interesting to see how changing environmental conditions, such as the disturbances associated with climate change, might exacerbate the stress experienced by social fish when they become isolated.”

Unless the isolation from their shoal-mates is thorough natural disturbances like tropical cyclones, it won’t be good for the fishes.

Nadler continued: “When category 4 Cyclone Nathan passed over Lizard Island last year, we saw a number of blue-green pullers living by themselves on small coral colonies. They were probably separated from their group by the sheer force of the storm and currents. Those isolated fish would have been quite scared out on the reef as they wouldn’t have had their buddies to protect them.

“The results of this study indicate that these fishes probably had elevated metabolic rates which could have had a devastating impact on their overall health and condition and, ultimately, their survival.”

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