Global warming causes fish bullying

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Global warming not only leads to increased coral bleaching, it can also cause smaller fish to be bullied to death, according to research published in a recent issue of the journal PLoS ONE.

This conclusion was made by Australian scientist Mark McCormick, studying the interactions among Ambon damselfishes (Pomacentrus amboinensis) on bleached coral reefs.

The author carried out the study in the northern Great Barrier Reef, where he placed one large and one small, tagged juvenile damselfish in sites containing one of three different substrates: healthy Pocillopora damicornis (a bushy hard coral), thermally bleached P. damicornis, and dead P. damicornis.

After the juvenile fish were introduced to their new habitats, the survival of the tagged fish was monitored 2"3 times per day by visual census

The author found that the damselfish stayed closer to shelter on live coral, while on bleached and dead coral they moved higher up the coral making them more vulnerable to predation.

At the same time, the larger damselfish of the experimental pair were pushing the smaller ones further away from them in habitats with bleached and dead coral.

This exposed the smaller fish to even higher predation risk, as they were even further away from shelter.

This was supported by the mortality rates of the damselfishes, which were up to four times higher on bleached and dead coral than on live coral.

An additional finding of the study that juvenile damselfish tended to remain where they settled, even if healthier coral was only a short distance away, compounds the problem.

According to Dr. McCormick, increasing the resilience of the system is the only way to reduce the effect of coral bleaching on fish communities.

"If we take away the stress of harvesting and pollution caused directly by human activities, the habitat may be better able to resist the stress of increased water temperature and the community may be able to bounce back more quickly from major environmental change."

For more information, see the paper: McCormick, MI (2009) Behaviourally mediated phenotypic selection in a disturbed coral reef environment. PLoS ONE 4, e7096. doi:10.1371/journal.pone.0007096