Coral bleaching is not the only threat to reef fish

Climate change poses a number of threats to the long-term sustainability of the Great Barrier Reef and the species that live there. Although the damage caused by more frequent bleaching of corals is well documented, the impact of tropical cyclones is less well studied because of their short duration and unpredictability.

As in the case of coral bleaching, climate change models predict that tropical cyclones of high intensity will occur more frequently as the planet warms up.

In an article published September 5 in the journal PLOS ONE, researchers at the University of Wollongong (UOW) studied the effect of two successive Category 4 cyclones – Cyclone Ita in April 2014 and Cyclone Nathan in March 2015 – on the social structure of corals. reef fish at Lizard Island.

Lead author Dr. Martin Hing of the UOW School of Sustainable Ecosystem Solutions said that cyclones can devastate the physical structure of coral reefs and the abundance and distribution of animals.

However, scientists know relatively little about how cyclones affect the social structures of species living on the reef or how different social strategies affect a species' ability to cope with disturbances.

"For social species, social organization is a key factor in terms of things like the foraging efficiency and how this particular species breeds – things that fuel their ultimate survival," said Mr. Hing.

"If this is interrupted, if their social structure is changed, their feed efficiency may decrease and they will have lower survival rates."

The researchers sampled group size and coral size of five species forming groups and eight species of paired Gobiodon fish (coral gobies) before and after cyclones Ita and Nathan. The group sizes of the species forming groups decreased after each cyclone, but showed signs of recovery four months after the first cyclone (Ita).

A similar increase in group size was not evident in species forming groups after the second cyclone (Nathan). On the other hand, there was no change in the average size of the group forming the pair after one or other of the cyclones.

"We found that the species forming groups decreased in the size of their group after each cyclone, while the species forming pairs did not increase or decrease in their average group size," said Mr. Hing.

"Between the two cyclones, the species forming groups recovered slightly and began to return to larger groups. This was quite unexpected as the average coral size was much smaller and we expected the smaller corals to generally not support larger groups of fish.

"Then, when the second hurricane arrived, they further decreased in group size.

"We sampled about 10 months after the second cyclone and the size of their group was still very small. After an isolated incident, the population could probably rebound, but it seems that the multiple impacts could have more lasting effects on them.

"As the frequency of more intense cyclones is likely to increase in the near future, this could lead to population decline and even localized extinctions of coral gobies forming a group."

The social organization is a key factor in the foraging and reproduction efficiency, which feed their ultimate survival.

One of the reasons why the impact of repeated tropical cyclones on reef inhabitants is less well known is the difficulty of knowing when and where cyclones will hit.

For this study, the UOW team was at Lizard Island before and after the two cyclones.

"This study was highly opportunistic and probably unique. We tried to do a longer-term study on the sociality of these fish, but the cyclones had different plans for us, "Hing said.

"Essentially, we started an experiment, then the cyclone came in and we thought," Okay, let's go back and see what we can find. "

"On the first trip, we tagged about 600 fish and we went to see if these same individuals were still in their original corals or if they had moved to other corals around the place. After the hurricane, we found only 11 of the 600 fish we tagged, which was very devastating.

"However, we recovered the data we collected and could compare before and after cyclones. This happened two years in a row.

"We were actually at Cairns airport when the second hurricane arrived. We made a trip to Lizard Island, but we went back.

About the study

"Repeated cyclones reveal potential causes of sociality in coral-dwelling Gobiodon fish" by Martin L. Hing, O. Selma Klanten, Mark Dowton, Kylie R. Brown and Marian Y.L. Wong, is published in PLOS ONE at 14:00 ET on September 5 (4:00 am EST Sept. 6) 2018.

This research was supported by the Hermon Slade Foundation.