Coral Fish

Coral reef fish make their way home

Coral reef fish hatchlings dispersed by ocean currents are able to make their way back to their home reefs again to spawn.

This finding is based on a study conducted by scientists from Australia, France, and the US, led by Dr Geoff Jones and Dr Glenn Almany of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University.

For the study, which took place in a marine protected area in Papua New Guinea, scientists tested a new method to trace fish from birth to spawning and detect the percentage of fish hatched on one reef that return there to spawn.

The techniques used in this study can reveal the extent to which fish populations on separate reefs are isolated breeding populations, or connected by fish movements.

Following two fish species, the clownfish (Amphiprion percula) and the vagabond butterflyfish (Chaetodon vagabundus), the scientists found that young of both species returned to their home reef about 60 percent of the time - a surprising result for fish larvae that had dispersed from a small reef habitat into a large area.

"If we understand how fish larvae disperse, it will enable better design of marine protected areas, and this will help in the rebuilding of threatened fish populations,” said Almany.

The two species in the study have different reproductive styles. Butterflyfish release eggs and sperm into the water, and the larvae drift and swim freely for more than a month before finding a home reef. In contrast, clownfish spawn eggs that are attached to the reef for a week before the larvae hatch and disperse in oceanic waters for 10 to 14 days. The larval clownfish must then find a reef, and a suitable anemone, that will be home for the remainder of its life.

Currents inevitably carry both species away from the parental reef (larval fish cannot swim well), but this study confirms that the majority of both species appear to find their way home after completing the oceanic larval phase.

But how do you tag fish larvae? Well, the process involves injecting minute quantities of harmless stable barium isotopes into the breeding female fish of both species. “The isotopes are passed to the offspring and incorporated into the ear bones - or otoliths - of the developing embryos,” said Thorrold, “thereby labeling the hatchlings at birth with the isotopes as permanent traceable tags.”

Two months after injecting females, the scientists returned and captured newly settled fish at the same reef to determine how many had returned to their home reef and how many had migrated from other nearby reefs.

Reef fish conservation programs utilizing marine protected areas are based on assumptions about how many fish migrate in from other areas and how many return to home areas to spawn. At a time of increasing pressures on coral reef ecosystems, this study provides an important piece for planning the optimum size of coral reef-protected areas and breeding populations.

“Just as importantly,” said Almany of their results, “40 percent of the juveniles came from other reefs that are at least ten kilometers (five miles) away, which indicates significant exchange between populations separated by open sea. This shows how marine protected areas can contribute to maintaining fish populations outside no-fishing zones.”

The successful test of this method in the field offers scientists a new way to evaluate the effectiveness of management models and practices based on direct information. Thorrold is continuing this work, using the maternal labeling technique to evaluate the degree of connectivity in other fish populations, including endangered Nassau grouper in the Caribbean.
Adapted from press release by Woods Hole Oceanographic Institution.

X-Ray International Dive Magazine
@ 2004-2007

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Isn't this just so amazing? I have always suspected that they are clever. The clown fish is one of my favourites when I go diving. On each dive in the Red Sea, one can always see clown fish in their anemones constantly throughout the dive. They seemed to be everywhere. Interestingly, I did not seem to see as many when I dived in Layang-layang. They are also very territorial and guide their own patch / anemone jealously. One of the things that I like to do is to swim quite closely to them with my face forward. They will get very excited and darts around the anemone. Quite often, they will try to attack me by charging at my mask. Usually, there will only be a pair of clown fish per anemone. However, at times you can see more than one pair. I read somewhere that the dominant pair are the bigger ones. The others are usually much smaller. This is because the dominant pair are always bullying the smaller ones, and thereby preventing them from feeding themselves. However, if the dominant pair dies off, the smaller fish will grow in size in a matter of days, and takes over the anemone. Bravo! The film "Finding Nemo" has certainly popularise the clown fish. Unfortunately, it has led people to taken corals and catching the clown fish for their own aquarium at home. This practice is to be discouraged as it leads to the destruction of what is already a very delicate marine environment.

Anyway, just thought that I let you folks know that these tiny creatures have "brains" and should be respected.Hurrah!