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Young Fish Return to "Home Reefs" to Settle Down

Scott Norris
for National Geographic News
May 4, 2007
 
After spending weeks adrift in the ocean as tiny larvae, juvenile coral reef fish often return to their "home reef" to settle down, researchers say.

The strong homing behavior shown by reef-dwelling butterflyfish in the waters near Papua New Guinea came as a surprise to scientists, and it may have important implications for the design of marine reserves.

Researchers and fisheries managers have long sought better information on how very young fish disperse in the ocean.

Tracking the fate of tiny larvae has been extremely difficult, since the fish are too small for even the smallest electronic tags.

So a team led by Glenn Almany of Australia's James Cook University used a new method for chemically "tagging" fish while still in the egg stage inside their mothers' bodies.

Later recovery of the tagged juveniles showed that as the fish matured, they returned to their place of birth.

On a very small reef, the team found, more than 60 percent of the young, recently settled fish were offspring of adult fish living in the same location.

"We were shocked to find that so many larvae returned to their parents' reef after 38 days in open waters," Almany said.

The team's report appears in this week's edition of the journal Science.

Parental Involvement

For their study the researchers tracked two fish species with different reproductive strategies.

"Each represents one of the two dominant modes of reproduction found in marine fishes," Almany noted.

Clown anemonefish lay their eggs on bare rocks, which are tended by the parents for several days.

After hatching, the free-swimming larvae typically settle down near sea anemones after just 11 days, making the fishes' stay-at-home tendency not particularly surprising.

(Read "No Nemo: Anemones, Not Parents, Protect Clownfish" [June 5, 2003].)

The untended eggs and larvae of vagabond butterflyfish, on the other hand, may spend four to six weeks drifting with the currents—a trait shared by many commercially important fish species.

Right now little is known about how far the butterflyfish larvae travel and how the juveniles find their way back to their parents' reef, Almany said.

"We can only guess about how they might locate the exact reef on which they were born," he said. "Perhaps some form of imprinting, such as a chemical 'memory' of home."

Not Reseeding Reefs?

Robert Steneck is a marine ecologist at the University of Maine who was not involved in the study.

He noted that previous studies had often assumed high rates of dispersal in coral reef fish, with local populations maintained by young fish arriving from across a broad region.

Similarly, the theory behind many marine reserves is that the offspring of fish in protected areas will help maintain populations far beyond reserve boundaries.

Almany's team, Steneck said, "found the best coral reef system, the best marking tool, and the right species of fish to empirically test the theories. Their results were very surprising but very important indeed."

The finding that fish populations on coral reefs may largely be locally generated means the benefits of reserves may not extend as far as had been hoped.

"The stated goal of many [marine] reserves is that they can be a source of larvae for effectively reseeding damaged reefs downstream," Steneck said.

"We might want to increase our efforts to protect reproducing fish outside of reserves," he continued, "if we don't want to see growing dead zones throughout the world's reefs."

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