It’s a case of “reef, sweet reef” for baby tropical fish, say researchers who have found a way of tracking the movements of two generations of fish. Their study shows that baby fish are able to find their way home to the reef their mother lived on.
“We have suspected this for a long time,” says Michael Berumen of the University of Arkansas in the US. “But it has spawned a big debate. We know fish are capable of returning to their home reef, but do they really? Until now, we didn’t know the answer to that.”
To see if this “self-recruitment” really does happen in the wild, Berumen and his colleagues in Australia and France traveled to Kimbe Island near Papua New Guinea. On the reef that surrounds the island (pictured, right), they collected 176 female clownfish and 123 female butterflyfish.
Clownfish spawn their eggs in a nest but the larvae can spend about 10 days floating around in open water before settling on a reef. Butterflyfish, like snappers, groupers and many other species targeted by the fisheries industry, are pelagic spawners – meaning they spray their eggs and sperm into open water. The juveniles do not settle on a reef until 38 days later.
The researchers injected both species with small amounts of a barium isotope. After traveling through the females’ bloodstream, the radioactive tag ends up in their eggs and eventually in an ear bone in their offspring.
“It’s a really neat technique that they’ve developed to actually tag fish through a whole reproductive season,” says Stephen Simpson of Edinburgh University in the UK. “Particularly for a species of pelagic spawners whose eggs are much more difficult to find.”
The scientists returned to the reef about one month after having released the tagged females and this time collected juveniles and counted how many carried the barium isotope. The team calculated that about 60% of the juveniles on the reef were the offspring of females from that reef.
“For pelagic spawners, this means the females spew their eggs into the water column and somehow the eggs hatch and the larvae find their way back to the reef, which they’ve never seen,” says Simpson.
In the case of Butterflyfish, “there are 5 to 6 weeks during which they are potentially out at sea,” says Berumen.
Smelly and noisy
How the fish find their way back to the reef is another question. According to Simpson, reef fish scientists have traditionally been divided between those who believe the dispersal of offspring is at the mercy of currents and those who believe it is driven by the behavior of the offspring. He belongs to the second group and has shown that reef fish are capable of recognizing the sound of their home reef. Other scientists have shown that fish can pick out their reef by its smell.
But where does this ability to sense the home reef come from? Simpson has a possible explanation: “You could imagine there is a suite of genes passed on to the embryos, who are therefore pre-programmed as to what they should do once their ears, eyes, and nostrils develop”.
The new research may not just be a curiosity. The knowledge of the area over which the reef fish travel could help design better marine reserves.
For example, the scientists say reserves that are too large may not enable fish from the protected areas to re-supply the surrounding areas, where fishing continues.