updated 4/3/2006 3:06:39 PM ET 2006-04-03T19:06:39

You might think fish were born swimmers. Not so, however.

When they hatch, fish larvae struggle to get around and to survive. About 99 percent of all fish mortality occurs in the larval stage.

Only when they grow some do fish get better at escaping predators. Adults make quick getaways using a burst-and-coast method, in which they gain a lot of speed quickly and then glide away. Many species don't have this skill at birth.

Scientists have long suspected that the lack of larval weight prevented the little ones from gaining enough momentum in the burst phase. And perhaps their very small size caused the water to act more like syrup against them, preventing a good coast.

But a new study of zebrafish larvae concludes it's their poorly developed side fins and ineffective swim bladder that hold them back. They simply can't keep their bodies horizontal. The finding might apply to other species, researchers say.

"Momentum can explain some of the poor swimming in larvae, but not all, and the difference in coasting ability cannot be explained by differences in body length either," said study leader Ulrike Muller of Wageningen University in the Netherlands. "Many fish larvae hatch without fully formed pectoral fins, and all hatch without a swim bladder, so similar problems could occur for them."

Muller presented her findings Monday at the Society for Experimental Biology's Annual Main Meeting in Canterbury.

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