Moreover, those changes among fish populations -- a desperate bid to adapt -- may be difficult or impossible to reverse.

Boom-and-bust cycles in over-fished species can wreak economic havoc on fishing communities, and can trigger a downward spiral toward extinction.

The study, experts say, could provide important clues on how to restore fish populations that have, in many cases, been reduced by 90 percent due to decades of industrial-scale fishing.

Scientists have long puzzled over the fact that populations of heavily harvested fish, from sardines to tuna, fluctuate in size more erratically than species that are not plucked from the sea for food.

To find out why, a team of researchers led by George Sugihara at the University of California in San Diego poured over a rare set of data tracking both fished and unfished species off the coast of California over a period of five decades.

In considering three possible explanations, they found no evidence for the first: that fluctuations in population simply mirrored the intensity of commercial fishing.

They did find that the young fish left behind as too small to bring to market were somewhat more vulnerable to the vagaries of the sea, whether changing sea surface temperatures, currents or winds.

The critical factor, however, was not the impact of environmental conditions on these age-imbalanced populations, but an intrinsic lack of stability caused by such "juvenescence," as scientists call it.

The disappearance of older, bigger fish from the population induced early maturation in the survivors in two ways, the study found.

In some cases the smaller fish actually adapted physically to new conditions, changes that could be reversed.

But the researchers also found evidence for a genetic impact, adding weight to a recent body of evidence suggesting that environmentally-driven evolutionary changes can occur far more quickly than once believed.

"The implication is that fisheries management need to give priority to precautionary measures," said Nils Stenseth and Tristan Rouyer, both from the University of Oslo, in a commentary, also published in Nature.

"When the ecological effects of fishing a particular population are observed, the evolutionary consequences may have already set it, and may be irreversible."

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