The body size of some whale species diminished by several meters decades before 20th century factory fishing caused their populations to collapse, researchers revealed.

If that pattern holds true for other commercially harvested marine species, a drop in body size could serve as early warning that protective measures are needed, they reported in the journal Nature Ecology & Evolution.

Once a population has crashed, recovery is difficult at best: 25 years after a moratorium on Canada's decimated northwest Atlantic Cod fishery, the sub-species has yet to bounce back. Hunted since the 18th century, many whale species were saved from extinction by a leaky 1982 moratorium that has still allowed more than 1,000 of the majestic sea mammals to be killed every year. Researchers led by Christopher Clements of the University of Zurich looked at annual records - including size and number caught, compiled by the International Whaling Commission and reaching back to about 1900, when new technologies emerged that turned the hunt into a harvest.

"We show that during this period of commercial whaling, the mean body size of caught whales declined dramatically, by up to four meters (13 feet), over a 70-year period," the study concluded. Industrial whaling wiped out nearly three million of the animals during the last century, according to a tally published recently in the journal Marine Fisheries Review. "Fishing pressure remained high until whale populations collapsed and become commercially untenable, whereupon whalers moved on to new species," the study said. For blue, fin and sei whales, body size started to shrink a couple of decades before the sudden drop off in numbers caught.