The five species they studied live up to 60 years, grow up to more than a metre (3.25 feet) in length but only become sexually mature in their teenage years.

A quick, devasting decline in numbers of adult fish over three generations can drive a species to the brink of extinction because so few are left that can reproduce, according to the IUCN's criteria.

Deep-sea fish species were previously ignored by trawlermen in their quest for big, easier-to-catch "table" fish such as cod, tuna and halibut in coastal shelf fisheries such as the Grand Banks.

As these fisheries began to collapse in the 1960s and '70s, the trawlers turned their attention to species that live on the continental slope, the downward ridge between the coastal shelf and extreme ocean.

In this case, the main targets are the Greenland halibut and redfish. The other species get scooped up as "bycatch" — they are turned into fish paste, fish fingers or, ironically, ground up into pellets for fish farms.

Even if fishing quotas are rigorously restored, it takes years to restore a fishery if adult stocks fall below a given threshold.

A 2000 study of 90 seriously overfished areas in the North Sea and Atlantic found that only seven had fully recovered after 15 years of fishing restrictions.

The IUCN's Red Book on biodiversity lists "critically endangered" as its highest level of peril for species in the wild. Only two categories (extinct in the wild and extinct everywhere) are higher.

"Critically endangered" means a species faces an extremely high risk of extinction in the wild. The giant panda and Bengal tiger are listed as "endangered," meaning that they face a very high risk of extinction in the wild.

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