FishBase uses that concept and divides the fish in groups under that category . The groupings are fairly wide for example medium doubling time is a period from 1.4 to 4.4 yrs and fast is less than 15 mths. It takes into account, I think, a number of factors including fecundity , age of sexual maturity ,life span, natural morts., etc. The time it takes for a population to double. It gives a more accurate measure of a species ability to grow its population.