Long post here but bare with me and all this can be found in any wildlife management or ecology textbook, and probably google and wikipedia its all 101 stuff:
You can not get an exact population count on wildlife. You have to make estimations based on the data you have available, its just one of those things and is why statistics are used extensively for wildlife populations, because they are only estimates. One of these methods is called the 'mark-recapture method'. This is what was used in this study and where this statement came from and where the number came from:
"Cresson told commissioners the recapture rate of tagged tripletail is
2 1/2 times higher than with other game fish, which, he said, is an indication of how vulnerable the fish are to over-harvest."
So, its obvious that studies were done, there goes that notion that 'there are no studies'
In this type of study, you catch all the individuals and mark them (tag in the ear of a deer, tag in the fish, clip the toe of a rat, color band a bird, etc.) then you go back and you 'trap' again. The numbers of recaptures is then plugged into a formula and you come out with a number which estimates the population. Its not that cut and dry for all species but these are the basics, and something that is migratory like a tripletail will be very hard to estimate the population, BUT what IS very very important here is that which is bolded above (2 2/2 times more susceptible to harvest)
And THAT is where the 'red flags' come up.
and to Spunt Drag and others who keep saying 'who cares about their minimum age at sexual maturity', that is one of the most important factors in setting regulations. You have to protect your stock. Its why there are minimum lenght requirements for game fish - plain and simple, a tripletail is not any different.