No, you cannot just use the maximum length. See info on https://www.fishbase.de/MANUAL/fishbasethe_maturity_table.htm

No, fish maturity depends on species of fish and a matured fish can be identified only during spawning period of that fish species by slightly pressing the abdomen, one can see milt oozing out in case of male fish and eggs in case of female fish

Length at first maturity Lm varies roughly between 50 and 65% of maximum length (Lmax) depending on whether the species is short- or long-lived (r or k strategist). If no maturity studies are available, half Lmax would be a relatively safe approximation, except for large, long lived species, such as salmon, sharks etc., where 2/3 of Lmax would provide a more precautionary approach.

Froese, R. and C. Binohlan. 2000. Empirical relationships to estimate asymptotic length, length at first maturity and length at maximum yield per recruit in fishes, with a simple method to evaluate length frequency data. J. Fish Biol. 56:758-773. provide guidance. As for the mechanism, Pauly is arguing that gill size (surface essential for gas exchange and metabolism) as a limiting factor vs. body growth (volume) is the trigger for maturation.

[https://www.seaaroundus.org/fishbase-and-sealifebase-2021-anniversary-symposium-magnifique/]

Assuming your sampling includes both juvenile and mature fish during the spawning season, collected around the spawning area, and that you are able to recognise macroscopic maturity stages in gonads, you may adjust maturity stage proportions at different toutal lengths to a logistic function to get an estimate of the first maturity length.

back to the roots!

We have meanwhile developed an App available for free in Google Play Store by the name of FishBase Guide, which gives you Lm, Lopt, Lmax and a visual indication of the vulnerability of the species when you type in a local name in any of the languages for which FishBase has recorded common names (325,000 so far) and indicate the country. You can provide feedback and also connect directly from the app to the species summary page in FishBase, if you want to know more (at the bottom of the screen on your phone). Should you use a common or commercial name, which is not yet recorded in FishBase, please signal that together with the scientific name (if available) and country so that we can continuously improve the coverage and service. Also, should you be aware of any publications with such common names for any country and script, which are not yet recorded, please share or at least alert us, so that we put them in. Hope the service is helpful.