Sizes also vary around the world, as they are legal definitions which are defined by the local regulatory authority.
[1] The idea behind this limitation is that only the older, mature fish get taken, leaving juveniles behind to continue breeding and propagating their species.
[2] Aboriginal Australians around Moreton Bay had a practice based on traditional knowledge which avoided this pitfall.
If the older fish were taken at this point, the mullet would swim away from the shore, to the open sea, and this food source would be lost forever.
This explanation does not align with scientific knowledge of fish migration, however, the practice did sustain the fishery for several thousand years.