Why do trucks carrying fish say "fish" on the trailer?

Ever notice that trucks that have fish in them seem to always say “fish” usually in black letters at the bottom of the trailer doors? Is that the law or something? I see it all the time on the highways, so it isn’t just a case of one or two trucks.

State law in Texas, if that’s where you are.

http://www.seafood-sales.com/faq2.htm

We did this one before. Yes, it’s a state law in many states. I’ll see if I can find the previous theads.

http://boards.straightdope.com/sdmb/showthread.php?t=241372&highlight=fish+ohio