I was curious so I googled. Here is a completely unscientific, unofficial random answer
http://answer.google.com/answers/threadview?id=539353
I'm posting this as a comment because I've spent an hour searching and
still can't find the exact regulation, so can't really post this as an
answer. However, I called my sister, who's been an over-the-road truck
driver for over 19 years, more than half of that time spent hauling
"reefers," or perishable goods in refrigerated boxes. She said that
trucks hauling fish are required to be especially identified because
of the danger of cross contamination if the containers are used for
other products, as well as odor control and other safety/hygienic
issues. She said the FDA regulates this, as they do other food
labeling/identifcation issues, and truckers hauling fish are subject
to inspection by the FDA as well as the normal DOT authorities, and
having the boxes labeled "FISH" facilitates this.