By some estimates, as many as 500,000 dolphins were killed by these nets every year. After a worldwide outcry by environmental groups, the use of drift nets was banned by several countries, and the U.S. banned the sale of any tuna caught using dolphin-slaying methods (including drift nets, purse nets, and the popular shenanigans net, which involved harpooning a bunch of dolphins and hastily stuffing them inside tuna costumes). Thus the "dolphin safe" label was born.
But the tuna industry wasn't about to just stop catching and selling their product, so they needed an equally effective alternative, which they found in the FAD -- the fish aggregation device. FADs are artificial structures that use vibrations and such to attract schools of fish, allowing fishermen to catch massive hauls of tuna. Dolphins aren't attracted to them, so everybody wins!
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Trust us -- you'll have a much better time if dolphins aren't attracted to you.
Well, not quite. While dolphins aren't sucked into FADs, absolutely everything else is. For every 1,000 tons of tuna, more than 100,000 random animals are unintentionally hauled up and killed, including endangered species of sharks, manta rays, and sea turtles. That's roughly 100 times the number of dolphins that were being caught in drift nets. So, in actuality, "dolphin safe" is just a euphemism for "wholesale massacre in a can."