Dolphins were trained to turn in trash that fell into the pools in exchange for fish. One dolphin was smart enough to hide pieces of paper under a rock, tearing off smaller pieces from the paper in order to get more fish out of it.

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At the Institute for Marine Mammal Studies in Mississippi, Kelly the dolphin has built up quite a reputation. All the dolphins at the institute are trained to hold onto any litter that falls into their pools until they see a trainer, when they can trade the litter for fish. In this way, the dolphins help to keep their pools clean.

Read more at the theguardian