Was this newspaper article just playing with words? Or is "mong" a real word, and if so, is it in current use?

Unlike Britons, to whom carp are inedible bottom feeders fit only for sport, Poles and other Eastern Europeans eat them as a matter of culinary tradition. And many of the half million Poles who have streamed into Britain in recent years love to serve them on Christmas Eve -- starting with a nice fish head soup.

The Poles would much prefer to buy their fish at market. Trouble is, British fishmongers no longer mong it.

I'd say it's a jocular back-fomration from monger, because the -er looks like a noun agentive suffix. If you look it up, it has an interesting etymology, from Old English mangere from Latin mango, mangōnis, 'dealer in slaves', possibly from Greek μαγγανον (manganon) 'any means for charming or bewitching; philtre'.