What's with all the mackerel?

I am a bit obsessed with Tube station facts.  One of my favourites is that more of the London Underground is above ground than under ground. 

The other day someone told me that Ohio is the only American state which doesn't contain any of the letters in the word mackerel.  It made me sit up, because I'm sure I've head a similar fact about Tube stations; namely that there is only one tube station which doesn't have any letters in common with the word mackerel.  *

I find this interesting, mildly, but the question which I find much more interesting and which has been burning a massive hole in my brain ever since  is WHAT'S WITH ALL THE MACKEREL?

Google isn't helping.  It's telling me a lot of other things about mackerel, like that the French word for mackerel (maquereau) is slang for a pimp, and all sorts of recipes for cooking them (mackerel stuffed ravioli, anyone?), but not why or how it became a linguistic yardstick of sorts.  It's driving me nuts.

*Or at least there used to be.  It was St John's Wood; but now there's Hoxton on the new East London line and that doesn't have any of them either.  Or any letters from the word badger, for what it's worth.