What's the Toughest Fish to Catch on a Fly?

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Over the weekend I did something I've been trying to do for a long long time. I caught a grass carp on a dry fly. Is it the the toughest gamefish on the planet? Probably not. Is it close? On a dry fly--in my mind it's pretty darn close. I caught not just just one, but two "grassies" and I lost a third massive fish most likely because I was on 4x this past Saturday. I actually gave up trying a few years ago, but at a family picnic over the weekend the chance presented itself and somehow it randomly happened.

I know, I know… All you carp neigh-sayers will say so-what, and all you carp freaks, if you have grass carp that readily eat dry flies I really don't want to hear about it. Plus I don't believe you.

In my circle of friends and anglers this was a big deal. You see here in Colorado we don't have permit, muskie (a few), milkfish, gt's, sheepshead, roosterfish or taimen to readily chase.

There are just few fish around that are that insanely hard to get to, have a really limited season on the fly, or are just picky, picky fish. The grass carp in my mind qualifies as one of the most difficult fish to stick on a fly. Most likely due to the fact that they are herbivores, are spooky as hell and talk to each other. The fact that these fish on Saturday actually ate dry flies and aggressively moved on them still has me completely perplexed. I'm chalking it up to pure luck and a little bit of determination.

It got me thinking though, there's got to be some regional fish that I'm not thinking about that I've never had the chance to try with flies. So I'm curious: What are your toughest, most difficult fish to catch on a fly? Lay 'em on me.