You mainly need to know your client.  There are a very limited number of calls for exclusively photo materials left these days.  There are three main fly fishing magazines: Fly Fisherman, American Angler, and Fly Rod & Reel.  Add to that Grey's Sporting Journal, which does one fly fishing issue a year, and the occasional (very, very occasional) two-page spread in Field & Stream.

The main three are always open to unsolicited cover submissions, but you need to submit some legitimate material or you'll get blocked as a spammer pretty fast.  Look up the Managing Editor's email address in the front of each book for cold-calling; that's the best way these days.  (Send only proofs under 800 px on the long side, and don't send more than a dozen to start with.  It goes without saying you should send only your best work).

Inside, American Angler has a two-page Table of Contents slot that favors darker and more contrasty images, and also the rear cover currently (until they start being able to sell that as ad space again).  Fly Fisherman has a set of proven photographers they go to on a frequent basis.  They have a financial interest in keeping their consistent, professional photo suppliers in business and so they prefer to do business with them where possible, rather than solicit one-off freelance images.

Your best bet with FF and FR&R is to submit photos for cover consideration and included with pieces.  Again, the market is so tight right now, they are trying to keep people they can count on in business so they aren't thrown on the mercies of the random one-off supplier's market.  From the editors' perspective it makes sense to have a one stop shop and people like the Becks, the Grossenbachers, Tosh and Denver have established those roles long ago.  That is a hard nut to crack (and honestly, I'd feel kinda bad about trying to crack it).

The other options are the occasional magazines like the Drake and the Flyfish Journal, which publish less frequently but are going to be more open to unsolicited cold calls.  Then of course there are the happy-to-take-your-free-offerings outlets like Catch Magazine and This is Fly, which mostly function on a pretty hypothetical "exposure" market.  I like Catch's ownership and think it's a great publication; TIF has its place in the market too.  But from a strictly financial perspective that is a non-starter.  Being featured in Catch or TIF is very unlikely to lead to paying work any time soon unless you are really good and willing to put in a lot of work immediately as a full-time pro.

Most photographers now are either established old-guard pros who work full time and do business with accounts they've had for years, or guys like me, who submit photos with their articles and provide the occasional one-off.  Before we all get worked up at the pros "hogging the market," keep in mind that their biggest asset is consistency: they can provide a huge range of good, publishable photos on a wide range of subjects.  Magazine editors need that, they need photos to be available quickly, and they are doing more business with these pros for both personal and financial reasons that make good sense.  You need to be aware of that going in, because your equivalent photos are going to get passed over in favor of established names and you can't take that personally.

Zach

You can query more than one but be prepared to face an awkward conversation if they all want the same photo.  You should never under any circumstances sell the same photo to more than one outlet at once.  I have seen this happen with a saltwater cover (bright blue background and a guy in an orange safety suit casting) and I've heard it got the photographer blacklisted.

Zach

well said! Zach