It's pretty obvious that you should fish a tropical fly line in Mexico and a cold water fly line in Alaska. It's also intuitive that when you're choosing a fly line for Bonefish, fly lines with 'Bonefish' in the name are probably a safe bet (SA Amplitude Bonefish, for example). But which line should you fish when conditions are moderate in a freshwater Bass fishing scenario in Texas? Here are some common questions:

  • If the air temp and water temp are 60 degrees, is that a cold water situation or a warm water situation and where's the crossover?
  • What happens if I use a cold water fly line in warm water conditions or vice versa?
  • How do I know if a fly line is built for tropical scenarios or cold water scenarios?

These are all valid questions and fly line temperature ranges aren't always talked about in a definitive way. This stuff can be confusing. We're here to flesh it out with advice from experts at RIO and Scientific Anglers so you don't have to guess next time you're wondering if you can fish an SA MPX fly line in the Louisiana marsh.

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Advice From RIO



In the video above, Simon Gawesworth talks about RIO's new 'tropical' Flats Pro and DirectCore species-centered fly lines. As you can see in the video, RIO labels each fly box very clearly with 'Tropical Series' directly above the name of the fly line. It's easy to tell that these lines are tropical lines... but what does that mean?

To make sure we got this right, we reached out to RIO directly to see how they define the difference between tropical and cold water lines. Here's what Chris Walker, R&D Engineering Manager at RIO Products, had to say:

  • "That’s a good question and, as you suggested, it’s not exactly a cut and dry answer. Typically we say RIO tropical lines perform best when air and water temperatures are above 75F. Under those conditions, they don’t require any straightening at all and come off the reel memory free. The cooler the conditions, the more likely a tropical line is to have some memory. That being said, I’ve fished tropical lines in air/water temperatures as cool as 50 degrees without serious issues. They may require a quick hand-stretching at the start, but they still fish pretty well."
  • "Our cold water lines perform best when water temperatures are 70F or cooler. I mention only water temperatures in this case because we get our share of 90 degree days with water temperatures in the 60s during hopper season in Idaho. Cold water lines stay slick in these conditions as long as they are staying wet. I’ve run into trouble when a cold water line stays on the deck of the boat for a while with air temps over 80F. They start to get “sticky” in these situations because the coating softens a little bit in the heat. They’re still fishable but don’t shoot as well. In fact, I have a friend who fished a RIO Gold for bonefish in the Bahamas for an entire week. I didn’t envy his choice of line, but he caught his share of fish."

Chris spells out the difference pretty clearly here: if air and water temps are over 75 degrees, use a tropical line. If water temps are cooler than 70 degrees AND your fly line isn't on the bow of a boat in hot summer temps, use a cold water line. Of course, there's overlap and multiple lines can be used in multiple scenarios.

Chris also provided us with some valuable info about what happens to a fly line if it's used outside of its designated temperature range. Tropical lines have 'memory' in cold temperatures (meaning they hold the rounded shape of being on the reel). This makes them difficult to cast. Cold water lines get 'sticky' in warm temperatures which also makes them difficult to cast. Anglers battle enough adversity on the water, a fly line that doesn't cast should be avoided when possible. Choosing the right line helps.

Advice from Scientific Anglers

So what happens if you're fishing an SA line? Are SA's temp recommendations and ranges the same as RIO's? The above chart comes from Scientific Anglers and spells it out. Here's what you should know:

  • Tropical lines work best when air temps > 70F and water temps > 60F
  • Cold water lines work best when air temps ≤ 60F and water temps ≤ 50F

That means there's some overlap which SA calls 'medium temp lines'. Fortunately, SA also clearly marks tropical lines with a label that says 'Tropi-Core technology for tropical climates' and cold water lines with 'For use in moderate and cold climates'.

Scientific Anglers Amplitude Smooth Anadro Fly Line, for example, is a line that works best in moderate and cold climates. That means the line performs best in SA's cold water and medium temp brackets on the chart above. Scientific Anglers Amplitude Smooth Infinity Salt Fly Line is built with Tropi-Core technology for tropical climates which means the line performs best in SA's tropical bracket.

Key Takeaways

SA's chart and RIO's temp recommendations are valuable resources when deciding which fly line to use in a specific situation. If you're chasing fish in a tropical environment, choose a tropical line. Similarly, cold water environments (freshwater trout fishing) necessitate a cold water line.

That being said, there is some crossover. Use your best temperature judgments and choose a line based on those estimates. If the weather turns colder or warmer than you originally anticipated you'll probably be alright. Just don't bring a tropical line to Iceland in April.


If you're still confused or want more information about how to choose a line for a specific fishery, don't hesitate to give us a call at (888) 413-5211 or email us anytime at [email protected].