What are they eating? That’s a question I ask myself every time I go fly fishing. Happily, as long as I can manage to land at least one trout of a reasonable size, I can use one of my most valuable fishing tools to take a sample and learn the answer – my throat pump.

I find it particularly useful during chironimid hatches since matching the colour and size of the naturals can be important, and hatches change throughout the day.

If I’d been doing well on a pattern but my catch rate has slowed, and I know that there are still fish around and chironomids are hatching, I’ll sample the next fish I land to see what’s currently on the menu and change tactics accordingly. 

Manufacturers of throat pumps, in some cases, still call them “stomach pumps”, but the goal is to sample what the trout ate most recently - the trout’s throat contents. So, a better name for them is “throat pump”. You risk harming the trout if you sample the contents of its stomach.

Collecting a throat sample is not difficult, and with practice a trout can be netted, throat sampled, and released unharmed in less than a minute without it ever leaving the water.

5) Gently insert the tube of the throat pump into the trout’s mouth until you feel it just slip into its throat. Stop there! Do not squirt water into the trout.

6) The tube sliding in easily is a good sign that the fish has been actively feeding. If the throat is tight the trout has probably not been actively feeding and you may want to consider stopping there and releasing it without taking a sample; otherwise you might harm it by having to apply undue force to get the tube into its throat – especially if it’s on the small side.

7) Release pressure on the bulb and slowly withdraw the pump tube. If there’s anything in the trout you’ll see it enter the tube.

8) Set the pump aside with the tube opening facing up so that you don’t lose any contents.

9) Release the trout, or if you plan to harvest it, bring it into the boat and quickly kill it.

10) Squirt the contents of the pump into your vial.

12) Throat samples from actively feeding trout should contain live prey (insects, crustaceans, zooplankton, molluscs, etc.).

13) If you have a camera or smart phone with a macro mode, record some photos of the live insects that you collected. They make a great reference for fly tying.

14) If the contents are cloudy or the insects, etc., that you find are dead, you may have pushed the tube too far down the throat into the stomach, and you’ve withdrawn stomach contents instead of throat contents.

15) If the tube is empty either the fish has not fed recently, or the prey is too big to fit into the pump tube. For example, dragonfly nymphs, snails, and cased caddis larvae can often be too big to go in. In that case you need to rely on previous experience to make an educated guess about what the trout might be eating.

You can further reduce potential harm to trout by modifying your pump's tube by using a small flame (e.g. lighter) to slightly melt and round off the sharp edge at the open end of the pump so that the tube will slide into the trout’s throat with less trauma.

If you don’t have a throat pump, and if you harvest a fish, you can open the esophagus and stomach when you clean it and see what’s in there. In fact, I always open those when cleaning fish. Sometimes, if I see an angler cleaning a fish on shore before I head out - especially on a lake I seldom fish - I’ll ask them if I can have the offal so I can have a look and see what that fish ate; it gives me a place to start.

Get yourself a throat pump and learn how to use it. It’s a great way to learn about trout prey and your catch rate will improve!