Fly fishing is traditionally done in streams, creeks, and rivers where water is constantly moving and is very clear. This means the gamefish have the advantage of sight. They can see the lures very clearly and many times react less on instinct and more on what they see. This is why fly fishermen try to "Match the Hatch" or use a fly that imitates the look of insects the gamefish are naturally feeding on. They do this by making educated guesses based the insects and larva they see, time of year and past experiences. Every once in a while they select the lure they think is best and catch only a few fish. This is when a fly fishermen pulls out a secret weapon, the stomach pump.
A stomach pump is an extremely simple tool that relies on the fact that fish do not have a very complex digestive system. If water is pushed into the stomach at a low pressure, usually the contents can be washed back out. The fish is not harmed as long as you use the correct sized tubing and don't force anything. The resulting stomach contents can then be examined to determine what the fish is actually eating. This works on trout but also on bass, bluegill, and crappie.
As a bass fishermen this would be useful in selecting lure sizes, colors, and shape. But it is not a necessity because many times largemouth bass feed by reaction rather then by sight. Where the stomach pump may be more useful is for largemouth bass and forage population management. Lets examine the 8 inch bass below. When I looked at the stomach contents what was seen is seven juvenile sunfish, several insect larva, and one freshwater shrimp. What that tells me is not only what the bass is feeding on but also where it is feeding. Based on the insect larva and the shrimp we can concluded that this bass is feeding near aquatic vegetation or structure and in shallow water. As a biologist I use this tool all the time. Many times I find what I am expecting; bluegill and shad. Occasionally I will see grasshopper and crawfish parts, small bass, or catfish, and every once in a while a mouse. Based on what we find we can decide what food chain improvements are working and what we can be do better. The goal is to have a diverse and plentiful food chain to grow the bass you desire.
Below is a picture of a pump you can purchase at any fly fishing outfitter.
A video of how to use the stomach pump on brown trout.
By: Steven Bardin