Here’s a useful video from Steve Galletta, of Bighorn Angler Fly Shop & Lodge in Fort Smith, Montana. You don’t see many anglers using a stomach pump, but it can provide great information about what fish are eating, especially if there are lots of bugs in the water at the same time.
Some folks worry that “gastric lavage” will hurt the trout, but there is no evidence that, of you use the right tools and perform the procedure carefully, it causes any lasting damage.
Here’s Steve’s instructions:
The anglers that know how to use a stomach pump use them religiously, and for others that have not seen it done before can find it hard to understand and are often intimidated to try it. When done right it doesn’t hurt the fish and is a very quick process. I will try to explain the steps here to go along with the video. I have found that trout in the 13″ – 17″ range are the easiest to pump and rainbows tend to be easier to pump than browns, since in my experience their stomach cavity seems easier to find.
- Fill the stomach pump full of water by slightly depressing the rubber ball.
- Insert the stomach pump into the trout – the key here is to slowly insert the tube into the trout’s mouth and push it straight in until you feel a small opening. This opening is the stomach cavity. After inserting the tube into the trout’s mouth you will feel pressure against the tube until you reach the stomach cavity. Stop there.
- At this point slightly compress the rubber ball so a portion of the water leaves the tube and is deposited in the trout’s stomach and then quickly decompress the ball again to suck the contents up into the tube. This should only take a second. The key here is to slightly depress the rubber ball, do not squeeze the ball completely or it will be more difficult to suck the stomach contents back up into the tube because you will loose the pressure in the tube.
- Remove the tube from the fish
- Squirt stomach pump contents out into your hand and evaluate what you see
- Match your imitation accordingly
For more information, check out his blog post on the subject.