Dragonflies spend most of their lives under water in a nymph stage. As nymphs, they are aggressive predators. They are carnivorous and primarily eat insects, but are known to eat vertebrates such as small fish and tadpoles.
An interesting fact about dragonfly nymphs is that they breath by sucking water into their abdomen and moving it over internal gills. The water can then be expelled under pressure to propel the creatures forward.
This find reminded me of a trout caught several winters back. After filleting it, I discovered two dragonfly nymphs that looked just like this one in its belly.