Newt diets

Rough-skinned newts (Taricha granulosa) are generalist predators, however, they tend not to eat a lot of fish. Adults feed mostly on soft-bodied, slow-moving prey. Adults will approach a prey item slowly and deliberately, and then quickly open their mouths. This is called suction-feeding and it is used to capture most prey, though large prey may be grasped with the jaws.

The adult diet consists of a wide range of aquatic and terrestrial invertebrates as well as amphibian eggs and larvae. Invertebrate prey includes crustaceans, insects, arachnids, mollusks, and leeches. Vertebrate prey consists of amphibian eggs and larvae of long-toed salamanders, northwestern salamanders, northern red-legged frogs, Pacific chorus frogs, and western toads. Small fishes may be eaten on the rare occasions when they can be caught.

Newts and fish

There is no way to reduce the newt population that would not be harmful to the fish, and it is not really necessary. If this is a regular breeding pond, then you will always have the newts. But since fish are rarely eaten, I would not worry. They already have a food preference other than trout and will continue to eat those prey.