Due to the intermittent nature of Pinnacles' streams, the three-spined stickleback is the only native fish here. A predatory feeder, it eats predominately aquatic insects, and reaches three inches in length when full-grown. The stickleback is often observed along the Bear Gulch and South Wilderness Trails. Other fish species may swim upstream into the park from the Salinas River during floods, but they generally do not survive through the summer.
In the early 1980's, non-native catfish inhabited the reservoir. This population was eradicated in the mid-1980's by draining the reservoir and electroshocking the remaining fish. In the mid-1990's non-native green sunfish infiltrated park streams. They were considered a major threat to red-legged frogs, and were removed by electroshocking. Currently the mosquitofish is the only non-native fish species here. Although its presence has a minor impact on red-legged frogs, eradicating it is currently impractical.