Kate Meyer/U.S. Forest Service

Rainbow trout is listed as both a native and non-native species because it is native to the lower Merced River and non-native at higher elevations.

When Euro-Americans arrived in the mid-1800s, fish could be found in the lower elevations of what is present day Yosemite. The recession of glaciers, more than 10,000 years earlier, created thousands of lakes—but most were separated from downstream fish populations by impassible barriers such as the magnificent waterfalls for which Yosemite is famous.

Native fish—including California roach, Sacramento pikeminnow, hardhead, and riffle sculpin—inhabited only the lower reaches of the Merced and Tuolumne rivers, with native rainbow trout and Sacramento sucker occurring as high as Yosemite Valley. Today, six non-native trout species and one non-native trout hybrid exist in Yosemite National Park. At least 265 lakes and ponds and approximately 550 miles of streams and rivers in the park contain fish.

U.S. Fish and Wildlife Service

Smallmouth bass is one of the six non-native fish species in Yosemite.

While native fish species existed in Yosemite National Park, the majority of fish found in the park today are non-native game species introduced primarily for recreational fishing. Fish stocking began in 1877. In those days, people transported fry into the region in jugs and coffee cans and by mule. That eventually gave way to aerial stocking beginning in the 1950s and continuing through 1990.

Conflict between the National Park Service’s mission to preserve the natural resources unimpaired for future generations and the continued maintenance of an unnatural fishery led to the adoption of the policy in 1969 to phase out and eventually end fish stocking in national parks. Limited fish stocking continued in the park until 1990. In early 1991, an agreement was reached between the NPS and the state Department of Fish and Game to stop fish stocking—ending over 100 years of fish stocking. From the first-known planting in 1877 until 1990, more than 33 million fish were stocked in Yosemite lakes and streams.

Although park scientists recognize sport fishing as a legitimate recreational activity in the park, the management of fisheries has changed dramatically in the past few decades. Just as park staff no longer feeds black bears for entertainment purposes, staff no longer plants fish in recognition that these activities have a detrimental effect on native species.  Non-native fish have an adverse effect on the park’s native aquatic ecosystems that evolved in the absence of fish. Those fishless habitats had been colonized by a diversity of aquatic species over thousands of years. Non-native fish may eat native wildlife or compete with them for resources. Aquatic invertebrates, amphibians, reptiles, and birds are among the wildlife affected by non-native fish. Predation by non-native fish is one of the primary reasons for the precipitous decline of Sierra Nevada yellow-legged frogs (Rana sierrae). Yosemite managers, therefore, face a balancing act: allowing recreational fishing to occur on a self-sustaining basis while restoring native species and their habitats that have been adversely affected by non-native fish.

Some populations of fish introduced into Yosemite’s lakes have not persisted because of severe climatic conditions, low nutrient availability associated with snowmelt over granitic watersheds, and a lack of spawning habitat. However, many lakes and streams do offer habitat that has allowed populations of fish to persist without stocking and Yosemite continues to offer high-quality recreational fishing opportunities throughout the park.