As defined by the World Wildlife Fund, the San Joaquin River watershed is part of the Sacramento-San Joaquin freshwater ecoregion, which supports almost 40 species of freshwater fish. These include several types of lampreys, sturgeons, sunfish, perch and various anadromous fish species such as salmon and steelhead. Some of these fish are believed to have descended from fishes of the Columbia Basin in geologically ancient times, when the upper reaches of the Sacramento River watershed were connected with that of the Snake River. Up to 75 percent of the historic species were endemic to the Sacramento-San Joaquin basin. Most native fish stocks have suffered because of predation by introduced species and dam construction. In a study from 1993 to 1995, it was found that the main stem of the San Joaquin River was mainly populated by fathead minnow, red shiner, threadfin shad and inland silverside, all of which were introduced. Downstream portions of the river's main tributaries were populated mainly by largemouth and smallmouth bass, redear sunfish and white catfish, while native species have survived relatively well in the upper reaches of the river and its tributaries, which also play host to introduced brown trout.