 Critical Findings ASSESSMENT * A Past View of Resources in the Lahontan Region * Deer Creek Watershed Conservancy Strategies for Improving Watersheds and Aquatic Biodiversity
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Fish Native fishes are much better known than their invertebrate food supply and are also at risk from changes in water availability and quality, habitat alteration, and introduction of exotic species. Of the forty species of fishes native to the Sierra Nevada, six are formally listed as threatened or endangered and twelve others are candidates for listing. Four other fishes are in decline within the Sierra Nevada but are less threatened elsewhere. Less than half of the native fish species of the Sierra Nevada have secure populations. The long-term causes of the declines are introductions of exotic fishes, dams and diversions, alterations of stream channels, and watershed disturbance (grazing, mining, roads, logging, etc.). These different problems occur throughout the range and usually operate in combination to degrade and dissect aquatic habitat. This habitat fragmentation, in turn, allows piecemeal extirpation of local populations. Chinook salmon are a principal example of the drastic declines in native fishes of the Sierra Nevada. In the ninteenth century, more than a million salmon spawned annually in the streams of the west slope, with some ascending to an elevation of 6,000 feet. However, the curtain of dams across the Sierra Nevada rivers blocked access to about 90% of the original spawning habitat (figure 8.1). 
Figure 8.1 (Actual View 26K) Two major changes in Sierra Nevada fish distribution. The shaded area shows streams and lakes that historically were without fish but that now mostly contain them. The dotted and heavy lines show current and historic distribution, respectively, of chinook salmon. (From volume II, chapter 33.) Consequently, spring-run chinook salmon, present in less than 10% of their original numbers, have been virtually eliminated from the Sierra Nevada except for those spawning in a few undammed tributaries to the Sacramento River (such as Deer Creek and Mill Creek). Fish are one of the most intensively managed components of the ecosystems of the Sierra Nevada. Occasional transfer of fish in buckets in the 1800s has exploded into hatchery production of millions of fish and mechanized stocking at hundreds of sites throughout the range. At least thirty non-native fishes have become established in the Sierra Nevada, and ten of these exotic species are now widespread and abundant. Before the active manipulation of fisheries, most of the Sierra Nevada above 6,000 feet lacked any fish fauna. Hundreds of miles of streams and almost all of the more than 4,000 natural lakes of the Sierra Nevada were dominated by invertebrates and frogs until widespread trout introductions began in the nineteenth century. Trout are now present almost everywhere in the range that is capable of supporting them. In Sequoia, Kings Canyon, and Yosemite National Parks fish stocking was terminated in the lakes in the 1980s. Recent regional surveys show that trout have disappeared naturally from 29%44% of these previously stocked lakes. Many high-elevation lakes outside the national parks are still regularly planted with trout to support recreational fisheries. This artificially maintained fishery provides substantial angler use, and fishing in the high-elevation lakes remains a major objective of those who visit these sites. Nevertheless, the predatory trout have greatly altered lake and stream ecosystems, resulting in local and rangewide changes in species assemblages of aquatic invertebrates. Introduced trout are also a factor contributing to the decline of some native amphibians, in particular the mountain yellow-legged frog, whose former distribution is almost perfectly coincident with the former fishless zone (figure 8.2). Further, the widespread use of fish poison for fisheries management in Sierra streams and lakes for more than forty-five years has had undetermined impacts on nontargeted organisms.  Figure 8.2 (Actual View 9K) Decline of the mountain yellow-legged frog and its association with the historic fishless area in the Sierra Nevada. Current known populations of frog are shown as dots compared to the frog's former range, which closely coincided with the historic fishless area. Most lakes in the historic fishless area now contain populations of non-native fishes, which were introduced for sports fishing and are implicated in the decline of the mountain yellow-legged frog. (From volume II, chapter 32.) Amphibians
 
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