Research - Category II: Aquatic Ecosystems

The Influence of Introduced Trout on the Diversity and Structure of Native Aquatic Invertebrate Communities in High Sierran Streams
(Funded 2000-2001)

Principal Investigator: David B. Herbst
Sierra Nevada Aquatic Research Laboratory
UC Santa Barbara
(760) 935-4536
herbst@lifesci.ucsb.edu

Executive Summary:
Resource management agencies are often confronted with conflicts between natural resource protection and public recreation in wildland areas. In some cases, these resource use conflicts have impeded the incorporation of ecological science into management policy (Sellars 1997). A prominent example of such resource use conflicts in the stocking of non-native trout for recreational fisheries versus the protection of unaltered habitat for native wildlife. Recently, the Sierra Nevada Ecosystem Project (SNEP) reviewed information on ecosystems in the Sierra Nevada and reported that aquatic systems the most altered and impaired habitats in this region (SNEP 1996). Degraded ecological conditions in streams and rivers have resulted from a century and a half o alternations to stream flows and stream water quality caused by mining, dams, diversions, and the destruction of riparian areas by reservoirs, road building, logging and overgrazing, as well as from the introduction of exotic fishes to lakes and streams. Decline and losses of over half the 70 species of native amphibians and fish found in the Sierra Nevada have been attributed to the introduction of exotic species, especially trout (Jennings 1996, Knapp 1996, Moyle et al. 1996a). Although the effects of introduced trout in native biodiversity have been documented for many Sierran Lakes, there is practically no information on the impacts of introduced trout on Sierra stream communities and ecosystems. The fauna of High Sierran streams is dominated by aquatic invertebrates which have evolved in a region that was completely devoid of fish until fish were first introduced about 100 years ago. Inadequate data on the distribution and diversity of aquatic invertebrates are a major obstacle to evaluating and monitoring the health of native aquatic species and habitats in the High Sierra. The communities of these mountain streams harbor high proportions of endemic species in insect groups such as the stoneflies and caddisflies, and these insects potentially are vulnerable to direct and indirect effects of fish predation (Erman 1996). A critical management challenge for aquatic habitats of the Sierra Nevada is to acquire more complete information whether and how trout affect stream ecosystems and biodiversity. Such information will prove very important in guiding policy on the management of fisheries and native biodiversity.

The goal of the proposed research is to evaluate the effects of exotic predatory trout on mountain stream communities, as well as to establish a baseline inventory of aquatic invertebrates in the High Sierra for comparisons to possible future changes. The objective of the proposed research is to compare the structure and diversity of invertebrate communities in 25 streams containing introduced fish and 25 streams lacking fish in the Yosemite National Park. In this comparative approach, each fishless stream will be paired with a nearby, similar stream containing introduced trout. The research approach will involve the sampling and description of the rare or endemic species; description of the movement behaviors and microdistributions of invertebrates; and characterization of the invertebrates' food resources, physical environment, impacts of trout on stream invertebrate communities, invertebrate communities, invertebrate food resources (e.g., algae, detritus), and physical-chemical conditions in stream habitats, as well as indicate environmental conditions that may modify the effects of fish on stream communities.

Fish stocking and its impacts on high elevation aquatic ecosystems is a controversial topic currently under review in California by a variety of public agencies, including the National Park Service and the U.S. Forest Service. The Forest Service, for example, is examining fish stocking policies in all 11 National Forests in the Sierra Nevada through the Framework for Conservation and Collaboration and through the development of a comprehensive Environmental Impact Statement. The California Department of fish and Game also has a mandate to protect native species while at the same time carrying out its historic mission of managing recreational fisheries, often via the stocking of state waters. The proposed research will provide essential scientific input to policy develpment through (1) the evaluation fo the impact of non-native trout on the biodiversity and ecological integrity of Sierran streams, (2) the development of criteria for identifying aquatic diversity management areas (sensu Moyle et al. 1996b) for conserving native biodiversity, and (3) the establishment of baseline biological indicators for monitoring the progress and success of policies and procedures designed to restore stream biodiversity.