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The influence of introduced trout on native aquatic invertebrate communities in a paired watershed study of High Sierran streams

  • Author(s): Herbst, David B
  • Silldorff, Erik L
  • Cooper, Scott D
  • et al.
Abstract

Resource management agencies are often confronted with conflicts between natural resource protection and public recreation in wildland areas. An example of such a conflict is the stocking of non-native trout for recreational fisheries versus the protection of unaltered freshwater habitat for native wildlife. Declines and losses of over half the species of native amphibians and fish found in the Sierra Nevada have been attributed to the introduction of exotic species, especially trout. Although the effects of non-native trout on native biodiversity have been documented for many Sierran lakes, there is practically no information on their impact on Sierran stream ecosystems and communities, which evolved in the absence of fish. These mountain streams harbor high proportions of endemic species in insect groups such as stoneflies and caddisflies, and these insects potentially are vulnerable to direct and indirect effects of fish predation. Inadequate data on the distribution and diversity of stream invertebrates is a major obstacle to evaluating and monitoring the health of native aquatic species and habitats in the High Sierra.

The objective of this research has been to compare the composition of invertebrate communities in streams lacking introduced trout with paired nearby streams containing trout. The studies involved surveys of aquatic invertebrate diversity in riffle and pool habitats of first and second order streams in Yosemite National Park, at elevations from 1350 to over 3000 meters. Invertebrate samples were collected along with an inventory of their food resources, water chemistry and physical habitat. Analyses of the data emphasizes contrasts of the impacts of trout on the organization and diversity of 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 and private organizations. The research results presented here provide essential scientific input to policy development through (1) the evaluation of 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 for conserving native species, and (3) the establishment of baseline biological indicators for the monitoring of programs designed to restore stream biodiversity.

The introduction of non-native trout into the Sierra Nevada mountains of California, USA, has caused important changes in the densities of many invertebrate and vertebrate species inhabiting lakes of this region. Researchers, however, know little about the effects of trout introductions on invertebrates in streams of the Sierra Nevada. In this report, we present the results from surveys of 21 fishless streams and 21 paired streams containing introduced trout in Yosemite National Park.

Results show that environmental conditions did not differ between the fishless and trout streams, but that algal biomass and macroalgal cover were significantly higher in the streams containing trout. The majority of taxa were unaffected by trout presence in Yosemite streams but enough were affected that we conclude trout have caused changes in benthic stream community structure and function in many but not all streams to which trout have been introduced. Moderate effects of trout on the structure of the invertebrate community were evident in significant and/or consistent changes in the density of 26 different taxa (20 decreasing and 6 increasing in the presence of fish). Among common taxa, trends were statistically significant for 10 taxa and qualitatively consistent for 8 taxa (statistical power insufficient). This represents 18 of 43 common taxa groups (over 40%). Another 8 uncommon taxa also showed distinct patterns of association with fishless or trout streams but were not abundant enough for statistical tests. The strongest effects of trout appeared to be on endemic taxa (such as the caddisfly Neothremma spp., the mayfly Edmundsius agilis, Triclad flatworms, and others), which may be vulnerable because of the absence of evolved selection for mechanisms of coexistence with fish predators.

The effects of trout on the function of benthic communities were also evident in increased (1) algal density and cover, (2) abundance of midges (which play dominant roles in the consumption of organic matter and algae and as prey to invertebrate predators), and (3) reduced density of the most common large predator (Doroneuria baumanni, likely the dominant aquatic predator prior to trout introductions). These changes suggest that trout alter the resource production and transformation in high Sierra streams.

Because the strongest effects from trout fell primarily on endemic species, we recommend that managers first focus on endemic invertebrates by conducting taxonomic verifications, detailed surveys of species-specific distributions and, when necessary, eradicating trout to increase and recover populations of threatened invertebrates. In addition, experimental removals of trout from stream segments should be combined with benthic invertebrate community monitoring before and after treatments to evaluate the potential for recovery.

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