FINAL T ECHNICAL MEMORANDUM ◦ SEPTEMBER 2020 Conceptual Ecological Model and Limiting Factors Analysis for Steelhead in the Los Angeles River Watershed PREPARED FOR PREPARED BY Council for Watershed Health Stillwater Sciences 177 E. Colorado Blvd., Suite 200 555 5th Street, 35th Floor Pasadena, CA 91105 Los Angeles, CA 90013 Stillwater Sciences Technical Memorandum Conceptual Ecological Model and Limiting Factors Analysis for Steelhead in the Los Angeles River Watershed Council for Watershed Health (CWH) contact: Andrea Dell’Apa Project Manager (213) 229-9945 [email protected] Stillwater Sciences contacts: AJ Keith Matt Drenner, PhD Wendy Katagi, CEP Aquatic Ecologist & Project Director Fisheries Biologist Project Manager (510) 821-1415 (213) 336-0001 x722 (213) 336-0001 x183 [email protected] [email protected] [email protected] Acknowledgements: Report preparation assistance was provided by Andrea Dell’Apa and Yareli Sanchez (Council for Watershed Health). Comments on the draft by Tim Brick (Arroyo Seco Foundation), Andrea Dell’Apa (Council for Watershed Health), Johnathan Perisho (Watershed Conservation Authority), Christian Romberger (California Department of Fish and Wildlife), and Kerwin Russell (Southwest Resource Management Association) greatly improved this final technical memo, as did technical input from Rosi Dagit (Resource Conservation District of the Santa Monica Mountains), Sabrina Drill (University of California Agriculture and Natural Resources), Anne Elston (Pacific States Marine Fisheries Commission), Travis Longcore (UCLA Institute of the Environment and Sustainability), and Christine Medak (U.S. Fish and Wildlife Service). This effort was supported by a Proposition 68 grant from the Wildlife Conservation Board (Grant Agreement WC-1922DC, Project ID 2019149). Suggested citation: Stillwater Sciences. 2020. Conceptual ecological model and limiting factors analysis for steelhead in the Los Angeles River watershed. Final Technical Memorandum. Prepared by Stillwater Sciences, Los Angeles, California for the Council for Watershed Health, Pasadena, California. Cover photos: Clockwise from upper left: perennial stream habitat in the upper Arroyo Seco; juvenile steelhead; th mainstem Los Angeles River at the 7 Street Bridge looking upstream; adult steelhead. September 2020 Stillwater Sciences i Technical Memorandum Conceptual Ecological Model and Limiting Factors Analysis for Steelhead in the Los Angeles River Watershed EXECUTIVE SUMMARY The LA River watershed historically supported a population of steelhead (Oncorhynchus mykiss) belonging to the Southern California Steelhead Distinct Population Segment (DPS), which is listed as endangered under the federal Endangered Species Act. Steelhead, the anadromous1 life history form of rainbow trout, are no longer present due to physical, chemical, and biological changes associated with urbanization of the LA River watershed. Restoring a steelhead population to the LA River watershed is a conservation goal that is consistent with the City of Los Angeles Mayor’s Office biodiversity goals, the National Marine Fisheries Service’s (NMFS) recovery goals for the DPS, County of Los Angeles biodiversity goals, U.S. Army Corps of Engineers (USACE) LA River and Arroyo Seco Watershed goals, and with the goals of numerous other agency and conservation organizations. A major challenge for recovery of steelhead is providing access to suitable spawning and rearing habitat that exists in the upper tributaries. The LA River Fish Passage and Habitat Structures (LAR FPHS) design project is intended as a first step in surmounting these challenges. The LAR FPHS design project includes preparation of designs to modify a 4.8-mile section of the existing concrete-lined LA River flood control channel to improve fish passage for steelhead migration to soft-bottom reaches of the LA River and upper tributaries and provide important habitat features to benefit steelhead and other native fish. The LAR FPHS design project and its proposed concepts are fully consistent with design recommendations included in Alternative #20 of the USACE Integrated Feasibility Report for the LA River Ecosystem Restoration Project. In particular, the specific location of the LAR FPHS Project and its objective to redesign the channel bed are compatible with the proposed alternative to modify the existing mainstem channel of the LA River within a section that flows from Taylor Yard to the Union Pacific LA Trailer and Container Intermodal Facility (LATC, also known as Piggyback Yard). The LAR FPHS also aligns with the USACE Arroyo Seco Watershed Ecosystem Restoration Study, which is entering the Feasibility Study Design phase, focusing on opportunities for fish passage, stream naturalization, and overall habitat restoration along the Arroyo Seco from its confluence with LA River to the central Arroyo Seco reach just below Devil’s Gate Dam. This document is intended to provide the ecological basis for the steelhead passage and habitat improvements that are central to the LAR FPHS design project. It provides an overview of the current and historical riverine habitat conditions in the LA River watershed, the processes that create(d) and maintain(ed) these conditions, and the influence of these conditions and processes on the historical and potential future population of steelhead in the watershed. The document also describes our understanding of the life cycle and ecological interactions of the steelhead population that formerly occupied the LA River watershed, based on data from other steelhead populations in California and the Pacific Northwest and current and historical information for the LA River watershed. This information is presented in the form of a conceptual ecological model that describes steelhead freshwater life history and the primary ecological influences on these life history events, and identifies the habitat constraints most likely to affect the success of each life stage and limit productivity of the population. The LA River occurs in a Mediterranean climate characterized by a wet and a dry season. Steelhead migrations (juvenile and adult) in the LA River would occur mainly during the wet season from approximately February through May when rain events and associated high flows 1 Anadromous fish migrate to the ocean as juveniles and return to freshwater as adults to reproduce. September 2020 Stillwater Sciences ii Technical Memorandum Conceptual Ecological Model and Limiting Factors Analysis for Steelhead in the Los Angeles River Watershed provide connectivity between the ocean and upstream habitat. Juvenile rearing would occur throughout the year in the watershed but would be limited to locations with suitable habitat and perennial flows (e.g., in the upper tributaries including the Arroyo Seco and Big Tujunga Creek). Under existing conditions, upstream migration of adults (and to a lesser extent downstream migration of juveniles) during the wet season is prevented due to the presence of numerous migration barriers. Major physical barriers (e.g., dams) prevent access to the tributaries of the LA River that contain the best habitat for spawning and rearing. Velocity barriers arise from concrete channelization of the LA River mainstem. These channelized sections create hydraulic conditions in which water velocity is too high for adults to migrate when water depth is suitable for migration, and vice versa, when water velocity is low enough for adult upstream migration, water depths are too shallow. Urbanization and associated modification of the natural drainage system has also reduced the duration of flow events large enough to support migration. Thus, the presence of migration barriers and altered flow patterns in the LA River are perhaps the most critical limiting factors for steelhead recovery. Water quality is another potential limiting factor identified for steelhead in the LA River. Stressful and even lethally high water temperatures in the LA River mainstem largely preclude steelhead rearing opportunities and could disrupt migration and hinder the physiological transition to salt water. Urbanization and wastewater treatment inputs contribute to high water temperatures within the mainstem. Suitable temperatures for spawning, early life stage development, and rearing occur within tributaries, which suggests that steelhead could successfully spawn and rear in these locations if access were provided. Other potential limiting factors include the amount of rearing habitat, the presence of non-native species, and artificial lighting. Rearing habitat is mostly absent or unsuitable in the mainstem LA River, lower tributaries, and the LA River estuary but suitable rearing habitat is present above barriers in the major tributaries. Non-native species in the mainstem and lower tributaries could act as predators or competitors with young steelhead, but the presence of non-natives generally decreases further upstream in the watershed where spawning and rearing would occur. Artificial lighting, especially in more urbanized areas such as the mainstem LA River, could disrupt life history transitions and behavioral patterns and result in increased predation of juveniles during their downstream migration. Under existing conditions, the mainstem LA River is not suitable for steelhead rearing but conditions improve in an upstream direction, with the best conditions in upper tributaries. Suitability of conditions in upper tributaries is further supported by recent surveys that found resident