Native Freshwater Mussels Northern California Climate Change Vulnerability Assessment Synthesis An Important Note About this Document: This document represents an initial evaluation of vulnerability for native freshwater mussels in northern California based on expert input and existing information. Specifically, the information presented below comprises vulnerability factors selected and scored by regional experts, relevant references from the scientific literature, and peer-review comments and revisions (see end of document for a glossary of terms and brief overview of study methods). The aim of this document is to expand understanding of habitat vulnerability to changing climate conditions, and to provide a foundation for developing appropriate adaptation responses. Peer reviewers for this document included Anonymous (Washington Department of Fish and Wildlife), Emilie Blevins (Xerces Society for Invertebrate Conservation), and Alexa Maine (Confederated Tribes of the Umatilla Indian Reservation). Vulnerability scores were provided by Keith Bensen (National Park Service). Upper Lake workshop participants provided additional comments on the climate change vulnerability of this species group. Table of Contents Species Group Description ..................................................................................................................1 Executive Summary ............................................................................................................................3 Sensitivity and Exposure ....................................................................................................................5 Sensitivity and future exposure to climate and climate-driven factors..................................................... 5 Sensitivity and future exposure to changes in natural disturbance regimes .......................................... 10 Dependency on habitat and/or other species ......................................................................................... 11 Sensitivity and current exposure to non-climate stressors ...................................................................... 12 Adaptive Capacity ............................................................................................................................ 14 Species group extent, integrity, connectivity, and dispersal ability ........................................................ 14 Intraspecific/life history diversity ............................................................................................................ 15 Resistance and recovery .......................................................................................................................... 16 Management potential ........................................................................................................................... 16 Public and societal value.............................................................................................................................. 16 Management capacity and ability to alleviate impacts ............................................................................... 18 Ecosystem services ...................................................................................................................................... 19 Recommended Citation .................................................................................................................... 19 Literature Cited ................................................................................................................................ 20 Vulnerability Assessment Methods and Application .......................................................................... 28 Species Group Description Freshwater mussels are bivalve mollusks that occupy a variety of freshwater habitats in northern California, including streams, rivers, and lakes (Howard & Cuffey 2003; Furnish 2007; Davis et al. 2013). Freshwater mussels are primarily sedentary filter/siphon feeders, consuming phytoplankton, bacteria, and algae suspended in the water column (Nichols & Garling 2000; 1 Climate change vulnerability assessment for the Northern California Climate Adaptation Project Copyright EcoAdapt 2019 Christian et al. 2004; Nichols et al. 2005; Vaughn et al. 2008). As they feed, freshwater mussels improve water quality through filtration and increase benthic species richness by cycling nutrients into the substrate (Howard & Cuffey 2006; Vaughn et al. 2008). In order to complete metamorphosis and disperse to new habitat, mussel larvae must attach themselves to a host fish, with host fish species varying by mussel species and habitat type (Brim Box et al. 2006; Haag 2012; O’Brien et al. 2013; Maine et al. 2016). This assessment considers the four native species that are currently distributed within northern California: the western pearlshell mussel (Margaritifera falcata), western ridged mussel (Gonidea angulata), winged floater (Anodonta nuttalliana), and California floater (Anodonta californiensis).1 These four species represent three of the four mussel genera found in western North America (Davis et al. 2013). Within their northern California distribution, the western pearlshell mussel is the most widespread of the three species (Blevins et al. 2017a; Xerces/CTUIR 2019). Winged/California floaters are less widely distributed, and western ridged mussel are a close third (Blevins et al. 2017a; Xerces/CTUIR 2019). Native Freshwater Mussel Species Key Life History Characteristics Summary Western pearlshell • Lives 40–60 years, occasionally reaching up to 100 years where mussel habitat/water quality parameters are not driving average lifespan down (Margaritifera • Inhabits perennial rivers and streams with cold, clear, highly oxygenated, and falcata) slightly acidic water from sea level up to nearly 2,440 m (8,000 ft) in elevation • Often found in boulder-strewn streams with substrates of cobble, gravel, sand, and silt • Dependent on salmonids as host fish, including cutthroat trout (Oncorhynchus clarkii), rainbow/steelhead trout (O. mykiss), Chinook salmon (O. tshawytscha), coho (O. kisutch), and brown trout (Salmo trutta; non- native) Western ridged • Lives 40–60 years mussel (Gonidea • Inhabits lowland and valley lakes, rivers, and creeks at elevations up to nearly angulata) 2,130 m (7,000 ft) • Tolerant of a relatively wide range of substrates • Dependent on inland fish species as hosts, including pit sculpin (Cottus pitensis), hardhead (Mylopharodon conocephalus), tule perch (Hysterocarpus traski) 1 The most current naming publication for mollusks, which is considered the taxanomic authority, lists Anodonta nuttalliana and Anodonta californiensis as separate species (Williams et al. 2017). Although genetic evidence suggests that these likely belong to a single clade or species (Mock et al. 2010), this assessment will follow the current naming conventions outlined in Williams et al. (2017). When referring to scientific publications that synonymize the two species, we will refer to them as California/winged floaters. 2 Climate change vulnerability assessment for the Northern California Climate Adaptation Project Copyright EcoAdapt 2019 Native Freshwater Mussel Species Key Life History Characteristics Summary Winged/California • Lives 15–30 years floater (Anodonta • Inhabits lakes, reservoirs, and slow-moving streams as well as low-gradient nuttalliana) rivers, typically at lower elevations • Typically found in mud, sand, or gravel substrates • Dependent on inland fish species as hosts, including pit sculpin, torrent sculpin (C. rhotheus), Sacramento pikeminnow (Ptychocheilus grandis), tule perch, three-spined stickleback (Gasterosteus aculeatus), redside shiner (Richardsonius balteatus), speckled dace (Rhinichthys osculus), hardhead, green sunfish (Lepomis cyanellus; non-native) Source(s): Murphy 1942; Karna & Millemann 1978; Brim Box et al. 2006; Haley et al. 2007; Haag 2012; O’Brien et al. 2013; Maine et al. 2016; Blevins et al. 2017b; Xerces/CTUIR 2019 Executive Summary The relative vulnerability of native Native Mussels Rank Confidence mussels in northern California was Sensitivity Moderate-High High evaluated as moderate-high by regional experts due to moderate- Future Exposure Moderate-High Moderate high sensitivity to climate and non- Adaptive Capacity Low-Moderate Moderate climate stressors, moderate-high exposure to projected future climate Vulnerability Moderate-High Moderate changes, and low-moderate adaptive capacity. Climate and climate-driven factors: • Streamflow, water temperature, precipitation amount, drought Disturbance regimes: Summary • Flooding Sensitivity & Non-climate stressors: Exposure • Dams and water diversions, pollution, other stream sediment sources Because they require abundant cool, clean water, native freshwater mussels are sensitive to climatic changes that reduce water availability (e.g., altered streamflow, reduced precipitation, increased drought) and decrease water quality (e.g., increased water temperature and sedimentation). Limited water availability can degrade or eliminate freshwater mussel habitat and impact host fish abundance and reproductive success, while warmer water temperatures can lessen fitness and survival of mussels by increasing thermal stress and reducing dissolved oxygen, particularly for mussels in smaller, high-gradient streams. Freshwater mussels reliant on salmonid host fish to reproduce are also affected by climate-driven changes in estuarine and marine environments