Giant Salmonfly
Pteronarcys californica
Other common names: California Salmonfly
NatureServe conservation status
Global (G-rank): G5
State (S-rank): S2
Utah Wildlife Action Plan status
- SGCN
External links
General information
The Giant salmonfly or California giant stonefly, is a species of giant stonefly (Pteronarcyidae) found in mountain streams and rivers of Western North America. Aquatic nymphs and terrestrial adults may be dark-orange to black-colored. However, nymphs may have subtle patterns on the abdomen, and adults often have bright orange coloration at the joints of their body segments. Adults have two pairs of large, many-veined wings that fold back over the body when at rest. Female salmonflies carry clusters of orange eggs resembling salmon roe, which they deposit in the river after mating (Birrell & Barth, 2024).
Phenology
Adults tend to emerge between May and July, with individuals emerging sooner in rivers or microclimates with warmer winter and spring temperatures (Gregory et al., 2000; Rockwell & Newell 2009; Anderson et al., 2019). Emergences of adults tend to be large and synchronous, stimulating aggressive feeding behavior by trout and migratory bird species (Townsend & Prichard, 1998; Stagliano, 2010). P. californica emergences are also highly anticipated by anglers (Stagliano, 2010).
Species range
P. californica is native to western North America and found along the Coast, Cascade, Rocky, and Sierra Nevada Mountains from Alaska and the Yukon south to Mexico and throughout most of the Basin and Range of the western United States (Stewart & Oswood, 2006). Occurrence records indicate P. californica occurs in Alaska, British Columbia, Alberta, Washington, Idaho, Montana, Oregon, Nevada, Utah, Wyoming, Colorado, California, and New Mexico (National Aquatic Monitoring Center (NAMC), 2024; iNaturalist, 2024). There are no specific population estimates for P. californica in Utah, and no programs currently exist for providing reliable statewide population data. However, surveys of individual populations within specific watersheds have been conducted from time to time (NAMC, 2024), along with independent observations made by community scientists (iNaturalist, 2024). Together, these efforts show that P. californica are broadly distributed in streams and rivers near many of the State’s mountain ranges (Fig. 1). Due to the lack of systematic, statewide surveys, however, the actual extent of their distribution is unknown and may be significantly larger (Birrell & Barth, 2024).
Migration
Nymphs were found to move a few (perhaps tens) meters each day with the fastest tagged individual moving 40 m in four days (or 22 m upstream in a single day) (Freilich, 1991). Small scale migration is heaviest near dawn and dusk.
Habitat
P. californica typically inhabit third- to seventh-order mountain streams with cool temperatures and large, unconsolidated cobbles. P. californica have been found in streams ranging from near sea level to 2500 m (Knight & Gaufin, 1966; Birrell et al., 2019; NAMC, 2024). Nymph densities are positively associated with substrate size, with individuals typically occupying sites with median particle diameters of at least 8 cm and substrates composed of less than 10% fine sediment (Brusven & Prather, 1974, Huff, 2006; Relyea, 2007; Kowalski & Richer, 2020). Though P. californica are not cold-water stenotherms, they tend to be rare in rivers with mean August temperatures > 19 °C (Huff, 2006; Anderson et al., 2019; Birrell & Frakes, 2024b) and mean maximum monthly summer temperatures > 23 °C (Birrell et al., 2023). They have never been recorded in streams with instantaneous temperatures > 25°C (Richards et al., 2013).
Susceptibility of P. californica to high temperature can be exacerbated by other stressors including hypoxia, low flows, and heavy metals (Frakes et al., 2021; 2022). They are strongly sensitive to heavy metals such as copper and cadmium, but are fairly tolerant to lead (Colborn, 1985; Frakes et al., 2022). Due to their general sensitivity to environmental degradation, P. californica are widely used as bioindicators of water quality and are listed as ‘very sensitive’ on multiple regional bioassessment indices (Colborn, 1985; Barbour et al. 1990; Fore et al. 1996;; Merritt et al. 2008). The susceptibility of P. californica to other factors, such as pH and salinity, is unknown (Birrell & Barth, 2024).
Food habits
Nymphs are considered major shredders of CPOM (coarse particulate organic matter) in stream systems (Cummins et al., 1973; Short and Maslen, 1977) but are poorly adapted to digest detrital polysaccharides (Martin et al., 1981). Gut content was found to be 75% diatoms, 15% vascular plant material, and 8% animal remains (Freilich, 1991). Richardson and Gaufin (1971) conducted feeding studies.
Ecology
Large leaf-shredders, like P. californica, are critical for maintaining the structure and function of lotic food webs, and their presence can lead to emergent ecosystem properties (Lecerf & Richardson, 2011). P. californica can be considered a keystone species because they increase food and nutrient availability for other species across multiple trophic levels (Stout, 1999; Lecerf & Richardson, 2011; Walters et al., 2018). For collector- and filterer-gatherers, P. californica are key producers of readily available food (Merritt et al., 2008). Pteronarcys nymphs typically break down 30-60% of their weight in detritus daily, > 80% of which is expelled as non-digested feces and fed on by other collectors (Poole, 1981, Perry et al. 1987). Over a nymph’s lifetime, this equates to the production 800-2000 kCal of energy for low-level consumers (Poole, 1981) and has been experimentally shown to significantly increase in-stream nutrient cycling (Short & Maslin, 1977). P. californica are also critical food sources for fish (Muttkowski, 1925). Indeed, in some rivers, P. californica can make up the > 60% of the annual diet of rainbow trout (Nehring, 2011). They are also consumed by numerous terrestrial animals, including spiders, snakes, frogs, birds, ground squirrels, and even, humans (e.g., indigenous peoples of Northeastern California, including the Pit River & Modoc tribes), during their large summer emergences (Muttkowski, 1925; Sutton, 1985; Rockwell et al., 2009). In rivers with large populations, emergences of adult P. californica can transfer more carbon into terrestrial ecosystems than the combined annual emergences of all other insects at a given site, suggesting that this species is key to supporting the total energy and nutrient budgets of riparian ecosystems (Walters et al., 2018). With so many species relying on P. californica in both terrestrial and aquatic systems, population declines and extirpations of the species are expected to have far-reaching ecological implications (Birrell & Barth, 2024).
Threats or limiting factors
Aquatic insects are faced with numerous threats stemming from increased human development, land-use change, and climate change, including warming temperatures, dewatering, hypoxia, sedimentation, habitat fragmentation and degradation, invasive species, and water pollution (DeWalt et al. 2005; Dudgeon et al., 2006; Strayer, 2006; Collen et al., 2012; Birrell et al., 2020; Costante et al., 2022). While all of these challenges are likely to affect P. californica populations, few studies have rigorously investigated the impacts of stressors on natural populations (but see Anderson et al., 2019; Kowalski & Richer, 2020). Indeed, additional, large-scale surveys and distribution models will be necessary to accurately assess and identify the importance of potential threats and to direct conservation priorities at range-wide to local scales. At present, however, current limited data must be relied upon to assess potential threats, and these are outlined and discussed below.
Case studies of P. californica declines in different regions point to various drivers of local extirpations and declines. In Utah, severe, periodic dewatering to support local agriculture and municipal uses have extirpated populations on the lower Blacksmith Fork and lower Spanish Fork Rivers (Birrell & Kowalski, unpublished data). Restoring these populations is an important and feasible conservation priority, as both rivers have intact populations immediately upstream of the dewatered reaches, which would likely reestablish the lower reaches once water availability is improved (Birrell & Kowalski, unpublished data). Drivers of local declines on the Provo River are unconfirmed, but are hypothesized to be driven by hydrological shifts and dewatering from two large, hypolimnetic-release dams, local water withdrawals systems, and trans-basin water diversions, along with nutrient pollution and hypoxia from urban and agricultural runoff (Birrell et al., 2019). In a 2022 survey, water temperatures and sediment levels on the Provo appeared to be conducive to P. californica survival, and problems with these factors are unlikely to be primary drivers of the decline (Birrell & Kowalski, unpublished data).
The local extinction of P. californica from the Logan River is more mysterious, as temperature, flow, oxygen, and sediment conditions appear to be within healthy levels (Vinson, 2008; Birrell & Kowalski, unpublished data). Indeed, the majority of the River is free-flowing and surrounded by National Forest lands, protecting it from excessive water withdrawals and stressors arising from human development. Some have speculated, however, that that populations have been extirpated due to excessive road-salting or applications of herbicides, though these hypotheses have not been tested (Vinson, 2008). In other states, declines of P. californica have been attributed to other stressors, including hydrological shifts and sedimentation (e.g., Gunnison and Colorado Rivers, Colorado; Kowalski & Richer, 2020), heavy metal pollution (e.g., Clark Fork River, Montana and Arkansas River, Colorado; Stagliano, 2010) and warming temperatures from river impoundments and climate change (Madison River, Montana; Anderson et al., 2019). Limiting these stressors, along with other potentially harmful and currently unmeasured factors, like insecticides, across rivers in Utah will be necessary to prevent further population declines (Birrell & Barth, 2024).