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Abstract

Examination of the available literature on the effects of squawfish (Ptychocheilus spp.) predation and competition on salmonid populations indicates that: (1) squawfish may prey extensively on young salmon in lakes, but there is little evidence that this predation has much impact on the number of returning adults; (2) squawfish do not appear to be significant predators of salmon and trout in streams except under highly localized, seasonal or unusual circumstances; and (3) there is little evidence to indicate that squawfish compete strongly with salmonids. Areas of research are suggested that would clarify the role of squawfish in regulating salmonid populations and elucidate their position in the aquatic ecosystems of western North America.
... Because predation is a challenge with which nearly all organisms must contend, it is often considered in the management of vulnerable populations of fishes (Zimmerman and Ward 1999;Link 2002). In the Sacramento River, California, this challenge may be amplified for populations of juvenile Chinook Salmon by climate change, a complex water diversion system, hatchery domestication effects, lack of juvenile rearing habitat, and non-native predatory fish species (Brown and Moyle 1981;NOAA Fisheries 2018). Factors such as climate change and habitat loss have served to limit the natural production potential of Chinook Salmon (Oncorhynchus tshawytscha) ...
... The middle Sacramento River contains two abundant piscivorous fish species: Sacramento Pikeminnow (Ptychocheilus grandis) and Striped Bass (Morone saxatilis). In the Sacramento River and other California watersheds, both predators consume vulnerable native species, including juvenile Chinook Salmon and Steelhead (Stevens 1966;Thomas 1967;Brown and Moyle 1981;Brown and Moyle 1997;Nakamoto and Harvey 2003;Sabal et al. 2016). Sacramento Pikeminnow are native to the Sacramento River drainage; Striped Bass were introduced in 1879 as a recreational and commercial species (Moyle 2002). ...
... Sacramento Pikeminnow and Striped Bass have both been considered as potential contributors via predation to native species decline in the highly modified Sacramento River system (CDFG 1999;Moyle 2002;Lindley and Mohr 2003;Bonham 2011). Pikeminnow species (Ptychocheilus spp.) have been shown to consume large amounts of outmigrating salmonids from a range of US West Coast watersheds under hydraulically favorable conditions, and, in response, predator control measures have been implemented with mixed results (Brown and Moyle 1981;Brown and Moyle 1997;Tucker et al. 1998;Friesen and Ward 1999;Zimmerman and Ward 1999;Moyle 2002;Nakamoto and Harvey 2003). Likewise, Striped Bass have also been shown to consume salmonids (Stevens 1966;Tucker et al. 1998), which modeling suggests may have population-scale effects (Lindley and Mohr 2003;Sabal et al. 2016). ...
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This study examined diets of two predatory fish species, the native Sacramento Pikeminnow (Ptychocheilus grandis) and the introduced Striped Bass (Morone saxatilis), in the Sacramento River, California, USA. Both species have been implicated in native species declines through predation, eliciting our investigation of their diets in the Sacramento River. Sampling occurred between March and November 2017, and was conducted via hook and line on a 35-km reach near Chico, California. Habitat types sampled include engineered structures (water diversions and beam bridges), rip-rapped channel edges, and natural riverbank. Stomach contents were collected via gastric lavage and later processed using visual, gravimetric, and genetic techniques. Diets of Sacramento Pikeminnow and Striped Bass were highly similar as determined through index of relative importance and PERMANOVA modeling. Water temperature was the only variable that significantly affected diet composition. Results reflect similar dietary niches for both species in the Sacramento River.
... Because predation is a challenge with which nearly all organisms must contend, it is often considered in the management of vulnerable populations of fishes (Zimmerman and Ward 1999;Link 2002). In the Sacramento River, California, this challenge may be amplified for populations of juvenile Chinook Salmon by climate change, a complex water diversion system, hatchery domestication effects, lack of juvenile rearing habitat, and non-native predatory fish species (Brown and Moyle 1981;NOAA Fisheries 2018). Factors such as climate change and habitat loss have served to limit the natural production potential of Chinook Salmon (Oncorhynchus tshawytscha) ...
... The middle Sacramento River contains two abundant piscivorous fish species: Sacramento Pikeminnow (Ptychocheilus grandis) and Striped Bass (Morone saxatilis). In the Sacramento River and other California watersheds, both predators consume vulnerable native species, including juvenile Chinook Salmon and Steelhead (Stevens 1966;Thomas 1967;Brown and Moyle 1981;Brown and Moyle 1997;Nakamoto and Harvey 2003;Sabal et al. 2016). Sacramento Pikeminnow are native to the Sacramento River drainage; Striped Bass were introduced in 1879 as a recreational and commercial species (Moyle 2002). ...
... Sacramento Pikeminnow and Striped Bass have both been considered as potential contributors via predation to native species decline in the highly modified Sacramento River system (CDFG 1999;Moyle 2002;Lindley and Mohr 2003;Bonham 2011). Pikeminnow species (Ptychocheilus spp.) have been shown to consume large amounts of outmigrating salmonids from a range of US West Coast watersheds under hydraulically favorable conditions, and, in response, predator control measures have been implemented with mixed results (Brown and Moyle 1981;Brown and Moyle 1997;Tucker et al. 1998;Friesen and Ward 1999;Zimmerman and Ward 1999;Moyle 2002;Nakamoto and Harvey 2003). Likewise, Striped Bass have also been shown to consume salmonids (Stevens 1966;Tucker et al. 1998), which modeling suggests may have population-scale effects (Lindley and Mohr 2003;Sabal et al. 2016). ...
... later Common to both these reaches are in-river diversion weir structures; the first located at the start of Weir2_RST -Butte1 reach, and the second located in the middle of Butte3 -Butte5 reach. Studies have shown that Striped Bass (Morone saxatilis) and Sacramento Pikeminnow (Ptychocheilus grandis) -both considered major predators of juvenile salmon in the CCV -tend to congregate below in-river diversion weirs and are effective at predating on disoriented salmon smolts that pass over these structures (Brown and Moyle 1981;Tucker et al. 2003;Sabal et al. 2016 Sturrock et al. (2015)) would be needed to put the spring smolt out-migration life history studied here in broader context. ...
... Predators can have profound negative impacts on prey species populations during seasonal migrations in rivers where patches of prey species migrate through fields of attracted predator (Petersen and DeAngelis, 2000). Predator-prey species interactions in rivers during seasonal migrations of prey species is documented in recreational and commercially important anadromous fishes, such as salmon species (Salmonidae) by multiple predator types (e.g., mammals, birds, and fishes; Yurk and Trites, 2000;Ruggerone, 1986;Brown and Moyle, 1981;Petersen and DeAngelis, 2000). Both ecological drivers and the WMS operation (i.e., increased flow of nutrient-rich water) could be impacting the surrounding Illinois River fish assemblage. ...
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Water management structures (WMS) are used to regulate water levels between large floodplain river backwaters and their adjacent river systems offering a balance between maintaining quality backwater habitat and providing benefits to the river systems. The design and operation of these WMS is dependent on unique management goals, and their impacts on the surrounding ecosystem are understudied. From 2016–2018 we quantified the response of the surrounding fish assemblages (i.e., backwater and river abundance and composition) to the WMS operation at the Emiquon Preserve, a 2723 ha restored floodplain lake located adjacent to the main channel of the Illinois River, U.S.A. Environmental conditions were recorded, and fish were collected using boat electrofishing on both sides of the WMS under three operational levels: closed and no flow, gravity flowing water, and mechanically pumping water to the river. Multivariate analyses determined that there was no difference in fish community composition within the Emiquon Preserve among operation levels. Within the river there were differences in community composition when the WMS was closed and mechanically pumping water. Environmental parameters varied seasonally and WMS operation level varied but indicated an increase in chlorophyll-a in the river when the WMS was mechanically pumping water, although the chlorophyll-a decreased within 200 m downstream of the WMS in the Illinois River. This study suggests that managed connections between restored backwater and habitats operated like this WMS have variable, spatially isolated impacts on fish communities despite supplying chlorophyll-a to large river ecosystems.
... Additional compounding stressors include anthropogenic structures in Mill Creek and the Sacramento River such as water diversion infrastructure, bridge pilings, rock revetment, and wing dams which increase the effectiveness of ambush predators (Sabal et al. 2016). These structures create unnatural locations where predators can lie and wait, striking naïve juvenile salmon that are potentially disoriented after swimming through these obstacles, as they pass by (Brown and Moyle 1981;Sabal et al. 2016). Striped bass have been found to be effective ambush predators (Tucker et al. 2002) and are estimated to significantly impact juvenile Chinook salmon populations (Lindley and Mohr 2003). ...
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Populations of wild spring-run Chinook salmon in California’s Central Valley, once numbering in the millions, have dramatically declined to record low numbers. Dam construction, habitat degradation, and altered flow regimes have all contributed to depress populations, which currently persist in only a few tributaries to the Sacramento River. Mill Creek (Tehama County) continues to support these threatened fish, and contains some of the most pristine spawning and rearing habitat available in the Central Valley. Despite this pristine habitat, the number of Chinook salmon returning to spawn has declined to record low numbers, likely due to poor outmigration survival rates. From 2013 to 2017, 334 smolts were captured and acoustic tagged while out-migrating from Mill Creek, allowing for movement and survival rates to be tracked over 250 km through the Sacramento River. During this study California experienced both a historic drought and record rainfall, resulting in dramatic fluctuations in year-to-year river flow and water temperature. Cumulative survival of tagged smolts from Mill Creek through the Sacramento River was 9.5% (±1.6) during the study, with relatively low survival during historic drought conditions in 2015 (4.9% ± 1.6) followed by increased survival during high flows in 2017 (42.3% ± 9.1). Survival in Mill Creek and the Sacramento River was modeled over a range of flow values, which indicated that higher flows in each region result in increased survival rates. Survival estimates gathered in this study can help focus management and restoration actions over a relatively long migration corridor to specific regions of low survival, and provide guidance for management actions in the Sacramento River aimed at restoring populations of threatened Central Valley spring-run Chinook salmon.
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The introduction of non-native species can negatively impact native species through reduced genetic fitness resulting from hybridization. The lack of spatiotemporal data on hybrid occurrences makes hybridization risk assessment difficult. Here, we developed a spatially-explicit Hybridization Risk Model (HRM) between native Oregon bull trout, an Endangered Species Act-listed Oregon species, and introduced brook trout by combining an intrinsic potential model (IPM) of brook trout spawning habitat and existing bull trout distribution and habitat use datasets in Oregon, United States. We created an expertbased brook trout IPM classification score (0–1) of streams based on the potential of geophysical attributes (i.e., temperature, discharge, gradient, and valley confinement) to sustain spawning habitats. The HRM included a risk matrix based on the presence/ absence of both species as well as the type of habitat (spawning versus other) at 100-m stream segment resolution. We defined the hybridization risk as “extreme” when stream reaches contained bull trout spawning habitat and brook trout were present with IPM moderate or greater scores (IPM >0.5). Conversely, “low” risk reaches contained historic or non-spawning bull trout habitat, brook trout were absent, and IPM scores were low (IPM<0.25). Our HRM classified 34 km of streams with extreme risk of hybridization, 115 km with high risk, 178 km with moderate risk, and 6,023 km with low risk. Our HRM can identify a differential risk of hybridization at multiple spatial scales when either both species coexist in bull trout spawning habitat or are absent. The model can also identify stream reaches that would have higher risk of hybridization, but where brook trout are not currently present. Our modeling approach can be applied to other species, such as cutthroat trout and rainbow trout, Chinook and coho salmon, or similar species occurring elsewhere that potentially hybridize in freshwaters.
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Abstract Complex predator–prey interactions over time have the potential to limit survival of threatened native species. Reservoirs created by large dams in temperate ecosystems are sites where both coldwater and warmwater fish species overlap in distributions, forming assemblages that would not occur under natural settings. For example, in many western North American reservoirs, juvenile native salmonids now overlap with native and non‐native predators such as Northern Pikeminnow Ptychocheilus oregonensis and bass Micropterus spp. Currently, native Northern Pikeminnow are considered by many to be the most formidable predator of salmon smolts in freshwater systems of the Pacific Northwest. However, their consumption of salmon fry and the role of non‐native warmwater predators remain unclear. Predation on fry has proved more difficult to identify than on smolts, due to smaller sizes and high digestibility, but is important for prioritizing management strategies. Here, we use multiple lines of evidence, including large datasets of stable isotopes and fish stomach contents, to identify which fish consume Chinook Salmon Oncorhynchus tshawytscha fry (
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Predation of juvenile salmonids within California’s Sacramento – San Joaquin Delta (the Delta) has been identified as a contributing factor to low survival during outmigration through the system. Artificial lighting at night (ALAN) may contribute to increased levels of salmonid predation by attracting predators and prey, increasing predator reaction distance, and foraging success. To assess ALAN effects on predator (piscivorous fishes) density and the relative predation risk of Chinook Salmon Oncorhynchus tshawytscha smolts in the Delta, we preformed field based experiments with introduced ALAN. We used ARIS (Adaptive Resolution Imaging Sonar) cameras to generate predator density estimates in light and dark treatments throughout nightly experiments at 30 minute intervals. We simultaneously deployed predation event recorders (PERs) to estimate the impact of ALAN intensity (lux) on relative predation risk of Chinook Salmon smolts. Early in the night (1‐3 hours past sunset), predator density and relative predation risk of smolts was unrelated to ALAN. However, late in the night (3‐5 hours past sunset) ALAN presence increased predator density and the relative predation risk of juvenile salmonids increased with increasing lux. Predation risk was also positively related to predator density, and increased late‐night predator density under ALAN, coupled with late‐night foraging benefits of ALAN, likely contributed to the lux risk relationship. The exact mechanism behind this discrepancy between early and late‐night trends is unknown and could be a result of our experimental design or the predator community sampled here. However, if these temporal trends prove robust to future investigations, late‐night lighting reduction campaigns during outmigration, could maximize the human benefits of ALAN, while minimizing the negative impacts on salmonids. Overall, our findings align with others and suggest that ALAN increases juvenile salmonid predation. Although many questions remain unanswered, it appears that reducing artificial illumination is a practical management strategy to reduce predation.