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The Sacramento–San Joaquin Delta has been invaded by several species of non-native predatory fish that are presumed to be impeding native fish population recovery efforts. Since eradication of predators is unlikely, there is substantial interest in removing or altering manmade structures in the Delta that may exacerbate predation on native fish (contact points). It is presumed that these physical structures influence predator-prey dynamics, but how habitat features influence species interactions is poorly understood, and physical structures in the Delta that could be remediated to benefit native fish have not been inventoried completely. To inform future research efforts, we reviewed literature that focused on determining the effects of predator-prey interactions between fish, based on contact points that are commonly found in the Delta. We also performed a geospatial analysis to determine the extent of potential contact points in the Delta. We found that the effects of submerged aquatic vegetation (SAV) and artificial illumination are well studied and documented to influence predation in other freshwater systems worldwide. Conversely, other common structures in the Delta—such as docks, pilings, woody debris, revetment, and water diversions—did not have the same breadth of research. In the Delta, the spatial extent of the different types of contact points differed considerably. For example, 22% of the Delta water surface area is occupied by SAV, whereas docks only cover 0.44%. Our conclusion, based on both the literature review and spatial analysis, is that the effects of SAV and artificial illumination on predation warrant the most immediate future investigation in the Delta.

https://doi.org/10.15447/sfews.2018v16iss4art3The Sacramento-San Joaquin Delta is a major survival bottleneck for imperiled California salmonid populations, which is partially due to a multitude of non-native fish predators that have proliferated there throughout the 20th century. Understanding the diets of salmonid predators is critical to understanding their individual impacts, role in the food web, and the implications for potential management actions. We collected the stomach contents of Striped Bass Morone saxatilis, Largemouth Bass Micropterus salmoides, Channel Catfish Ictalurus punctatus and White Catfish Ameiurus catus sampled from three 1-km reaches in the lower San Joaquin River in 2014 and 2015 during the peak juvenile salmon outmigration period. We tested each stomach (n = 582) for the presence of juvenile Chinook Salmon Oncorhynchus tshawytscha and other prey items using a genetic barcoding technique. Channel Catfish had significantly higher frequency of Chinook Salmon in their stomachs (27.8% of tested Channel Catfish contained Chinook Salmon DNA), compared to the other three predators (2.8% to 4.8%). However, non-native fish species occurred at greater frequencies in the diets of all four predator species than salmon. Using depletion estimation from electrofishing, we were able to generate population densities for Striped Bass and Largemouth Bass in our reaches. Largemouth Bass were evenly distributed throughout all three reaches, at a mean density of approximately 333 (± 195 SE) per km of river. Striped Bass were patchily distributed, ranging from 21 to 1,227 per km. Extrapolating the frequency of salmon detected in stomachs to the predator abundance estimates, we estimate that the population of Largemouth Bass we sampled consumed between 3 and 5 Chinook Salmon per day per 1-km study reach (consumption rate of 0.011 salmon per predator per day), whereas the Striped Bass population consumed between 0 and 24 Chinook Salmon per day (0.019 salmon per predator per day).

Water diversions that support agricultural and municipal use result in fish mortality through entrainment and impingement. Additionally, this infrastructure may attract both predators and prey fishes, thereby increasing predation rates and prey mortality near these anthropogenic contact points. The Sacramento–San Joaquin Delta (the Delta) in California's Central Valley is a tidal freshwater ecosystem that exports large volumes of water for municipal and agricultural use while at the same time providing valuable migratory and rearing habitat for imperilled fishes. Emigrating juvenile salmonids experience high mortality in the Delta, with predation by non‐native fishes contributing substantially.Therefore, this study had three main objectives. First, we determined if small water diversions aggregated piscivorous fishes like other similar structures in freshwater ecosystems. Second, we determined how small diversions may influence juvenile salmon predation risk in conjunction with other known predation risk factors (e.g. predator abundance, temperature and depth). Third, we assessed the predator assemblage, abundance and distribution to determine the likely predator composition in objectives one and two.Throughout the spring of 2021, we used ARIS (adaptive resolution imaging sonar; Sound Metrics) sonars to compare piscivore abundance at 30 water diversions in the north Delta to shorelines adjacent to diversions that did not contain these structures. We used predation event recorders (PERs) to assess the predation risk juvenile salmonids were exposed to, with linear distance (m) from diversions, and other predation risk factors in the north Delta. Finally, we used a boat electrofishing survey to determine the piscivore assemblage and compare spatial trends in black bass (Micropterus spp.) CPUE and relative abundance throughout these waterways.Piscivore abundance was greater near small water diversions than at adjacent shorelines and the predation risk of juvenile salmonids increased with diversion proximity. Additionally, predation risk increased with increasing piscivore abundance and decreasing water depth. The north Delta predator assemblage was dominated by black basses (Micropterus spp.), which likely drove the negative relationship of predation risk with water depth, given habitat requirements of these species. Furthermore, increasing smallmouth (Micropterus dolomieu) and spotted bass (Micropterus punctulatus) abundance in our northern study sites may have weakened temperature effects on predation, given metabolic requirements of these species.Our work demonstrated that small water diversions are likely to increase mortality of endangered salmonids, and that the north Delta predator assemblage was different than recorded by previous work in this system, changing predation risk factors. Although more work is needed to determine the population level impacts of diversions, the ubiquitous distribution of these structures warrants management solutions to reduce mortality from this source.These results indicate that in addition to entrainment and impingement, water diversions may increase mortality of small‐bodied fishes by attracting predators and elevating predation risk. Given the continual human demand for freshwater, predator–prey interactions should be considered along with entrainment and impingement when assessing intake infrastructure mitigation, especially when diversions co‐occur along migratory routes and essential habitat of imperilled fishes. [ABSTRACT FROM AUTHOR]

Outmigration survival of acoustic-tagged, hatchery-origin, late-fall-run Chinook salmon (Oncorhynchus tshawytscha) smolts from the Sacramento River was estimated for 5 years (2007-2011) using a receiver array spanning the entire outmigration corridor, from the upper river, through the estuary, and into the coastal ocean. The first 4 years of releases occurred during below-average river flows, while the fifth year (2011) occurred during above-average flows. In 2011, overall outmigration survival was two to five times higher than survival in the other 4 years. Regional survival estimates indicate that most of the improved survival seen in 2011 occurred in the riverine reaches of the outmigration corridor, while survival in the brackish portions of the estuary did not significantly differ among the 5 years. For the 4 low-flow years combined, survival rate in the river was lower in the less anthropogenically modified upper reaches; however, across all regions, survival rate was lowest in the brackish portion of the estuary. Even in the high-flow year, outmigration survival was substantially lower than yearling Chinook salmon populations in other large rivers. Potential drivers of these patterns are discussed, including channelization, water flow, and predation. Finally, management strategies are suggested to best exploit survival advantages described in this study.

Low survival rates of Chinook salmon (Oncorhynchus tshawytscha) smolts in California's Central Valley have been attributed to multiple biological and physical factors, but it is not clear which factors have the largest impact. We used 5 years of acoustic telemetry data for 1709 late-fall Chinook salmon smolts to evaluate the effect of habitat- and predation- related covariates on outmigration survival through the Sacramento River. Using a Cormack-Jolly-Seber mark-recapture model, we estimated survival rates both as a function of covariates (covariate model) and as a function of river location and release year (spatial-temporal model). Our covariate model was overwhelmingly supported as the preferred model based on model selection criteria, suggesting the covariates adequately replicated spatial and temporal patterns in smolt survival. The covariates in the selected model included individual fish covariates, habitat-specific covariates, and temporally variable physical conditions. The most important covariate affecting salmon survival was flow. We describe the importance of these parameters in the context of juvenile salmon predation risk and suggest that additional research on predator distribution and density could improve model estimates. Si les faibles taux de survie des saumoneaux de saumon chinook (Oncorhynchus tshawytscha) dans la vallee centrale de la Californie ont ete attribues a differents facteurs biologiques et physiologiques, l'identite des facteurs exerqant la plus grande influence demeure incertaine. Nous avons utilise 5 annees de donnees de telemetrie acoustique pour 1709 saumoneaux de saumon chinook de la fin de l'automne pour evaluer l'effet de covariables associees a l'habitat et a la predation sur la survie durant la devalaison dans le fleuve Sacramento. En utilisant un modele de marquage-recapture de Cormack-Jolly-Seber, nous avons estime les taux de survie en fonction de covariables (modele des covariables) et en fonction de l'emplacement dans le fleuve et de l'annee du lacher (modele spatiotemporel). Le modele de covariables s'est avere, de loin, le modele a privilegier sur la base de criteres de selection de modeles, ce qui donne a penser que les covariables reproduisent adequatement les motifs spatiaux et temporels de survie des saumoneaux. Les covariables dans le modele selectionne comprennent des covariables associees aux poissons individuels, des covariables propres a l'habitat et des conditions physiques variables dans le temps. La covariable ayant la plus importante influence sur la survie des saumons est le debit. Nous decrivons l'importance de ces parametres dans le contexte du risque de predation de saumons juveniles et suggerons que d'autres travaux sur la repartition et la densite des predateurs pourraient ameliorer les estimations decoulant du modele. [Traduit par la Redaction], Introduction Salmon smoltification and outmigration from freshwater rearing habitats is a time of increased mortality as fish undergo physiological changes and encounter new stressors (Connor et al. 2003; Welch et [...]

Historically, marine survival estimates for salmon have been confounded with freshwater seaward migration (outmigration) survival. Telemetry studies have revealed low and variable survival during outmigration, suggesting marine mortality may not be the primary source of variability in cohort size as previously believed. Using a novel combination of tagging technologies, survival during these two life stages was decoupled over 5 years for Sacramento River Chinook salmon (Oncorhynchus tshawytscha). Outmigration survival ranged from 2.6% to 17%, and marine survival ranged from 4.2% to 22.8%. Influential environmental drivers in both life stages were also compared with smolt-to-adult ratios (SAR) for three Chinook salmon populations over 20 years. Streamflow during outmigration had higher correlation with SAR ([r.sup.2] > 0.34) than two marine productivity indices ([r.sup.2] < 0.08). The few SAR estimates that were poorly predicted by flow occurred during years with the lowest marine productivity, suggesting most interannual SAR fluctuations are explained by outmigration survival, but abnormally poor marine conditions also reduce SAR. The outsized influence of flow on SAR provides managers with a powerful mitigation tool in a watershed where flow is tightly regulated. Historiquement, une certaine confusion, associee a la survie durant la migration vers la mer (devalaison), caracterise les estimations de la survie en mer des saumons. Des etudes de telemetrie ont revele des taux de survie faibles et variables durant la devalaison qui indiqueraient que, contrairement a la croyance anterieure, la mortalite en mer pourrait ne pas etre la premiere source de variabilite de la taille des cohortes. L'utilisation d'une combinaison novatrice de technologies de marquage a permis de decoupler la survie durant ces deux etapes du cycle de vie pendant une periode de 5 ans pour des saumons quinnats (Oncorhynchus tshawytscha) du fleuve Sacramento. La survie durant la devalaison allait de 2,6 % a 17 %, alors que la survie en mer allait de 4,2 % a 22,8 %. Les facteurs environnementaux exercant une influence pendant les deux etapes du cycle de vie ont aussi ete compares aux rapports saumoneaux-adultes (RSA) pour trois populations de saumons quinnats sur 20 ans. Le debit durant la devalaison presente une correlation plus forte avec le RSA ([r.sup.2] > 0,34) que deux indices de productivite marine ([r.sup.2] < 0,08). Les quelques estimations du RSA que le debit ne predit pas bien sont pour les annees ou la productivite marine etait la plus faible, ce qui donne a penser que la plupart des fluctuations interannuelles du RSA s'expliquent par la survie durant la devalaison, mais que des conditions marines anormalement mauvaises reduisent egalement le RSA. L'influence demesuree du debit sur le RSA fournit aux gestionnaires un puissant outil d'attenuation dans un bassin versant ou le debit fait l'objet d'une regularisation serree. [Traduit par la Redaction], Introduction Convention is that variability in salmon cohort success is set during the early marine residence period. To date, direct evidence of how outmigration (freshwater plus estuarine) survival might be [...]

We evaluated the interacting influences of river flows and tides on travel time, routing, and survival of juvenile late-fall Chinook salmon (Oncorhynchus tshawytscha) migrating through the Sacramento-San Joaquin River Delta. To quantify these effects, we jointly modeled the travel time, survival, and migration routing in relation to individual time-varying covariates of acoustic-tagged salmon within a Bayesian framework. We used observed arrival times for detected individuals and imputed arrival times for undetected individuals to assign covariate values in each reach. We found travel time was inversely related to river inflow in all reaches, yet survival was positively related to inflow only in reaches that transitioned from bidirectional tidal flows to unidirectional flow with increasing inflows. We also found that the probability of fish entering the interior Delta, a low-survival reach, declined as inflow increased. Our study illustrates how river inflows interact with tides to influence fish survival during the critical transition between freshwater and ocean environments. Furthermore, our analytical framework introduces new techniques to integrate formally over missing covariate values to quantify effects of time-varying covariates. Nous avons evalue l'interaction des influences des debits de riviere et des marees sur le temps de deplacement, l'itineraire et la survie de saumons quinnats (Oncorhynchus tshawytscha) juveniles de fin d'automne migrant dans le delta des fleuves Sacramento et San Joaquin. Pour quantifier ces effets, nous avons modelise conjointement le temps de deplacement, la survie et l'itineraire de migration par rapport a differentes covariables variant dans le temps de saumons dotes d'etiquettes acoustiques dans un cadre bayesien. Nous avons utilise les temps d'arrivee observes pour les individus detectes et impute des temps d'arrivee pour les individus non detectes afin d'affecter des valeurs aux covariables dans chaque tronqon. Nous constatons que le temps de deplacement est inversement relie au debit entrant dans tous les tronqons, alors que la survie n'est positivement reliee au debit entrant que dans les tronqons ou des debits tidaux bidirectionnels passent a un ecoulement unidirectionnel quand les debits entrants augmentent. Nous constatons aussi que la probabilite que les poissons entrent dans le delta interieur, un tronqon caracterise par une faible survie, diminue quand le debit entrant augmente. L'etude illustre comment les debits entrants de rivieres interagissent avec les marees pour influencer la survie des poissons durant le passage critique du milieu d'eau douce au milieu oceanique. En outre, notre cadre d'analyse presente de nouvelles methodes permettant l'integration formelle sur des valeurs de covariables manquantes pour quantifier les effets de covariables variant dans le temps. [Traduit par la Redaction], Introduction Anadromous salmonids have evolved diverse life history strategies that capitalize on spatial and temporal variation in their habitat to maximize productivity. Understanding how salmonids use habitat over space and [...]

Abstract--Recent acoustic tagging of juvenile Chinook salmon (Oncorhynchus tshawytscha) in the southern portion of California's Sacramento-San Joaquin Delta has revealed extremely low survival rates ( ********** Predation on juvenile Chinook [...]

Migratory salmon move nutrients both in and out of fresh waters during the different parts of their life cycle. We used a mass-balance approach to quantify recent changes in phosphorus (P) fluxes in six coastal California, USA, watersheds that have recently experienced dramatic decreases in salmon populations. As adults, semelparous Chinook (Oncorhynchus tshawytscha) and coho (Oncorhynchus kisutch) salmon imported 8.3 and 10.4 times more P from the ocean, respectively, than they exported as smolts, while iteroparous steelhead (i.e., sea-run rainbow trout, Oncorhynchus mykiss) imported only 1.6 times more than they exported as kelts and smolts. Semelparous species whose life histories led them to import more nutrients were also the species whose populations decreased the most dramatically in California in recent years. In addition, the relationship between import and export was nonlinear, with export being proportionally more important at lower levels of import. This pattern was driven by two density-dependent processes--smolts were larger and disproportionately more abundant at lower spawner abundances. In fact, in four of our six streams we found evidence that salmon can drive net export of P at low abundance, evidence for the reversal of the 'conveyor belt' of nutrients. Les saumons migrateurs deplacent des nutriments tant vers les eaux douces qu'hors des eaux douces durant les differentes etapes de leur cycle biologique. Nous utilisons une methode de bilan massique pour mesurer les changements recents dans les flux de phosphore (P) dans six bassins versants cotiers de Californie, E.-U., qui ont connu des declins spectaculaires de leurs populations de saumons. Au stade adulte, les saumons chinook (Oncorhynchus tshawytscha et coho (Oncorhynchus kisutch) semelpares importent respectivement 8,3 et 10,4 fois plus de P de l'ocean qu'ils n'en exportent au stade saumoneau; en revanche, les truites arc-en-ciel anadromes (Oncorhynchus mykiss) iteropares importent seulement 1,6 fois plus qu'ils en exportent comme becards et saumoneaux. Les especes semelpares dont le cycle biologique les amene a importer plus de nutriments sont aussi les especes dont les populations ont decru de facon la plus spectaculaire en Californie ces dernieres annees. De plus, la relation entre l'importation et l'exportation n'est pas lineaire et l'exportation est proportionnellement plus importante aux faibles niveaux d'importation. Ce patron s'explique par deux processus dependants de la densite--les saumoneaux sont de plus grande taille et de facon disproportionnee plus abondants aux faibles abondances de reproducteurs. En fait, dans quatre des six cours d'eau, nous trouvons des preuves que les saumons peuvent controler l'exportation nette de P aux faibles abondances; ce sont des indications du renversement de la ' courroie de transmission ' des nutriments. [Traduit par la Redaction], Introduction Species can control fluxes of nutrients and energy across ecosystem boundaries through their own movements (Lundberg and Moberg 2003; Polis et al. 2004). Nutrients can be liberated in the [...]

The Sacramento-San Joaquin Delta is a major survival bottleneck for imperiled California salmonid populations, partially because a multitude of non-native fish predators have proliferated there throughout the 20th century. Understanding the diets of salmonid predators is critical to understanding their individual effects, role in the food web, and the implications for potential management actions. We collected the stomach contents of Striped Bass Morone saxatilis, Largemouth Bass Micropterus salmoides, Channel Catfish Ictalurus punctatus and White Catfish Ameiurus catus sampled from three 1-km reaches in the lower San Joaquin River in 2014 and 2015 during the peak juvenile salmon out-migration period. Using a genetic barcoding technique, we tested each stomach (n = 582) for the presence of juvenile Chinook Salmon Oncorhynchus tshawytscha and other prey items. Channel Catfish had significantly higher frequency of Chinook Salmon in their stomachs (27.8% of tested Channel Catfish contained Chinook Salmon DNA), compared to the other three predators (2.8% to 4.8%). However, non-native fish species occurred at greater frequencies than salmon in the diets of all four predator species. Using depletion estimation from electrofishing, we were able to generate population densities for Striped Bass and Largemouth Bass in our reaches. Largemouth Bass were evenly distributed throughout all three reaches, at a mean density of approximately 333 (± 195 SE) per km of river. Striped Bass were patchily distributed, ranging from 21 to 1,227 per km. Extrapolating the frequency of salmon detected in stomachs to the predator abundance estimates, we estimate that the population of Largemouth Bass we sampled consumed between 3 and 5 Chinook Salmon per day per 1-km study reach (consumption rate of 0.011 salmon per predator per day), whereas the Striped Bass population consumed between 0 and 24 Chinook Salmon per day (0.019 salmon per predator per day). [ABSTRACT FROM AUTHOR]

Disparate bodies of literature implicate risk avoidance and energy conservation as important drivers of animal movement decisions. Theory posits that these phenomena interact in ecologically consequential ways, but rigorous empirical tests of this hypothesis have been hampered by data limitations. We fuse fluid dynamics, telemetry, and attack data to reconstruct risk and energy landscapes traversed by migrating juvenile salmon and their predators. We find that migrants primarily use midriver microhabitats that facilitate migration at night. During daylight, predators become more aggressive in the midriver, and prey reduce midriver use in favour of nearshore microhabitats, resulting in increased energy expenditure and decreased migration efficiency. Predators attack most when migrants are not prioritising threat avoidance and during ephemeral periods of low lighting. Our findings suggest that predator-prey interactions result from an interplay between landscapes of fear and energy, which can determine the degree to which predators affect prey through mortality or fear., (© 2025 John Wiley & Sons Ltd.)