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Flows released from reservoirs are often modified to mitigate the negative ecosystem effects of dams. We estimated the effects of two experimental flows, fall-timed floods and elimination of sub-daily variation in flows on weekends, on growth rates of rainbow trout (Oncorhynchus mykiss) in the Colorado River downstream from Glen Canyon Dam. Experimental flow effects were compared to effects of water temperature, phosphorous concentration, solar insolation, and competition, by fitting mixed effect von Bertalanffy models to ~ 10 000 observations of growth from mark-recapture between 2012 and 2021. There was strong support for models predicting faster growth during intervals with higher solar insolation, and lower water temperature and competition for prey. Effects of phosphorus and experimental flows were small and uncertain. Drought-related increases in dam release temperatures during summer and fall were predicted to result in severe weight loss for larger trout and could eventually threaten the viability of the population and the fishery it supports. The effects of water temperature and competition on fish growth substantially exceeded the effects of controlled floods and steadier flows. Key words: Drought, fish growth, water quality, controlled floods, hydropeaking, water temperature, Introduction Dams and river regulation have fundamentally altered flow regimes of the world's rivers (Petra and others 2009). Environmental flows (e-flows) seek to replicate aspects of the quantity, timing, or [...]

Drift-feeding fish are typically considered size-selective predators. Yet, few studies have explicitly tested which aspect of prey 'size' best explains size selection by drift-foraging fish. Here, we develop a Bayesian discrete choice model to evaluate how attributes of both prey and predator simultaneously influence size-selective foraging. We apply the model to a large dataset of paired invertebrate drift (n = 784) and rainbow trout (Oncorhynchus mykiss) diets (n = 1028). We characterized prey 'size' using six metrics (length, width, area, hemispherical area, volume, mass) and used pseudo-[R.sup.2] to determine which metric best explained observed prey selection across seven taxa. We found that rainbow trout are positively size-selective, they are selecting prey based upon differences in prey width, and size-selectivity increases with fish length. Rainbow trout demonstrated strong selection for the adult and pupae stages of aquatic insects relative to their larval stages. Our study provides strong empirical evidence for size-selective foraging in rainbow trout and demonstrates prey selection is based primarily upon width, not length or area as has been widely reported. Les poissons qui s'alimentent d'organismes a la derive sont typiquement consideres comme etant des predateurs selectifs selon la taille. Peu d'etudes ont toutefois verifie explicitement quel aspect de la << taille >> des proies explique le mieux la selection des tailles par ces poissons. Nous developpons un modele bayesien de choix discrets pour evaluer l'influence d'attributs tant des proies que des predateurs sur l'alimentation selective selon la taille. Nous appliquons le modele a un grand ensemble de donnees jumelees sur les invertebres a la derive (n = 784) et les regimes alimentaires de truites arcen-ciel (Oncorhynchus mykiss) (n = 1028). Nous caracterisons la << taille >> des proies a l'aide de six parametres (longueur, largeur, aire, aire hemispherique, volume et masse) et utilisons le pseudo-[R.sup.2] pour determiner quel parametre explique le mieux la selection de proies entre sept taxons. Nous constatons que les truites arc-en-ciel font preuve de selection positive en fonction de la taille, qu'elles selectionnent leurs proies sur la base de differences de largeur des proies et que la selectivite de la taille augmente avec la longueur des poissons. Les truites arc-en-ciel presentent une forte propension a selectionner les adultes et pupes d'insectes aquatiques plutot que leurs larves. L'etude fournit de preuves empiriques solides d'une alimentation selective selon la taille chez les truites arc-en-ciel et demontre que la selection de proies est principalement basee sur la largeur, non sur la longueur ou l'aire, comme cela a ete largement rapporte. [Traduit par la Redaction], Introduction Foraging is essential to animal survival and reproduction (Stephens et al. 2008). Because of the fundamental role that foraging decisions play in the dynamics of animal populations, foraging ecology [...]

Drift studies are central to stream and river ecological research. However, a fundamental aspect of quantifying drift--how net clogging affects the accuracy of results--has been widely ignored. Utilizing approaches from plankton and suspended sediment studies in oceanography and hydrology, we examined the rate and dynamics of net clogging across a range of conditions. We found that nets clog nonlinearly over time and that suspended solid concentrations and net mesh size exerted a strong effect on clogging rates. Critically, net clogging introduced unpredictable biases in resultant data due to the inaccuracies in water volume estimates introduced by progressive clogging. This renders the widespread approach to linearly 'correct' for clogging inadequate. Using a meta-analysis of 77 drift studies spanning 25 years, we demonstrate that the detrimental effects of net clogging are routinely unappreciated, even though the results of most of these studies were likely affected by clogging. We close by describing an approach for avoiding net clogging, which will increase the accuracy and reproducibility of results in future freshwater, lotic drift studies. Les etudes de la derive constituent une composante centrale de la recherche sur l'ecologie des cours d'eau et rivieres. Toutefois, un aspect fondamental de la quantification de la derive, l'incidence de l'encrassement des filets sur l'exactitude des resultats, est rarement pris en consideration. En utilisant des approches tirees d'etudes du plancton et des sediments en suspension en oceanographie et en hydrologie, nous avons examine le taux et la dynamique de l'encrassement des filets dans un eventail de conditions. Nous avons constate que l'encrassement des filets n'est pas lineaire dans le temps et que les concentrations de solides en suspension et la taille des mailles du filet ont une grande incidence sur les taux d'encrassement. Fait crucial, l'encrassement des filets introduit des biais imprevisibles dans les donnees en raison de l'inexactitude des estimations du volume d'eau causee par l'encrassement graduel. Cela fait en sorte que l'approche repandue de ' correction ' lineaire pour tenir compte de l'encrassement est inadequate. En utilisant une metaanalyse de 77 etudes de la derive couvrant 25 ans, nous demontrons que les effets deleteres de l'encrassement des filets sont regulierement sous-apprecies, meme si l'encrassement a vraisemblablement une incidence sur les resultats de la plupart de ces etudes. Enfin, nous decrivons une approche permettant d'eviter l'encrassement des filets qui accroitra l'exactitude et la reproductibilite des resultats de futures etudes de la derive en milieux lotiques d'eau douce. [Traduit par la Redaction], Introduction Drift in stream and river ecosystems is essential to the maintenance, dispersal, and life histories of many freshwater invertebrate populations and also provides a key food source for drift-feeding [...]

The cold and clear water conditions present below many large dams create ideal conditions for the development of economically important salmonid fisheries. Many of these tailwater fisheries have experienced declines in the abundance and condition of large trout species, yet the causes of these declines remain uncertain. Here, we develop, assess, and apply a drift-foraging bioenergetics model to identify the factors limiting rainbow trout (Oncorhynchus mykiss) growth in a large tailwater. We explored the relative importance of temperature, prey quantity, and prey size by constructing scenarios where these variables, both singly and in combination, were altered. Predicted growth matched empirical mass-at-age estimates, particularly for younger ages, demonstrating that the model accurately describes how current temperature and prey conditions interact to determine rainbow trout growth. Modeling scenarios that artificially inflated prey size and abundance demonstrate that rainbow trout growth is limited by the scarcity of large prey items and overall prey availability. For example, shifting 10% of the prey biomass to the 13 mm (large) length class, without increasing overall prey biomass, increased lifetime maximum mass of rainbow trout by 88%. Additionally, warmer temperatures resulted in lower predicted growth at current and lower levels of prey availability; however, growth was similar across all temperatures at higher levels of prey availability. Climate change will likely alter flow and temperature regimes in large rivers with corresponding changes to invertebrate prey resources used by fish. Broader application of drift-foraging bioenergetics models to build a mechanistic understanding of how changes to habitat conditions and prey resources affect growth of salmonids will benefit management of tailwater fisheries. Les conditions d'eau froide et claire presentes en aval de nombreux grands barrages creent les conditions ideales pour le developpement de peches aux salmonides d'importance economique. Si bon nombre de ces peches en aval de barrages ont connu des baisses d'abondance et d'embonpoint de grandes especes de truites, les causes de ces baisses demeurent incertaines. Nous elaborons, evaluons et appliquons un modele bioenergetique d'approvisionnement a la derive afin de cerner les facteurs qui limitent la croissance des truites arc-en-ciel (Oncorhynchus mykiss) dans un grand cours d'eau en aval d'un barrage. Nous examinons l'importance relative de la temperature, de la quantite de proies et de la taille des proies en elaborant des scenarios dans lesquels ces variables, individuellement ou combinees, sont modifiees. La croissance predite concorde avec les estimations empiriques du poids selon l'age, particulierement pour les groupes d'ages plus jeunes, ce qui demontre que le modele decrit avec exactitude l'effet de l'interaction de la temperature et des conditions relatives aux proies sur la croissance des truites arc-en-ciel. Des scenarios de modelisation qui gonflent artificiellement la taille et l'abondance des proies demontrent que la croissance des truites arc-en-ciel est limitee par la rarete de grosses proies et la disponibilite globale de proies. Par exemple, le fait de transferer 10 % de la biomasse de proies dans la classe de longueur de 13 mm (grande) sans accroitre la biomasse globale de proies se traduit par une augmentation de 88 % du poids maximum durant la vie des truites arc-en-ciel. En outre, des temperatures plus chaudes se traduisent par une croissance predite plus faible pour un degre de disponibilite de proies egal ou plus faible que le degre actuel; cependant, la croissance est semblable pour toutes les temperatures pour des degres de disponibilite de proies plus eleves. Les changements climatiques modifieront vraisemblablement les regimes d'ecoulement et de temperature dans les grandes rivieres, entrainant des changements correspondants aux ressources de proies invertebrees utilisees par les poissons. L'application elargie de modeles bioenergetiques d'approvisionnement a la derive pour etablir une comprehension mecaniste de l'influence des modifications des conditions de l'habitat et des ressources en proies sur la croissance des salmonides sera utile pour la gestion des peches en aval de barrages. [Traduit par la Redaction], Introduction Tailwaters below large dams are often characterized by cold and clear hypolimnetic water releases, creating habitat conditions ideal for salmonids. These conditions support salmonid populations that serve as economically [...]

Rainbow trout (Oncorhynchus mykiss) have been purposely introduced in many regulated rivers, with inadvertent consequences on native fishes. We describe how trout growth rates and condition could be influencing trout population dynamics in a 130 km section of the Colorado River below Glen Canyon Dam based on a large-scale mark-recapture program where ~8000 rainbow trout were recaptured over a 3-year period (2012-2014). There were strong temporal and spatial variations in growth in both length and weight as predicted from von Bertalanffy and bioenergetic models, respectively. There was more evidence for seasonal variation in the growth coefficient and annual variation in the asymptotic length. Bioenergetic models showed more variability for growth in weight across seasons and years than across reaches. These patterns were consistent with strong seasonal variation in invertebrate drift and effects of turbidity on foraging efficiency. Highest growth rates and relative condition occurred in downstream reaches with lower trout densities. Results indicate that reduction in rainbow trout abundance in Glen Canyon will likely increase trout size in the tailwater fishery and may reduce downstream dispersal into Grand Canyon. Les truites arc-en-ciel (Oncorhynchus mykiss) ont ete deliberement introduites dans de nombreuses rivieres regularisees, entrainant des consequences imprevues sur les poissons indigenes. Nous decrivons l'influence potentielle des taux de croissance et de l'embonpoint des truites sur la dynamique de la population de truites dans un troncon de 130 km du fleuve Colorado en aval du Glen Canyon, a la lumiere d'une campagne de marquage-recapture a grande echelle dans le cadre duquel ~8000 truites arc-en-ciel ont ete reprises sur une periode de 3 ans (2012-2014). Des variations temporelles et spatiales marquees de la croissance de la longueur et du poids concordant avec les predictions des modeles de von Bertalanffy et bioenergetiques, respectivement, ont ete notees. Le coefficient de croissance et la variation annuelle de la longueur asymptotique constituaient d'autres indices de variations saisonnieres. Les modeles bioenergetiques ont montre une plus grande variabilite de la croissance en poids d'une saison et d'une annee a l'autre qu'entre les differents troncons. Ces motifs concordent avec de fortes variations saisonnieres de la derive d'invertebres et des effets de la turbidite sur l'efficacite de l'approvisionnement. Les taux de croissance et l'embonpoint les plus grands ont ete observes dans les troncons inferieurs caracterises par des densites de truites plus faibles. Les resultats indiquent que la reduction de l'abondance des truites arc-en-ciel dans le Glen Canyon se traduira vraisemblablement par l'augmentation de la taille des truites dans la peche en aval et pourrait en reduire la dispersion dans le Grand Canyon. [Traduit par la Redaction], Introduction Body size and growth exert a strong influence on fish population dynamics (Xu et al. 2010), and growth rate is often used as a surrogate measure of fitness (Schluter [...]

Humans have exaggerated natural habitat fragmentation, negatively impacting species dispersal and reducing population connectivity. Habitat fragmentation can be especially detrimental in freshwater populations, whose dispersal is already constrained by the river network structure. Aquatic insects, for instance, are generally limited to two primary modes of dispersal: downstream drift in the aquatic juvenile life stages and flight during the terrestrial winged adult stage. Yet the impacts of large hydropower dams can make rivers uninhabitable for incoming (drifting) juvenile insects, with remaining refugia found only in tributaries. The ability of adult aquatic insects to traverse such river stretches in search of suitable tributary habitat likely depends on factors such as species‐specific dispersal ability and distance between tributaries. To explore the intersection of natural and human‐induced habitat fragmentation on aquatic insect dispersal ability, we quantified population genetics of three taxa with varying dispersal abilities, a caddisfly (Hydropsychidae, Hydropsyche oslari), a mayfly (Baetidae: Fallceon quilleri), and a water strider (Veliidae: Rhagovelia distincta), throughout tributaries of the Colorado River in the Grand Canyon, Arizona, USA. Using 2bRAD reduced genome sequencing and landscape genetics analyses, we revealed a strong pattern of isolation by distance among mayfly populations. This contrasts with caddisfly and water strider populations, which were largely panmictic. Analysis of thousands of informative single nucleotide polymorphisms showed that realized dispersal ability may not be accurately predicted by species traits for these widespread species. Principal components analysis revealed a strong division between caddisfly populations upstream and downstream of Havasu Creek (279 km through the 390 km study reach), suggesting that the geography of the Grand Canyon imposes a dispersal barrier for this species. Our use of genetic tools in the Grand Canyon to understand population structure has enabled us to elucidate dispersal barriers for aquatic insects. Ultimately, these data may be useful in informing effective conservation management plans for understudied organisms of conservation interest. [ABSTRACT FROM AUTHOR]

Most of the world's large rivers are dammed for the purposes of water storage, flood control, and power production. Damming rivers fundamentally alters water temperature and flows in tailwater ecosystems, which in turn affects the presence and abundance of downstream biota.We collaborated with more than 200 citizen scientists to collect 2,194 light trap samples across 2 years and more than 2,000 river km. Samples contained 16,222 net‐spinning caddisfly (Hydropsyche) individuals across six species. We used these data to model the distribution of Hydropsyche throughout the Colorado River Basin in the western U.S.A. to identify the roles of water temperature, flows, and species‐specific morphology in determining aquatic species distributions throughout a large arid watershed that has been heavily altered by damming.We predicted that water temperatures would determine Hydropsyche presence and abundance to a greater extent than diel variation in river stage associated with hydropower production. Among many species, adult female Hydropsychids are morphologically adapted to swim to deep‐water oviposition sites. We predicted that the presence of this ability would negate the otherwise deleterious effects of high stage change on caddisfly egg mortality.We found that distributions of the two most widespread species, Hydropsycheoccidentalis and Hydropsyche oslari (92% of total Hydropsyche captured), were both predicted by water temperatures. However, we also found that the abundance of H. oslari decreased by as much as 10‐fold as diel stage change increased, despite the presence of female morphological adaptations for deep‐water oviposition. We found sexual dimorphism and evidence for deep‐water swimming adaptations in 5/6 species.Our results show that net‐spinning caddisflies have species‐specific responses to environmental variation and suggest that environmental flows designed to reduce diel stage change and destabilise water temperatures may improve habitat quality for these ubiquitous and important aquatic insects. [ABSTRACT FROM AUTHOR]

Structural connectivity and dispersal ability are important constraints on functional connectivity among populations. For aquatic organisms that disperse among stream corridors, the regional structure of a river network can, thus, define the boundaries of gene flow. In this study, we used mitochondrial DNA (mtCO1 barcoding gene) to examine the genetic diversity and population structure of a caddisfly with strong dispersal capabilities, Hydropsyche oslari (Trichoptera:Hydropsychidae), in the topologically-diverse Colorado River Basin. We expected to find less genetic differentiation among populations of H. oslari within the Upper Basin, which has a dense dendritic network of perennial tributaries that allow for greater potential dispersal and gene flow, than among populations within the arid and sparse river network of the Lower Basin. We also expected to find genetic differentiation among H. oslari in the Upper and Lower Basins because contemporary populations are geographically distant from each other and have been separated by a >300-km-long reservoir (Lake Powell) for ½ a century. Consistent with these predictions, we found that populations of H. oslari within the Upper Basin had more shared haplotypes and less nucleotide diversity (π = 0.001–0.008) than H. oslari within the Lower Basin (F ST = 0.01, π = 0.014–0.028). However, populations were genetically more structured in the Upper Basin (F ST = 0.47) than in the Lower Basin (F ST = 0.01). We also found that populations in the Upper and Lower Basin are entirely genetically differentiated (S nn = 1), suggesting that these 2 populations were isolated thousands of years before the 1963 closure of Glen Canyon Dam and subsequent filling of Lake Powell. The most similar haplotypes among the 2 basins represent a 5.4% difference, which indicates the presence of a species complex within H. oslari. [ABSTRACT FROM AUTHOR]