Nathan Franssen

U.S. Fish and Wildlife Service · Dexter Fish Technology Center

Fish assemblage structure, rarefied species richness, species diversity and evenness of assemblages upstream of a reservoir in Oklahoma, U.S.A., were compared pre and post-impoundment as well as in contemporary collections from streams above and below the reservoir. There were significant shifts in assemblage structure between historical and contem...

Understanding population-level responses to novel selective pressures can elucidate evolutionary consequences of human-altered habitats. Stream impoundments (reservoirs) alter riverine ecosystems worldwide, exposing stream fishes to uncommon selective pressures. Assessing phenotypic trait divergence in reservoir habitats will be a first step in ide...

Anthropogenic habitat alteration creates novel environments that can alter selection pressures. Construction of reservoirs worldwide has disturbed riverine ecosystems by altering biotic and abiotic environments of impounded streams. Changes to fish communities in impoundments are well documented, but effects of those changes on native species persi...

Understanding how altered flow regimes mediate interactions among native and nonnative species is necessary for the conservation of aquatic systems. Anthropogenic alteration of natural flows and establishment of nonnative fishes coincided with near extirpation of Colorado pikeminnow (Ptychocheilus lucius) from the San Juan River, NM, USA. Despite m...

Anthropogenic influences have disproportionally affected freshwater ecosystems, and a loss of biodiversity is forecasted to greatly reduce ecosystem function and services. Loss of species may destabilize communities by limiting the stabilizing forces of compensatory dynamics and/or statistical averaging, both of which are effects that can buffer va...

Environmental temperature shapes the ontogeny of ectotherms by influencing rates of growth and development which can be key determinants of survival. Whereas the escalating impacts of water management on freshwater ecosystems is well documented, the effects of cold‐water releases from dams—which can alter downstream temperatures—remains relatively...

Fish passages are constructed to facilitate movement around barriers, but few are quantitatively evaluated for non-salmonids. We quantified the efficiency of a selective, nature-like fish passage for three native fishes, Colorado pikeminnow (Ptychocheilus lucius), flannelmouth sucker (Catostomus latipinnis), and razorback sucker (Xyrauchen texanus)...

A poor understanding of factors leading to species decline can result in inefficient or ineffective species restoration. Endangered Colorado pikeminnow ( Ptychocheilus lucius) was nearly extirpated from the San Juan River, NM, USA and recent efforts to reestablish the species via hatchery augmentation of juveniles has yet to reach the target number...

Time-series data offer wide-ranging opportunities to test hypotheses about the physical and biological factors that influence species abundances. Although sophisticated models have been developed and applied to analyze abundance time series, they require information about species detectability that is often unavailable. We propose that in many case...

A poor understanding of factors leading to species decline can result in inefficient or ineffective species restoration. Endangered Colorado Pikeminnow ( Ptychocheilus lucius) was nearly extirpated from the San Juan River, NM, USA and recent efforts to reestablish the species via hatchery augmentation of juveniles has yet to reach the targeted numb...

Extensive efforts to reduce the population of invasive channel catfish (Ictalurus punctatus) in the San Juan River, USA, have increased the relative abundance of juveniles, with little understanding about potential trophic interactions between these juveniles and native fishes. Gut contents of juvenile (<250 mm total length) and adult (300 mm) chan...

Nonnative species are often perceived to cause the decline or impede management and recovery of native species, yet the ability to quantify the ecological impacts of nonnative species is often difficult. Disentangling the consequences of other stressors (e.g., habitat loss, climate change) and nonnative invasions on native fish communities might be...

Understanding the effects of hydrology on fish populations is essential to managing for native fish conservation. However, despite decades of research illustrating streamflow influences on fish habitat, reproduction, and survival, biologists remain challenged when tasked with predicting how fish populations will respond to changes in flow regimes....

• Fishes with periodic life histories are long-lived, slow to mature, and have intermittent periods of successful recruitment, limiting the ability of managers to recover their populations rapidly. • Endangered razorback sucker (Xyrauchen texanus) in the Colorado River Basin, USA, is a periodic strategist whose persistence is largely dependent on h...

Motivation: We compiled a global database of long-term riverine fish surveys from 46 regional and national monitoring programmes and from individual academic research efforts, with which numerous basic and applied questions in ecology and global change research can be explored. Such spatially and temporally extensive datasets have been lacking for...

Hatchery augmentation of dwindling wild populations has become a common strategy to avoid extinction of endangered fishes. While benign hatchery conditions can lead to low persistence of stocked individuals, experiments have demonstrated flow‐conditioning Razorback Sucker Xyrauchen texanus can increase swimming performance and reduce downstream mov...

The establishment of nonnative predators can have devastating consequences for native fish communities, but predation rates are often difficult to quantify due to spatial and temporal variation in predator foraging behavior. Predation by Channel Catfish Ictalurus punctatus throughout the Colorado River basin potentially threatens the recovery of na...

Distinguishing hatchery‐reared fishes from wild conspecifics can be required to quantify the success of augmentation programmes. This study estimated the probability of identifying calcein‐marked, hatchery‐reared Colorado pikeminnow Ptychocheilus lucius Girard from external and internal structures. Both control and marked fish held in the laborator...

• Disruption of ecosystems by human activities has caused worldwide extinction threats, which has prompted conservationists to implement captive breeding programmes that aid the recovery of imperilled species. Understanding factors that limit the survival of hatchery‐spawned fishes after stocking is critical to future conservation efforts using cap...

Hatcheries and stocking programs have become necessary to repatriate or augment populations of imperiled fishes worldwide. Over nearly two decades, millions of endangered juvenile Colorado Pikeminnow Ptychocheilus lucius have been stocked into the San Juan River (CO, NM and UT, USA); however, recruitment of these individuals to adult life stages (a...

Over evolutionary time, predator-prey interactions have shaped and constrained functional and behavioral traits of piscivorous fishes. The endangered Colorado Pikeminnow Ptychocheilus lucius, a large endemic piscivore of the Colorado River Basin, encounters a substantially altered prey base that differs in behaviors and morphologies compared to the...

Human transformation of aquatic systems and the introduction of nonnative species increasingly threaten the persistence of imperiled freshwater fishes. In response, large‐scale mechanical removal of nonnative fishes has been implemented throughout parts of the Colorado River Basin to aid recovery endangered fishes, but effects of these efforts can...

Stable isotope ecology has made great strides in quantifying energy transfer through food webs. However, trophic inferences gleaned from field-collected data can be limited when isotopic turnover and isotopic discrimination factors (Δ¹³C or Δ¹⁵N) are unknown. We quantified isotopic turnover and discrimination factors using an isotopic diet switch i...

Adaptive phenotypic divergence can arise when environments vary in ways favoring alternative phenotypic optima. In aquatic habitats, the costs of locomotion are expected to increase with water velocity, generally favoring a more streamlined body and the reduction of traits that produce drag. However, because streamlining in fish may come at the cos...

Imperilment of native fishes worldwide, and particularly in the American Southwest, has prompted management actions to protect and recover threatened populations. Implementation of management activities, however, often proceeds without clear understandings of ecological interactions between native fishes and other biotic and physical components of...

Understanding how habitat heterogeneity influences the structure of communities has been a longstanding goal of ecologists. Identifying how stream channel complexity affects fishes will be particularly important in systems simplified by anthropogenic activities and encroachment of non‐native riparian vegetation. Here, we assessed how large‐scale lo...

Establishment of nonnative fishes has contributed to the decline of native fishes worldwide. Efficacy of mechanical removal of nonnative fishes in large streams has been difficult to ascertain, and responses by native fishes after removal is equivocal. We summarize results of efforts on the San Juan River, New Mexico, Colorado, and Utah, to suppres...

Colorado pikeminnow (Ptychocheilus lucius) has been extirpated from a large portion of its historical range in the Colorado River basin, USA. A repatriation effort via stocking of juvenile P. lucius in the San Juan River, NM, CO and UT has resulted in limited recruitment of individuals into an adult population. Understanding biotic and abiotic fact...

Disturbance of freshwater ecosystems through cultural eutrophication has resulted in an increased global occurrence of harmful algal blooms (HABs). Ecosystem disrupting algal blooms (EDABs) are a subset of HABs that produce extensive disturbances across entire ecosystems. Prymnesium parvum is an EDAB species that has invaded freshwater systems worl...

Understanding patterns of animal distribution and abundance based on their movements is important to identify the habitats and factors that maximize growth and reproductive success. Despite stocking age-0 hatchery-reared Colorado Pikeminnow Ptychocheilus lucius for over 10 years in the San Juan River of Colorado, New Mexico, and Utah, the populatio...

Associations between the morphology of animals and their ecology have contributed to our understanding of phenotypic diversity by helping to relate form and function. Most early studies on fishes used traditional measurements of linear distances on the body or fins to quantify morphological variation among taxa. More recently, geometric morphometri...

Understanding population-level responses to human-induced changes to habitats can elucidate the evolutionary consequences of rapid habitat alteration. Reservoirs constructed on streams expose stream fishes to novel selective pressures in these habitats. Assessing the drivers of trait divergence facilitated by these habitats will help identify evolu...

Predators have long been recognised as important drivers of community structure, and there is a growing literature that suggests predation can affect invasion success by limiting survival or recruitment of potential invaders. Effects of apex predators are well known, but there is increasing evidence that mesopredators may also affect invasion succe...

The costs and benefits of investing in expensive sensory systems are shaped by environments that vary in the ease with which sensory information can be accessed. Fish provide an excellent model system in which to address questions of sensory evolution; while fishes rely heavily on vision, their visual environment is far more diverse and challenging...

Rivers and streams are among the most threatened ecosystems worldwide, and their fish assemblages have been modified by anthropogenic habitat alteration and introductions of non-native species. Consequently, two frequently observed patterns of assemblage change over time are species loss and biotic homogenization. In the present study, we compared...

1. Reservoirs modify riverine ecosystems worldwide, and often with deleterious impacts on native biota. The immediate effects of reservoirs on native fish species below dams and in impounded reaches have received considerable attention, but it is unclear how reservoirs may affect fish species at larger spatial and temporal scales. Documented declin...

Red shiners (Cyprinella lutrensis) are among the most widespread, ecologically general, and environmentally tolerant fish species in North America, and are highly invasive where they have been introduced outside their native range. However, long-term data on fish assemblages showed that red shiners gradually (1980s to 2006) disappeared from creeks...

Communication is shaped and constrained by the signaling environment. In aquatic habitats, turbidity can reduce both the quantity and quality of ambient light and has been implicated in the breakdown of visual signaling. Here, we examined the relationship between turbidity (quantified with long-term data) and the expression of carotenoid-based nupt...

La depredación intensa sobre peces en estadio larval y juvenil por piscívoros introducidos puede ser perjudicial para el reclutamiento de las especies nativas amenazadas o en peligro de extinción. El pez introducido lobina negra (Micropterus salmoides) en el río San Juan de los estados de Nuevo México, Colorado y Utah es raramente recolectado como...

Defining the trophic position of stream organisms is a first step in understanding the ecology of lotic systems. Whereas trophic positions of stream fishes have been traditionally assigned based on dietary analysis, stable isotope ratios may provide additional information on the validity of this approach and may be used to verify energy acquisition...

1] Nutrient dynamics in rivers are central to global biogeochemistry. We measured ammonium (NH 4 +) uptake, metabolism, nitrification, and denitrification in the thalweg, the river region of greatest flow, of the Kansas River (discharge = 14,360 L/s). We estimated gross and net uptake with a depleted 15 N-NH 4 + release, metabolism with diel O 2 me...

Abstract –  In the absence of other life-history constraints, fishes that can feed at low trophic levels (i.e., omnivores/detritivores) are predicted to be successful invaders because their food resources during the colonization and integration phases of the invasion are rarely limiting. Accordingly, we hypothesized that trophic position of non-nat...

1. Floods are major disturbances to stream ecosystems that can kill or displace organisms and modify habitats. Many studies have reported changes in fish assemblages after a single flood, but few studies have evaluated the importance of timing and intensity of floods on long‐term fish assemblage dynamics. 2. We used a 10‐year dataset to evaluate th...

Synopsis Spatial patterns of resource use by small-bodied fishes in the San Juan River were examined using stable isotopes. Using δ15N of fishes as an index of trophic position, our data suggest both native and non-native fishes primarily consumed macro-invertebrates. The δ13C of these fishes further suggested a detritus-based food web, from which...

Thesis (M.S.)--Kansas State University, 2006. Includes bibliographical references (leaves 20-25).