Pacific Northwest United States Research Articles

Soil Carbon Loss by Wind Erosion of Summer Fallow Fields in Washington's Dryland Wheat Region

Core Ideas Wind erosion results in loss of soil and associated C. Loss of C was measured from eroding fallow fields in eastern Washington. Eroded sediment was enriched in C compared with the parent soil. Carbon loss ranged from 0.4 to 18.5 kg C ha‐1 across 13 erosion events. Wind erosion of cropland negatively affects soil quality and productivity in the winter wheat (Triticum aestivum L.)–summer fallow (WW‐SF) region of the inland Pacific Northwest United States. Loss of soil diminishes the finite resource base and concurrent loss of soil organic C affects the inherent physical, chemical, and biological properties of the soil. This study aimed to quantify soil organic C loss from windblown summer fallow soils. Creep and Big Spring Number Eight samplers were used to trap sediment above an eroding soil during major wind events over an 8‐yr period. The C content of trapped sediment ranged from 6 to 11.6 g C per kg sediment. Enrichment ratios for C ranged from 0.6 to 1.9, indicating that the trapped sediment was generally enriched in C compared with the parent soil. Averaged across all sites and wind events, soil C loss from fields ranged from 0.4 to 18.5 kg C ha–1. The ongoing decline in soil organic C since the advent of dryland farming in the region 140 yr ago is most commonly attributed to degradation by microbes and oxidation. However, our data, combined with historic accounts of wind erosion in the WW‐SF region, strongly suggest that loss of soil organic C may also be caused by wind erosion. Conservation tillage has proven an effective strategy for reducing wind erosion in the WW‐SF region and thus farmers are encouraged to further adopt conservation tillage practices to retain soil C and thereby reduce the degradation of agricultural soils by wind erosion.

Managing Moist Forests of the Pacific Northwest United States for Climate Positive Outcomes

The moist forests of the Pacific Northwest United States (PNW) are among the most naturally carbon rich ecoregions in the world. However, regional in-forest carbon storage levels are currently well below ecological potential. Recent climate policy proposals have renewed and deepened debates over forest sector climate strategies. This paper begins with a review of regionally applicable forest carbon life cycle assessments (LCAs) in an effort to provide some clarity around how these studies are conducted, and why their results may vary. The review highlights the importance of assumptions made during carbon accounting across the wood product lifespan and how the inclusion or exclusions of variables, such as product substitution and leakage, influence study results and subsequent management recommendations. Next we discuss the influence of climate change on forest management and planning. We conclude with a review of regional-specific factors to consider when developing optimal forest climate strategies in the moist forests of the PNW. These strategies include, but are not limited to; extending harvest rotations, shelterwood and select tree harvests (in lieu of full harvest), and managing forests for increased structural, age, and species complexity.

Long-term effects of vegetation management on biomass stock of four coniferous species in the Pacific Northwest United States

Silvicultural treatments applied during the early stages of stand development can have long-lasting impacts on forest ecosystem structure. Forest vegetation management (VM) is an important component of many reforestation programs and although several studies have demonstrated the benefits of VM on planted conifer growth and survival, few reports have been published on the long-term effects of VM treatments on total ecosystem biomass accumulation. In this study we assessed the effects of two contrasting VM treatments on total tree and ecosystem biomass stock for Douglas-fir, western hemlock, western redcedar, and grand fir growing in Oregon’s central Coast Range (CR) and Douglas-fir and western redcedar growing in Oregon’s Cascade foothills (CF). The assessments were made at age 16 years, 11 years after treatment application ended. The study contained two vegetation management treatments: control (C) and vegetation management (VM). Both the C and VM plots received a pre-planting herbicide site preparation treatment. The VM plots had, additionally, sustained vegetation control using herbicides during the first 5 years after planting. At age 16 years, at the CR site, the VM treatment increased the biomass stock of crop trees by 26.5, 91.2, 44.7, and 96.1 Mg ha−1 for Douglas-fir, western hemlock, western redcedar, and grand fir, respectively. At the same age, at the CF site, the VM treatment increased crop tree biomass stock by 48.1 Mg ha−1 for Douglas-fir and 42.2 Mg ha−1 for western redcedar. When other ecosystem components were considered, however, total ecosystem biomass did not differ between C and VM treated plots for western hemlock, western redcedar and grand fir at the CR site largely due to the development of an abundant hardwood midstory. On the other hand, VM treatments increased the ecosystem biomass stock of Douglas-fir and western redcedar at the site with a low abundance of hardwood midstory (CF site). Midstory biomass of C plots at the CR site averaged 52.9, 64.7, and 36.0 Mg ha−1, for western hemlock, western redcedar, and grand fir, respectively. At the CF site, midstory biomass of C plots averaged 1.2 and 5.9 Mg ha−1, for Douglas-fir and western redcedar, respectively. The results of this study demonstrate that sustained VM treatments during the first 5 years of stand establishment increases the biomass stock of crop trees, directing site resources towards planted crop trees.

Effect of Vegetation Management and Site Conditions on Volume, Biomass and Leaf Area Allometry of Four Coniferous Species in the Pacific Northwest United States

Allometric equations are useful tools for calculating tree and stand-level attributes, such as above-ground biomass or stem volume, using simple measurements that can be obtained from stand inventory data. These equations tend to be species-specific and can be affected by site conditions and silvicultural treatments. Forest vegetation management treatments (VM) are an important component of reforestation programs in the Pacific Northwest of the United States; however, no study has investigated the impact of these treatments on crop tree allometry. In this study we assessed the long-term effects of two contrasting VM treatments on the allometry of sixteen-year-old Douglas-fir, western hemlock, western redcedar, and grand fir trees growing in Oregon’s central Coast Range (CR) and fifteen-year-old Douglas-fir and western redcedar trees growing in Oregon’s Cascade foothills (CF). The VM treatments included a control which received only a pre-planting herbicide application and a VM treatment consisting of five consecutive years of vegetation control after planting. The equations developed in this study were species-specific and were not affected by VM with the exception of western redcedar foliage biomass. For western redcedar, tress of similar diameter had more foliage biomass when growing on plots without VM after planting. The allometry of Douglas-fir and western redcedar was also found to be affected by site, such that trees of similar diameter and height had larger stem volume when growing at the CR site than the CF site. This difference in stem volume was found to be the result of differences in stem tapering. There was a strong relationship between stand basal area and leaf area index that was the same for all species tested and was unaffected by site. The equations presented in this study are useful for calculating stem volume, leaf area and individual tree and component biomass for stands of the studied species that are of similar age.

Glacier Recession and the Response of Summer Streamflow in the Pacific Northwest United States, 1960–2099

AbstractThe Pacific Northwest is the most highly glacierized region in the conterminous United States (858 glaciers; 466 km2). These glaciers have displayed ubiquitous patterns of retreat since the 1980s mostly in response to warming air temperatures. Glacier melt provides water for downstream uses including agricultural water supply, hydroelectric power generation, and for ecological systems adapted to cold reliable streamflow. While changes in glacier area have been studied within the region over an extended period of time, the hydrologic consequences of these changes are not well defined. We applied a high‐resolution glacio‐hydrological model to predict glacier mass balance, glacier area, and river discharge for the period 1960–2099. Six river basins across the region were modeled to characterize the regional hydrological response to glacier change. Using these results, we generalized past and future glacier area change and discharge across the entire Pacific Northwest using a k‐means cluster analysis. Results show that the rate of regional glacier recession will increase, but the runoff from glacier melt and its relative contribution to streamflow display both positive and negative trends. In high‐elevation river basins enhanced glacier melt will buffer strong declines in seasonal snowpack and decreased late summer streamflow, before the glaciers become too small to support streamflow at historic levels later in the 21st century. Conversely, in lower‐elevation basins, smaller snowpack and the shrinkage of small glaciers result in continued reductions in summer streamflow.

Regional-scale management maps for forested areas of the Southeastern United States and the US Pacific Northwest

Forests in the United States are managed by multiple public and private entities making harmonization of available data and subsequent mapping of management challenging. We mapped four important types of forest management, production, ecological, passive, and preservation, at 250-meter spatial resolution in the Southeastern (SEUS) and Pacific Northwest (PNW) USA. Both ecologically and socio-economically dynamic regions, the SEUS and PNW forests represent, respectively, 22.0% and 10.4% of forests in the coterminous US. We built a random forest classifier using seasonal time-series analysis of 16 years of MODIS 16-day composite Enhanced Vegetation Index, and ancillary data containing forest ownership, roads, US Forest Service wilderness and forestry areas, proportion conifer and proportion riparian. The map accuracies for SEUS are 89% (10-fold cross-validation) and 67% (external validation) and PNW are 91% and 70% respectively with the same validation. The now publicly available forest management maps, probability surfaces for each management class and uncertainty layer for each region can be viewed and analysed in commercial and open-source GIS and remote sensing software.

Geoelectric Hazard Maps for the Pacific Northwest

AbstractMaps of extreme value, horizontal component geoelectric field amplitude are constructed for the Pacific Northwest United States (and parts of neighboring Canada). Multidecade long geoelectric field time series are calculated by convolving Earth surface impedance tensors from 71 discrete magnetotelluric survey sites across the region with historical 1‐min (2‐min Nyquist) geomagnetic variation time series obtained from two nearby observatories. After fitting statistical models to 1‐min geoelectric amplitudes realized during magnetic storms, extrapolations are made to estimate threshold amplitudes that are only exceeded, on average, once per century. One hundred‐year geoelectric exceedance amplitudes range from 0.06 V/km at a survey site in western Washington State to 9.47 V/km at a site in southeast British Columbia; 100‐year geoelectric exceedance amplitudes equal 7.10 V/km at a site north of Seattle and 2.28 V/km at a site north of Portland. Systematic and random errors are estimated to be less than 20%, much less than site‐to‐site differences in geoelectric amplitude that arise from site‐to‐site differences in surface impedance. Maps of 100‐year exceedance amplitudes are compared with the peak geoelectric amplitudes realized during the March 1989 magnetic superstorm; it is noted that some storms of relatively modest intensity can generate localized geoelectric fields of relatively high amplitude. The geography of geoelectric hazard across the Pacific Northwest is closely related to known geologic and tectonic structures.

Vector-Borne Viruses of Pulse Crops, With a Particular Emphasis on North American Cropping System

Due to their nutritional value and function as soil nitrogen fixers, production of pulses has been increasing markedly in the United States, notably in the dryland areas of the Northern Plains and the Pacific Northwest United States (NP&PNW).There are several insect-transmitted viruses that are prevalent and periodically injurious to pulse crops in the NP&PNW and elsewhere in North America. Others are currently of minor concern, occurring over limited areas or sporadically. Others are serious constraints for pulses elsewhere in the world and are not currently known in North America, but have the potential to be introduced with significant economic consequences. Managing plant viruses and the diseases they cause requires effective diagnostics, knowledge of virus vectors, virus transmission biology and ecology. A comprehensive compendium to inform producers and researchers about viruses currently and potentially affecting pulses in North America is needed. Here we provide an overview of insect transmitted viruses and their biology, followed by descriptions of the structure, infection biology, host ranges, symptoms, interspecific interactions, and current management options including host plant resistance and vector control for 33 viruses affecting or potentially affecting pulses in the United States and Canada. These are organized based on their transmission biology into persistently transmitted (families Geminiviridae, Luteoviridae and Nanoviridae), semi-persistantly transmitted (Secoviridae), and nonpersistantly transmitted (Betaflexiviridae, Bromoviridae and Potyviridae) viruses. We conclude with an overview of the principles of managing insect-transmitted viruses and an outline of areas requiring further research to improve management of viruses in pulses currently and into the future.

Adult stakeholder perceptions of 4th grade participation in children’s forest environmental education programming

An environmental education (EE) consortium located in the pacific northwest United States coalesced an external evaluation team to research its K-5 EE programming. The purpose of this article is to describe one phase of this program evaluation process including the exploration of adult stakeholder perceptions of EE programming accessed by 4th grade children. The author highlights the goals of the consortium and the context for the program evaluation, and the relevant literature addressing adult perspectives on EE is reviewed. The study framing, method, and qualitative results are reported. Findings suggest that adult stakeholders perceived positive impacts from 4th grade participation in EE, though the programming was viewed as being most beneficial to children coming from economically disadvantaged families. Implications for EE practice and research are discussed.

Evaluation of Multimedia Medication Reconciliation Software: A Randomized Controlled, Single-Blind Trial to Measure Diagnostic Accuracy for Discrepancy Detection.

The Veterans Affairs Portland Healthcare System developed a medication history collection software that displays prescription names and medication images. This article measures the frequency of medication discrepancy reporting using the medication history collection software and compares with the frequency of reporting using a paper-based process. This article also determines the accuracy of each method by comparing both strategies to a best possible medication history. Randomized, controlled, single-blind trial. Three community-based primary care clinics associated with the Veterans Affairs Portland Healthcare System: a 300-bed teaching facility and ambulatory care network serving Veteran soldiers in the Pacific Northwest United States. Of 212 patients with primary care appointments, 209 patients fulfilled the study requirements. Patients randomized to a software-directed medication history or a paper-based medication history. Randomization and allocation to treatment groups were performed using a computer-based random number generator. Assignments were placed in a sealed envelope and opened after participant consent. The research coordinator did not know or have access to the treatment assignment until the time of presentation. The primary analysis compared the discrepancy detection rates between groups with respect to the health record and a best possible medication history. Of 3,500 medications reviewed, we detected 1,435 discrepancies. Forty-six percent of those discrepancies were potentially high risk for causing an adverse drug event. There was no difference in detection rates between treatment arms. Software sensitivity was 83% and specificity was 91%; paper sensitivity was 81% and specificity was 94%. No participants were lost to follow-up. The medication history collection software is an efficient and scalable method for gathering a medication history and detecting high-risk discrepancies. Although it included medication images, the technology did not improve accuracy over a paper list when compared with a best possible medication history. ClinicalTrials.gov Identifier: NCT02135731.

Translating climate change policy into forest management practice in a multiple-use context: the role of ethics

Managers of public multiple-use landscapes are charged to balance a spectrum of interests and objectives, a task that has become increasingly challenging in light of global climate change. Forests supply a diverse array of social, economic, and environmental goods and benefits, but also stand to contribute to climate change mitigation by sequestering and storing carbon. The scientific dimensions of management decisions made against this backdrop are well appreciated, but their ethical complexity tends to be, at best, understated. Focusing on the issue of carbon storage for climate change mitigation in federal forests of the United States Pacific Northwest, we employ the method of argument analysis to highlight the role of normative or ethical judgments in multiple-use forest management. We demonstrate that such decisions are logically predicated on normative judgments about which public interests merit recognition and prioritization in the decision context. We show that a generalized commitment to multiple-use is insufficient as a normative basis for management decisions, and that more ethically explicit judgments are required to reach actionable conclusions about appropriate management objectives.

Effects of controlled atmosphere, edible coating, and 1-methylcyclopropene on improving storage quality of ‘Bartlett’ pears after long-term storage

ABSTRACTRecent trends towards greater fresh market use of ‘Bartlett’ pears has increased the need to extend its storage life to prolong the packing and marketing season in the United States Pacific Northwest region. Sixteen and 38%, respectively, of control fruit developed senescence disorders following 5 and 6 months of storage at −1.1°C. Commercial standard controlled atmosphere (CA) conditions (O2 at 1.5 kPa and CO2 < 1 kPa) or edible coating (Semperfresh™, SF) prevent the appearance of senescence disorders for 5 months, but 9% and 16% of fruit, respectively, developed senescence disorders after 6 months. The combination of CA+SF completely inhibited senescence disorders for 6 months. Treatment with CA and SF, alone or in combination, maintained high-storage quality and developed ripening capacity with characteristic melting texture during storage. Senescence disorders were inhibited for 6 months by 0.3 µL l−1 1-methylcyclopropene (1-MCP), alone or combination with CA or CA+SF. In part these pears developed ripening capacity after 6 months of storage. The combination of CA+SF+1-MCP maintained the highest storage quality with dark green colour and hard firmness, which might be associated and proportional with reductions in ethylene synthesis and respiration rate after long-term storage.

High prevalence of Helicobacter pylori clarithromycin resistance mutations among Seattle patients measured by droplet digital PCR.

Treatment of Helicobacter pylori infection is often empiric; however, current guidelines for management of Helicobacter pylori infection advise against the use of standard triple therapy (clarithromycin, amoxicillin, and proton-pump inhibitor) when clarithromycin resistance exceeds 20%. We developed and tested a new culture-free assay to detect clarithromycin resistance-conferring mutations to determine the prevalence of H.pylori clarithromycin resistance in patients from the United States Pacific Northwest. Droplet digital PCR (ddPCR) was used to detect the H.pylori 23S rRNA gene, and resistance-conferring mutations, in archived, formalin-fixed, paraffin-embedded (FFPE) gastric tissue and to retrospectively determine the prevalence of clarithromycin-resistant H.pylori among 110 patients at an academic medical center in the Northwest United States between 2012 and 2014. Of 102 patients with the H.pylori 23S rRNA gene detected by the ddPCR assay, 45 (44%) had clarithromycin resistance mutations. Thirty-three of the 45 patients with clarithromycin resistance mutations had a mix of wild-type and resistance alleles. Prevalence of clarithromycin resistance mutations differed among racial groups and was highest among Asians, with mutations detected in 14 (67%) of the 21 patient samples. The prevalence of clarithromycin resistance detected in this region exceeds 20%, indicating that standard triple therapy should not be the first-line antibiotic treatment for H.pylori infection. Culture-free assays for detecting clarithromycin resistance mutations can be performed on archived tissue samples and will aid in informing tailored treatment for effective H.pylori eradication.

Arbuscular Mycorrhizal Fungal Communities in Organic and Conventional Onion Crops in the Columbia Basin of the Pacific Northwest United States

Onions are highly responsive to arbuscular mycorrhizal fungi (AMF), but little is known about AMF communities in onion crops (∼10,000 ha) in the semiarid, irrigated region of the Columbia Basin of Washington and Oregon. AMF communities and root colonization were compared in organic and conventional onion fields, and between paired conventional fields that were fumigated or not with metam sodium. AMF were detected in all fields at all sampling times, with no differences in root colonization of onions used to bait soil from organic versus conventional and fumigated versus nonfumigated fields. However, AMF colonization of roots of onion plants sampled midsummer was greater in organic versus conventional fields (67 versus 51%) and less in fumigated versus nonfumigated conventional fields (45 versus 67%). Pyrosequencing identified four AMF genera (Glomus, Claroideoglomus, Paraglomus, and Diversispora) and four dominant operational taxonomic units (OTUs) (Glomus mosseae (Funneliformis mosseae), Glomus Whitfield type 17, Claroideoglomus lamellosum, and Glomus MO_G17). AMF community structure in roots of onion plants collected from crops midsummer was different in organic versus conventional crops, with greater AMF diversity and richness in organic than conventional crops. There was no effect of organic versus conventional crops on dominant OTUs, but several low-abundance OTUs in organic fields were not detected in conventional fields. There was no consistent effect of metam sodium chemigation on AMF communities in onion crops. Overall, cropping practices in organic versus conventional onion production, and the use of metam sodium soil fumigation by center-pivot chemigation do not appear to be major drivers of AMF communities.

Estimating aboveground live understory vegetation carbon in the United States

Despite the key role that understory vegetation plays in ecosystems and the terrestrial carbon cycle, it is often overlooked and has few quantitative measurements, especially at national scales. To understand the contribution of understory carbon to the United States (US) carbon budget, we developed an approach that relies on field measurements of understory vegetation cover and height on US Department of Agriculture Forest Service, Forest Inventory and Analysis (FIA) subplots. Allometric models were developed to estimate aboveground understory carbon. A spatial model based on stand characteristics and remotely sensed data was also applied to estimate understory carbon on all FIA plots. We found that most understory carbon was comprised of woody shrub species (64%), followed by nonwoody forbs and graminoid species (35%) and seedlings (1%). The largest estimates were found in temperate or warm humid locations such as the Pacific Northwest and southeastern US, thus following the same broad trend as aboveground tree biomass. The average understory aboveground carbon density was estimated to be 0.977 Mg ha−1, for a total estimate of 272 Tg carbon across all managed forest land in the US (approximately 2% of the total aboveground live tree carbon pool). This estimate is more than twice as low as previous FIA modeled estimates that did not rely on understory measurements, suggesting that this pool may currently be overestimated in US National Greenhouse Gas reporting.

Effects of Climatic Conditions and Management Practices on Agricultural Carbon and Water Budgets in the Inland Pacific Northwest USA

AbstractCropland is an important land cover influencing global carbon and water cycles. Variability of agricultural carbon and water fluxes depends on crop species, management practices, soil characteristics, and climatic conditions. In the context of climate change, it is critical to quantify the long‐term effects of these environmental drivers and farming activities on carbon and water dynamics. Twenty site‐years of carbon and water fluxes covering a large precipitation gradient and a variety of crop species and management practices were measured in the inland Pacific Northwest using the eddy covariance method. The rain‐fed fields were net carbon sinks, while the irrigated site was close to carbon neutral during the winter wheat crop years. Sites growing spring crops were either carbon sinks, sources, or neutral, varying with crops, rainfall zones, and tillage practices. Fluxes were more sensitive to variability in precipitation than temperature: annual carbon and water fluxes increased with the increasing precipitation while only respiration increased with temperature in the high‐rainfall area. Compared to a nearby rain‐fed site, irrigation improved winter wheat production but resulted in large losses of carbon and water to the atmosphere. Compared to conventional tillage, no‐till had significantly lower respiration but resulted in slightly lower yields and water use efficiency over 4 years. Under future climate change, it is expected that more carbon fixation by crops and evapotranspiration would occur in a warmer and wetter environment.

Identifying blueberry germplasm that is slow to get Blueberry shock virus in the Pacific Northwest United States

Identifying blueberry germplasm that is slow to get <i>Blueberry shock virus</i> in the Pacific Northwest United States

Pacific lamprey recolonization of a Pacific Northwest river following dam removal

AbstractRecolonization of Pacific lampreys Entosphenus tridentatus into historically used freshwater habitats in the United States Pacific Northwest was evaluated in the White Salmon River basin after removal of Condit Dam. Pacific lamprey population declines are of concern, and passage barrier removal is often recommended for conservation. Condit Dam on the White Salmon River in Washington was a complete barrier to fish migrating upstream for nearly 100 years, was breached in 2011, and was removed by 2012. Distribution of larval Pacific lampreys was estimated before and after removal of Condit Dam using either backpack or deepwater electrofishing. Larval detection probabilities were calculated for the basin, and sample efforts were refined to ensure at least 80% confidence that larvae were absent when not detected. Pacific lampreys were not present upstream of Condit Dam before it was removed but were present in areas downstream of the dam. After dam removal, Pacific lamprey larvae were collected upstream of the former dam site from four reaches of the mainstem White Salmon River, indicating a recent recolonization event. Pacific lampreys were absent from the river mouth area before the dam was removed but were found in newly created habitat at the mouth after dam removal. Pacific lampreys naturally recolonized the White Salmon River basin within a few years after dam removal. Removing dams and providing passage opportunity can allow Pacific lampreys to distribute into vacant areas and may help reverse population declines.

Precipitation extremes and their relation to climatic indices in the Pacific Northwest USA

There has been focus on the influence of climate indices on precipitation extremes in the literature. Current study presents the evaluation of the precipitation-based extremes in Columbia River Basin (CRB) in the Pacific Northwest USA. We first analyzed the precipitation-based extremes using statistically (ten GCMs) and dynamically downscaled (three GCMs) past and future climate projections. Seven precipitation-based indices that help inform about the flood duration/intensity are used. These indices help in attaining first-hand information on spatial and temporal scales for different service sectors including energy, agriculture, forestry etc. Evaluation of these indices is first performed in historical period (1971–2000) followed by analysis of their relation to large scale tele-connections. Further we mapped these indices over the area to evaluate the spatial variation of past and future extremes in downscaled and observational data. The analysis shows that high values of extreme indices are clustered in either western or northern parts of the basin for historical period whereas the northern part is experiencing higher degree of change in the indices for future scenario. The focus is also on evaluating the relation of these extreme indices to climate tele-connections in historical period to understand their relationship with extremes over CRB. Various climate indices are evaluated for their relationship using Principal Component Analysis (PCA) and Singular Value Decomposition (SVD). Results indicated that, out of 13 climate tele-connections used in the study, CRB is being most affected inversely by East Pacific (EP), Western Pacific (WP), East Atlantic (EA) and North Atlaentic Oscillation (NAO).

Effects of dietary lipid and light source on steatitis in steelhead, Oncorhynchus mykiss

The primary lipid source in feeds used at Pacific salmon (Oncorhynchus spp.) and steelhead (anadromous O. mykiss) conservation hatcheries in California and the Pacific Northwest United States is marine fish oil which is characterized by high levels of long-chain polyunsaturated fatty acids (LC-PUFAs). As these fatty acids are prone to peroxidation, the high marine fish oil content of salmonid feeds is likely a contributing factor in a disease called steatitis which is an ongoing issue at some hatcheries. Dermal lesions associated with steatitis have been observed when affected fish were transferred from enclosed hatchery buildings to outdoor rearing units exposed to sunlight. Therefore, a two-factor study examining possible interactive effects of dietary lipid source and ultraviolet (UV) radiation on growth responses, histology and tissue fatty acid profiles of juvenile steelhead was conducted. For 10weeks, fish were fed diets containing canola oil (CO), fish oil (FO) or oxidized fish oil (OFO) while exposed to fluorescent or UV light. Each treatment was randomly assigned to triplicate groups of 50 fish initially weighing 1.6g/fish. The main effects of lipid and light source on growth responses and survival were not statistically significant, but percent weight gain was significantly affected by interaction of the main effects. Whole body histopathology revealed significantly higher (more severe) steatitis scores in fish fed FO or OFO compared with fish fed CO and in fish exposed to UV light compared with fish exposed to fluorescent light. Whole body lipid concentration and tissue fatty acid profile were significantly affected by lipid and light source but not their interaction. These results suggest reducing dietary LC-PUFAs and minimizing UV light exposure may reduce steatitis in steelhead.