Lost River Sucker Deltistes Luxatus Research Articles

Does release size into net‐pens affect survival of captively reared juvenile endangered suckers in Upper Klamath Lake?

AbstractObjectiveHigh juvenile mortality prevents recruitment into the adult populations of endangered Shortnose Sucker Chasmistes brevirostris and Lost River Sucker Deltistes luxatus in Upper Klamath Lake, Oregon. To address the lack of recruitment, the U.S. Fish and Wildlife Service implemented the Sucker Assisted Rearing Program (SARP). Managers developing the rearing program lack information about how length at release relates to survival. To determine how initial length affects survival of captively reared juvenile suckers, we introduced juvenile suckers from the SARP into three net‐pens in Upper Klamath Lake.MethodsThe juvenile suckers ranged from 102 to 284 mm standard length, and each fish was tagged with a passive integrated transponder (PIT) tag. Fish were monitored continuously by PIT antennas and mortality was inferred when movements ceased.ResultEstimated survival over 57 days was high in all net‐pens (0.79–1.00) and remained high at two net‐pens for 76 and 86 days. Adjusted survival curves resulting from a stratified Cox model with standard length as a covariate, indicated that length positively influenced predicted survival by as much as 41% at one site. During the study, pH and dissolved oxygen regularly exceeded no‐effect thresholds at two sites and briefly reached lethal thresholds at the same two sites but did not coincide with the observed mortalities. Slower growth and the lowest survival were observed at the third site, where water quality never exceeded thresholds.ConclusionA larger release size and the location of the net‐pen can improve the survivability of juvenile suckers in net‐pens in Upper Klamath Lake.

Water and Endangered Fish in the Klamath River Basin: Do Upper Klamath Lake Surface Elevation and Water Quality Affect Adult Lost River and Shortnose Sucker Survival?

AbstractIn the western United States, water allocation decisions often incorporate the needs of endangered fish. In the Klamath River basin, an understanding of temporal variation in annual survival rates of Shortnose Suckers Chasmistes brevirostris and Lost River Suckers Deltistes luxatus and their relation to environmental drivers is critical to water management and sucker recovery. Extinction risk is high for these fish because most individuals in the populations are approaching their maximum life span and recruitment of new fish into the adult populations has never exceeded mortality losses in the past 22 years. We used a time series of mark–recapture data from the years 1999–2021 to analyze the relationship between lake level, water quality covariates, and survival of adult Shortnose Suckers and two spawning populations of Lost River Suckers in Upper Klamath Lake, Oregon. We compared competing model hypotheses in a maximum likelihood framework using Akaike's information criterion and then ran the top environmental covariates in a Bayesian framework to estimate how much of the variation in survival was explained by these covariates as compared to random variation. The complementary analyses found almost unequivocal support for our base model without environmental covariates. Estimated adult sucker survival was high across the time series and consistent with sucker life history (mean annual survival = 0.82–0.91). This suggests that adult suckers were generally robust to interannual variation in lake levels as well as consistently poor water quality within the years of our data set. Recovery time is limited, as a declining survival trend for adult suckers in recent years may be due to the onset of senescence. The successful recovery of suckers in Upper Klamath Lake may rely on shifting research from the causes of adult mortality and its relationship with lake surface elevation to the causes of poor recruitment into adult populations.

Effect of Temperature on Survival of Lost River Suckers with a Natural Infection of Ichthyobodo spp.

AbstractTo compensate for low natural survival of endangered Lost River Suckers Deltistes luxatus, the U.S. Fish and Wildlife Service and the Klamath Tribes have initiated captive rearing programs. We conducted laboratory experiments intended to determine the temperature for optimum growth of juvenile Lost River Suckers; however, due to an unanticipated infection with Ichthyobodo spp., we instead estimated survival in conjunction with temperature and parasite loads. Ichthyobodo spp. are common parasites that infest fish skin and gills; they are known to be present in Upper Klamath Lake and have been found in fish at both the federal and tribal fish‐rearing facilities. All fish held at mean temperatures of 16°C and 19°C survived the entire 65‐d experiment, whereas the median time to death was 27 d at 22°C, 8 d at 24°C, and 7 d at 26°C. Gill samples from all Lost River Suckers tested positive for Ichthyobodo spp. DNA, with a trend toward higher copy numbers in suckers that were held at a mean temperature of 22°C or warmer compared to those held at 19°C or cooler. Fish kept at all temperatures grew slowly. These results suggest that survival of Lost River Suckers with a natural infection of Ichthyobodo spp. was significantly impacted by temperature.

Successful Collection and Captive Rearing of Wild-Spawned Larval Klamath Suckers

AbstractShortnose Chasmistes brevirostris and Lost River Suckers Deltistes luxatus endemic to the Klamath River Basin on the California–Oregon border have experienced dramatic population declines in parallel with many other Catostomid species. Captive propagation has become a key element of many endangered fish recovery programs, although there is little evidence of their success in restoring or recovering fish populations. We initiated a novel rearing program for Klamath suckers in 2016 with the goal of developing a husbandry strategy that better balances the ecological, genetic, and demographic risks associated with captive propagation. We collected 4,306 wild-spawned Klamath sucker larvae from a major spawning tributary May–June 2016 and reared them at a geothermal facility established through a partnership with a local landowner and aquaculture expert. Mortality during collection was <1%. We reared larvae in glass aquaria for 17–78 d until they reached approximately 30 mm total length, upon which we moved them to round fiberglass tanks for 14–46 d or until reaching approximately 60 mm total length. Overall survival of larvae to ponding for final growout was 71%. Larval tank-rearing survival was 98% for 37 d until an isolated fish health incident affected three aquarium populations, reducing survival to transfer to 75%. Survival after transfer to round fiberglass tanks for 14–46 d was 94%. This study outlines the first successful collection and early life-history husbandry of wild-spawned endangered Klamath suckers that we are aware of.

Mortality of Endangered Juvenile Lost River Suckers Associated with Cyanobacteria Blooms in Mesocosms in Upper Klamath Lake, Oregon

AbstractUnsustainably high mortality within the first 2 years of life prevents endangered Lost River Suckers Deltistes luxatus in Upper Klamath Lake, Oregon, from recruiting to spawning populations. Massive blooms of the cyanobacterium Aphanizomenon flos‐aquae and their subsequent death and decay in the lake (bloom‐crashes) are associated with high pH, low percent oxygen saturation, high total ammonia concentrations, and spikes in the cyanotoxin microcystin. Poor water quality within the lake is considered the most likely cause of juvenile sucker mortality, but mechanisms causing the high mortality are not known. We introduced PIT‐tagged age‐1 Lost River suckers into three continuously monitored mesocosms in Upper Klamath Lake to determine the timing of juvenile sucker mortality relative to pH, temperature, and dissolved oxygen. Mortality was inferred from a lack of movement detected on remote PIT tag detection equipment within each mesocosm. Mortality was compared among mesocosms and an indoor tank‐held control group. We fitted time‐varying Cox hazard models to test hypotheses about short‐term and chronic effects of single and co‐occurring water quality parameters on the daily hazard rate. Presumed healthy or moribund fish that were collected pre‐season, mid‐season, or at the end of the study were examined macroscopically and histologically to generate inferences about the causes of mortality. Models did not indicate a plausible association between water quality variables and mortality. Hypoxia preceded periods of higher mortality at two of three sites but did not co‐occur with mortality. Hatchery‐reared Lost River Suckers confined to mesocosms may not represent the behavior of wild fish, and it is unclear whether the same factors affect the mortality of wild age‐0 suckers.

Evaluation of Practical Diets for Juvenile Lost River Suckers

AbstractA 9‐week feeding trial was conducted to evaluate one custom formulation and six commercially available diets when fed to juveniles of the endangered Lost River Sucker Deltistes luxatus. The seven treatments included the following: two catfish Ictalurus spp. diets, a tilapia Oreochromis spp. diet, a shrimp diet, a general warmwater fish diet, a general fish diet, and a feed formulated at the Abernathy Fish Technology Center (Abernathy sucker diet). The general fish diet produced fish that were significantly larger than the fish fed the other diets. The Lost River Suckers fed the general fish diet or the Abernathy sucker diet averaged 99.0% survival for the duration of the trial, which was significantly higher than in Lost River Suckers fed all other diets except for one catfish diet. Spinal deformities were significantly less frequent in fish fed the general fish diet and the Abernathy sucker diet. Whole‐body proximate and fatty acid composition were affected by diet. Whole‐body fatty acid profiles generally mirrored the diet profiles. The general fish diet appears to be an effective diet for juvenile Lost River Suckers, although the Abernathy sucker diet should be considered for further development as it yielded high survival and no incidence of spinal deformity.

Population Viability of Endangered Lost River Sucker and Shortnose Sucker and the Effects of Assisted Rearing

Abstract The Lost River Sucker Deltistes luxatus and Shortnose Sucker Chasmistes brevirostris are two narrowly endemic fish species in the upper Klamath Basin of southern Oregon and northern California. Both species have been federally listed as endangered pursuant to the U.S. Endangered Species Act since 1988 because of dramatic declines in abundance and distribution. In Upper Klamath Lake, Oregon, both species have only recruited a single cohort to the adult populations since that time. Most individuals in this population are at or older than the expected life span of the species. Consequently, the U.S. Fish and Wildlife Service and the Klamath Tribes have initiated assisted rearing efforts to stabilize the population. However, it is unclear how quickly these populations might become extirpated and how assisted rearing might alter population trajectories. We modeled the potential for extinction and recovery of the populations of endangered Lost River Sucker and Shortnose Sucker in Upper Klamath Lake. We simulated population trajectories over the next 50 y with a stochastic population viability assessment approach. Projections indicate that if population trajectories do not change, the Shortnose Sucker population may decline by 78% to number < 5,000 in 10 y and become completely extirpated within the next 30 (18.6% probability) to 40 y (99% probability). The two Lost River Sucker populations have a greater likelihood to remain extant after 50 y, with only 1% probability of extinction given our scenarios and assumptions, but the populations are likely to number fewer than 1,000 individuals. Our results also suggest that rearing of Klamath Lake sucker species in a controlled environment for augmenting the natural population will be effective in reducing extirpation probabilities over the next 50 y if survival to recruitment can be achieved, but a long-term effort of at least 40 y will be required. The necessity of long-term augmentation to ensure population persistence in the absence of natural recruitment underscores the urgent need to determine and address the causes of recruitment failure in the wild.

Colonial Waterbird Predation on Lost River and Shortnose Suckers in the Upper Klamath Basin

AbstractWe evaluated predation on Lost River Suckers Deltistes luxatus and Shortnose Suckers Chasmistes brevirostris by American white pelicans Pelecanus erythrorhynchos and double‐crested cormorants Phalacrocorax auritus nesting at mixed‐species colonies in the Upper Klamath Basin of Oregon and California during 2009–2014. Predation was evaluated by recovering (detecting) PIT tags from tagged fish on bird colonies and calculating minimum predation rates, as the percentage of available suckers consumed, adjusted for PIT tag detection probabilities but not deposition probabilities (i.e., probability an egested tag was deposited on‐ or off‐colony). Results indicate that impacts of avian predation varied by sucker species, age‐class (adult, juvenile), bird colony location, and year, demonstrating dynamic predator–prey interactions. Tagged suckers ranging in size from 72 to 730 mm were susceptible to cormorant or pelican predation; all but the largest Lost River Suckers were susceptible to bird predation. Minimum predation rate estimates ranged annually from <0.1% to 4.6% of the available PIT‐tagged Lost River Suckers and from <0.1% to 4.2% of the available Shortnose Suckers, and predation rates were consistently higher on suckers in Clear Lake Reservoir, California, than on suckers in Upper Klamath Lake, Oregon. There was evidence that bird predation on juvenile suckers (species unknown) in Upper Klamath Lake was higher than on adult suckers in Upper Klamath Lake, where minimum predation rates ranged annually from 5.7% to 8.4% of available juveniles. Results suggest that avian predation is a factor limiting the recovery of populations of Lost River and Shortnose suckers, particularly juvenile suckers in Upper Klamath Lake and adult suckers in Clear Lake Reservoir. Additional research is needed to measure predator‐specific PIT tag deposition probabilities (which, based on other published studies, could increase predation rates presented herein by a factor of roughly 2.0) and to better understand biotic and abiotic factors that regulate sucker susceptibility to bird predation.Received January 19, 2016; accepted June 25, 2016 Published online October 7, 2016

Evaluation of Dietary Lipid Sources for the Juvenile Lost River Sucker

AbstractThe establishment of a propagation program is among the recommended actions of the U.S. Fish and Wildlife Service recovery plan for the endangered Lost River Sucker Deltistes luxatus. As an initial step in diet development, a feeding trial was conducted to identify appropriate dietary lipid sources to feed juvenile Lost River Suckers at a rate of 0.47 g/fish. Six dietary treatments were produced by adding the following to a basal diet: (1) marine fish oil (Fish) as a control, (2) flaxseed oil (Flax), (3) soybean oil (Soy), or a 1:1 mixture of (4) fish and flax oil (Fish/Flax), (5) fish and soy oil (Fish/Soy), or (6) flax and soy oil (Flax/Soy). At the conclusion of the feeding trial weight gain of fish fed the control diet (Fish) was not significantly different than that of fish fed the other dietary treatments. However, weight gain of fish fed Soy was significantly higher than that of fish fed Fish/Flax. Whole‐body lipid was significantly higher in fish fed Soy than in fish fed Fish, Flax, Fish/Flax, or Fish/Soy, but not significantly different than that in fish fed Flax/Soy. Generally, whole body fatty acid profiles reflected the fatty acid profiles of their respective dietary treatments. These results suggest soybean oil is an appropriate dietary lipid source for the Lost River Sucker, but more research on quantitative dietary requirements is needed for successful propagation of this species.

Evaluation of First Feeds for Larval Lost River Suckers

AbstractThe Lost River Sucker Deltistes luxatus is endemic to the Klamath Basin in southern Oregon and northern California. Population declines led to listing this species as endangered in 1988. Its native habitat has been greatly reduced and degraded putting their future in question. If this species is to be reared in captivity to prevent extinction, the development of culture methodologies is necessary. Successful initial feeding methods are one of the first areas of culture that need to be developed. Lost River Suckers begin feeding during the larval life stage. Larval fish often require live prey at the onset of feeding; however, it may be possible to start larvae on formulated microdiets. We investigated the use of live Artemia and two commercial microdiets (Hikari and Otohime brands) as first feeds over the course of a 41‐d feeding trial. Starting on day 21 of the trial, feed sizes were increased, and the Artemia‐fed fish were offered a commercial microdiet (Otohime) while being co‐fed Artemia through day 26 of the trial (6 d cofeeding). After day 26 the Artemia‐fed fish were only offered the microdiet. The response variables of total length, weight, survival, whole‐body lipid content, and whole‐body fatty acid profile were measured during and at termination of the trial. Fish grew significantly faster when started on Artemia and also displayed significantly higher survival compared with fish fed only commercial microdiets. Fish started on Artemia transitioned well to Otohime B1, and maintained 95.2% (SD, 2.2) survival over the 41‐d feeding trial. The fatty acid profiles of the fish generally reflected the profiles of the diets they were fed. This study demonstrated that live Artemia followed by a transition to Otohime commercial feed is an effective feeding regimen for the duration of the larval phase of this species.

Effects of Lake Surface Elevation on Shoreline‐Spawning Lost River Suckers

AbstractWe analyzed remote detection data from PIT‐tagged Lost River Suckers Deltistes luxatus at four shoreline spawning areas in Upper Klamath Lake, Oregon, to determine whether spawning of this endangered species was affected by low water levels. Our investigation was motivated by the observation that the surface elevation of the lake during the 2010 spawning season was the lowest in 38 years. Irrigation withdrawals in 2009 that were not replenished by subsequent winter–spring inflows caused a reduction in available shoreline spawning habitat in 2010. We compared metrics of skipped spawning, movement among spawning areas, and spawning duration across 8 years (2006–2013) that had contrasting spring water levels. Some aspects of sucker spawning were similar in all years, including few individuals straying from the shoreline areas to spawning locations in lake tributaries and consistent effects of increasing water temperatures on the accumulation of fish at the spawning areas. During the extreme low water year of 2010, 14% fewer female and 8% fewer male suckers joined the shoreline spawning aggregation than in the other years. Both males and females visited fewer spawning areas within Upper Klamath Lake in 2010 than in other years, and the median duration at spawning areas in 2010 was at least 36% shorter for females and 20% shorter for males relative to other years. Given the imperiled status of the species and the declining abundance of the population in Upper Klamath Lake, any reduction in spawning success and egg production could negatively impact recovery efforts. Our results indicate that lake surface elevations above 1,262.3–1,262.5 m would be unlikely to limit the number of spawning fish and overall egg production.Received May 16, 2014; accepted January 29, 2015

Development of a quantitative assay to measure expression of transforming growth factor β (TGF-β) in Lost River sucker (Deltistes luxatus) and shortnose sucker (Chasmistes brevirostris) and evaluation of potential pitfalls in use with field-collected samples

The Nature Conservancy is in the process of restoring the Williamson River Delta in an attempt to recreate important juvenile habitat for the endangered shortnose sucker Chasmistes brevirostris and the endangered Lost River sucker Deltistes luxatus. Measurement of TGF-β mRNA expression level was one of the indicators chosen to evaluate juvenile sucker health during the restoration process. TGF-β mRNA expression level has been correlated with disease status in several laboratory studies and TGF-β mRNA expression level has been used as a species-specific indicator of immune status in field-based fish health assessments. We describe here the identification of TGF-β and a possible splice variant from shortnose sucker and from Lost River sucker. The performance of a quantitative RT-PCR assay to measure TGF-β mRNA expression level was evaluated in field-collected spleen and kidney tissue samples. The quality of extracted RNA was higher in tissues harvested in September compared to July and higher in tissues harvested at lower temperature compared to higher temperature. In addition, the expression level of both TGF-β and 18S as assessed by qRT-PCR was higher in samples with higher quality RNA. TGF-β mRNA expression was lower in kidney than in spleen in both Lost River sucker and shortnose sucker.

Tag Loss and Short‐Term Mortality Associated with Passive Integrated Transponder Tagging of Juvenile Lost River Suckers

AbstractPassive integrated transponder (PIT) tags are commonly used to mark small catostomids, but tag loss and the effect of tagging on mortality have not been assessed for juveniles of the endangered Lost River sucker Deltistes luxatus. I evaluated tag loss and short‐term (34‐d) mortality associated with the PIT tagging of juvenile Lost River suckers in the laboratory by using a completely randomized design and three treatment groups (PIT tagged, positive control, and control). An empty needle was inserted into each positive control fish, whereas control fish were handled but not tagged. Only one fish expelled its PIT tag. Mortality rate averaged 9.8 ± 3.4% (mean ± SD) for tagged fish; mortality was 0% for control and positive control fish. All tagging mortalities occurred in fish with standard lengths of 71 mm or less, and most of the mortalities occurred within 48 h of tagging. My results indicate that 12.45‐ × 2.02‐mm PIT tags provide a viable method of marking juvenile Lost River suckers that are 72 mm or larger.Received February 18, 2011; accepted July 27, 2011

Larval Fish Transport and Retention and the Importance of Location for Juvenile Fish Recruitment in Upper Klamath Lake, Oregon

AbstractIn the Upper Klamath Lake basin of Oregon, we applied the member‐vagrant hypothesis to larval lake fishes, documented marsh and hydrographic retention for larval shortnose suckers Chasmistes brevirostris and Lost River suckers Deltistes luxatus, and examined the effect of larval retention on the shoreline abundance of the juveniles of five species of fish in August. Emigration of larval suckers was highest from natal rivers, and immigration was highest in nonnatal areas downstream of the lake. Lake retention was facilitated by a wind‐generated gyre and advection by an eastern boundary current. Marshes and other shoreline irregularities acted as traps that slowed river and eastern boundary current advection and retained larvae, especially for shortnose suckers, for up to 3 weeks. The interaction of behavior, seasonal spawning, and seasonal system dynamics appears to drive retention patterns. For both species of suckers and three minnow species (fathead minnow Pimephales promelas, blue chub Gila coerulea, and tui chub G. bicolor), the abundance of juveniles along the shoreline increased with distance from the lake outlet and was weakly related to lake elevation, a surrogate for available habitat. For these five species, 45.5% of the variation in August juvenile abundance was determined by distance from the lake outlet. This work suggests that the nonbehavioral component of freshwater retention is complex and depends on hydrographic features, the boundary conditions of hydrographic features, and shoreline habitat.

Response of Larval Lost River and Shortnose Suckers to Wetland Restoration at the Williamson River Delta, Oregon

AbstractPopulation declines of the federally endangered Lost River sucker Deltistes luxatus and shortnose sucker Chasmistes brevirostris have been linked to several factors, including the loss of larval nursery habitat associated with lake fringe and riparian wetlands. Restoration of deltaic wetlands in the Williamson River delta, Oregon, is regarded as one strategy that may increase larval nursery habitat and survival. This study examined larval sucker presence, growth, development, and feeding in pilot restoration wetlands at the Williamson River delta to determine whether wetland restoration provides habitat that is conducive to larval rearing. We compared results from the restoration wetlands to those in riparian and lacustrine wetland reference sites during 2003 and 2004. The seasonal timing of larval Lost River and shortnose suckers captured in the restoration wetlands was similar to that in reference sites during the April‐July study period. The frequency of occurrence in the restoration wetlands was comparable to that in reference sites; larvae were collected in 90‐100% of transects during periods of peak abundance. These data are an indication that restored wetlands provide critical habitat for larval suckers. Larval sucker length, developmental phase, and gut fullness in the restoration wetlands indicated that larvae were feeding, growing, and hence, rearing, in those areas. Water temperatures in the restoration wetlands were 3‐4°C higher than those in reference sites, especially early in the season, which may have increased the restoration sites' suitability for larval rearing compared with reference sites. Our results indicate that initial wetland restoration efforts at the Williamson River delta have successfully created suitable rearing habitat for larval Lost River and shortnose suckers and suggest that further large‐scale wetland restoration in the delta will increase larval rearing opportunities and contribute to the recovery of these two endangered species.

Effects of Habitat Volume and Fathead Minnow Introduction on Larval Survival of Two Endangered Sucker Species in Upper Klamath Lake, Oregon

AbstractWe used laboratory and field studies to evaluate the degree to which the abundance of introduced fathead minnow Pimephales promelas and volume of nursery habitat were limiting factors for survival of larval shortnose suckers Chasmistes brevirostris and Lost River suckers Deltistes luxatus in Upper Klamath Lake. In 20‐h experiments in which we manipulated water depth and vegetation, fathead minnow fed on sucker larvae, but deeper water and the presence of vegetation reduced predation, suggesting that predator and prey densities and cover influenced predation. Older larvae had higher survival than younger larvae in shallow water, and vegetation increased survival for all larvae in shallow water. We also used larval production estimates from 11 years of field surveys to calculate annual larval sucker survival. Average annual larval sucker survival from 10 to 15 mm total length was about 18% (range, 4.4‐48.0%). The shoreline abundance of adult fathead minnow had a negative relationship with—and appeared to set an upper limit on—annual larval sucker survival, which was consistent with the density relationship found in laboratory studies. In addition, the average shoreline abundance of fathead minnow was about three times that of any other species and had negative relationships with the abundance of all other species and lake elevation. Recent management of Upper Klamath Lake has kept June lake elevations near the 1981‐2004 mean, but it is unclear whether fluctuations in the lake's fishes are due to lake management, to larger‐scale processes such as climate, to biotic interactions resulting from environmental changes, or to a combination of those factors. This study illustrates the need to better understand the biological trade‐offs with management‐manipulated variables such as lake elevation. In addition, to manage the competing demands on Upper Klamath Lake, it will be important to recognize that high survival of larval suckers will be less frequent than in the period preceding fathead minnow presence.

Patterns of Intra‐ and Interspecies Genetic Diversity in Klamath River Basin Suckers

AbstractThe Klamath River basin, Oregon, is home to four catostomid species: The Klamath smallscale suckerCatostomus rimiculus, the Klamath largescale suckerCatostomus snyderi, the shortnose suckerChasmistes brevirostris, and the Lost River suckerDeltistes luxatus. A decline in Lost River and shortnose sucker abundances resulted in these species being listed as endangered under the U.S. Endangered Species Act in 1988. The primary aims of this study were to measure interspecific and intraspecific genetic variation in Klamath River basin suckers by examining 15 microsatellite loci. Over 300 suckers were sampled from four subbasins within the Klamath River basin (Lost River, upper Klamath–Sprague rivers, lower Klamath River, and upper Williamson River) and one region outside of the basin (Rogue River). A hierarchical analysis of population subdivision based on groups generated from microsatellite genetic distance (RST) data was performed by use of analysis of molecular variance. Assignment of individuals to known populations was performed with the program WHICHLOCI. The program STRUCTURE was used to assign individuals probabilistically to homogeneous clusters without population or species identifiers. Klamath smallscale and Lost River suckers were genetically distinct from each other and from shortnose and Klamath largescale suckers. Sympatrically occurring shortnose and Klamath largescale suckers from the Lost and upper Klamath–Sprague River subbasins were genetically indistinguishable from one another, which indicates that interspecific hybridization has occurred between these two species. Managing for the long‐term survival and recovery of endangered Klamath River basin suckers should focus on preserving the genetic diversity of all four sucker species. Continued protection of endangered shortnose suckers requires the protection of Klamath largescale suckers, since each of these species may be a crucial contributor to the evolutionary legacy of the other. Maintenance of healthy and abundant sucker spawning and rearing habitat should minimize the chance of losing genetically unique populations while maintaining genetically diverse populations and facilitating the recovery of all Klamath River basin suckers.

Associations between Water Quality and Daily Growth of Juvenile Shortnose and Lost River Suckers in Upper Klamath Lake, Oregon

Poor water quality from hypereutrophic Upper Klamath Lake in south-central Oregon has been suspected of contributing to the recruitment failure of two endangered endemic fish species, the Lost River sucker Deltistes luxatus and the shortnose sucker Chasmistes brevirostris. We used otolith daily increment widths as a proxy for juvenile somatic growth to construct two growth models: (1) a linear mixed-effects (LME) model examining the lifetime effects of lakewide averages of potentially stressful daytime water temperature, pH, and nighttime dissolved oxygen (DO), and (2) a simple linear regression model examining the effects of locally measured water temperature, pH, and daytime DO on growth of fish over 3 d before the fish's capture. Graphical relationships between daily growth and biweekly un-ionized ammonia failed to show a sublethal effect on the growth of suckers captured in areas where un-ionized ammonia surpassed levels lethal to both species. For both species, our LME models indicated that at temperatures greater than approximately 22°C, low nighttime DO (less than 4 mg/L for Lost River suckers and less than 1 mg/L for shortnose suckers) caused enough stress to reduce growth, whereas at temperatures less than approximately 22°C, any stress from low nighttime DO was not reflected in reduced growth. We attribute the pattern to the species' tolerance of low DO, the short duration of nighttime events, the fish's increased oxygen demand at higher temperatures, and growth compensation due to increased food resources associated with low DO. The combination of low DO and high temperature has also been implicated in adult fish kills in Upper Klamath Lake. Because 34% of the time lakewide August average temperatures exceeded 22°C, extended periods of warm temperatures and high primary production could affect the sizes of recruits surviving into fall. Both growth models suggested that shortnose suckers might be more tolerant of poor water quality than Lost River suckers.

Subchronic Toxicity of Low Dissolved Oxygen Concentrations, Elevated pH, and Elevated Ammonia Concentrations to Lost River Suckers

Abstract The Lost River sucker Deltistes luxatus is a federally listed endangered fish species that inhabits hypereutrophic Upper Klamath Lake in southern Oregon. We conducted four subchronic toxicity tests with Lost River suckers by continuously exposing them to low dissolved oxygen (DO) concentrations for 14 d (late juveniles); elevated pH for 30 d (late larvae); elevated ammonia concentrations for 30 d (late larvae); and sublethal ammonia concentrations for 14 d followed by sublethal low DO concentrations for 14 d (early juveniles). Mortality thresholds were approximately 1.4 mg/L in the DO test, more than 10.0 in the pH test (the highest pH tested), and between 0.37 and 0.69 mg NH3-N/L in the ammonia test. Contrary to the common expectation for fish exposed to toxicants, Lost River suckers generally did not display sublethal responses (i.e., reduced growth, altered whole-body ion content, and reduced swimming performance) to low DO concentrations, elevated pH, or elevated ammonia concentrations. In th...

Effects of Ambient Water Quality on the Endangered Lost River Sucker in Upper Klamath Lake, Oregon

Abstract Populations of the Lost River sucker Deltistes luxatus have declined so precipitously in the Upper Klamath Basin of Oregon and California that this fish was recently listed for federal protection as an endangered species. Although Upper Klamath Lake is a major refuge for this species, fish in the lake occasionally experience mass mortalities during summer and early fall. This field study was implemented to determine if fish mortalities resulted from degraded water quality conditions associated with seasonal blooms of phytoplankton, especially Aphanizomenon flos-aquae. Our results indicated that fish mortality did not always increase as water temperature, pH, and un-ionized ammonia concentration increased in Upper Klamath Lake. Little or no mortality occurred when these water quality variables attained their maximum values. On the other hand, an inverse relation existed between fish mortality and dissolved oxygen concentration. High mortality (>90%) occurred whenever dissolved oxygen concentration...