Lake trout suppression in Yellowstone Lake: developing benchmarks for harvest and a sampling design to measure efficacy
Introduced lake trout threaten to extirpate native Yellowstone cutthroat trout, a keystone species in the Yellowstone Lake ecosystem of Yellowstone National Park. A National Park Service (NPS) lake trout suppression program has been on-going since 1994; however, the effort has not resulted in a lake trout population decline. Consequently, recovery of the cutthroat trout is lacking. In August 2008, a panel of 15 independent scientists convened and evaluated the program. It was determined that because of the lack of an adequate monitoring design, existing data and analyses are insufficient for guiding the program. A top recommendation was that NPS address this issue and ultimately determine the level of harvest required to reduce lake trout abundance and set quantifiable benchmarks for the number of lake trout to be removed annually.
Statistical catch at age and matrix population models were used to assess the efficacy of the lake trout suppression program and quantify targets for exploitation and fishing effort. A large increase in fishing effort in 2014 resulted in high lake trout mortality and probably suppressed lake trout population growth. In 2014, fishing effort was 74,500 100-m net nights, which exceed the recommended target of 45,000 100-m net nights. The fishing effort in 2014 resulted in an instantaneous fishing mortality of 1.12 (0.98-1.26; 95% CI) and a population growth rate of 0.63 (0.44-0.82). Thus, lake trout abundance is predicted to decline if the amount of fishing effort in 2014 is maintained.
Electroshocking to induce mortality of lake trout embryos in Yellowstone Lake
Lake trout have been intentionally or inadvertently introduced into many lakes throughout the West, and their establishment often causes declines in native species abundances. For example, introduced lake trout threaten to extirpate native Yellowstone cutthroat trout in Yellowstone Lake, Yellowstone National Park. Consequently, it was deemed that suppression of the lake trout was needed to conserve Yellowstone cutthroat trout in Yellowstone Lake. Gillnetting is the primary method used to suppress lake trout in Yellowstone Lake and this method has been used since the program began in 1995. Unfortunately, lake trout are not the only fish species collected in gill nets. Some Yellowstone cutthroat trout are captured in gill nets and die; thus, the exploration of alternative methods to suppress lake trout to minimize bycatch of the targeted species is gaining popularity.
Currently, the use of electricity as an alternative suppression method has received considerable attention. An electrofishing grid was developed and implemented in 2013 in Swan Lake, Montana, that caused greater than 90% mortality in lake trout embryos up to 20 cm in the substrate. The electrofishing grid was also developed for the National Park Service, Yellowstone Lake, but never implemented because of the Federal Government shutdown in autumn 2013. Part of this project will be to experimentally evaluate the efficacy of the electrofishing grid in Yellowstone Lake. In addition, suction dredging and tarping spawning areas will be evaluated as novel methods for inducing mortality in lake trout embryos.
Reproductive readiness and behavioral ecology of wild hatchery-reared pallid sturgeon in the Missouri River above Fort Peck Reservoir, Montana
Pallid sturgeon Scaphirhynchus albus are an endangered species indigenous to the warm turbid waters of the Yellowstone, Missouri, and Mississippi rivers. The population declines observed in pallid sturgeon are probably a function of habitat alteration and fragmentation from the construction and operation of dams on the large rivers they inhabit. The pallid sturgeon population in the upper Missouri River, upstream of Fort Peck Reservoir, has experienced significant decline such that only a few (< 20) wild fish remain in the population. To augment the declining population, stocking of age-1 hatchery-reared pallid sturgeon produced from wild broodstock began in 1998 (i.e., 1997 year-class) to prevent extirpation of the species in the upper Missouri River. Whether stocked pallid sturgeon will reproduce or have similar behavior as wild-born pallid sturgeon during spawning migrations is unknown. The wild hatchery-reared (WHR) pallid sturgeon are reaching sexually maturity, which provides the opportunity to study how the reproductive behavior of mature WHR pallid sturgeon compares to wild born pallid sturgeon.
The objectives of this study are to 1) determine age and size of WHR pallid sturgeon at first sexual maturity, 2) determine the spawning periodicity of WHR pallid sturgeon, 3) determine if mature WHR pallid sturgeon use habitat and move similarly to wild-born adult pallid sturgeon, 4) determine if experimental discharge releases from upstream reservoirs provide a cue for pallid sturgeon to migrate further upstream during spawning migrations, and 5) locate spawning sites. To accomplish these objectives, six wild-born adults and 28 WHR pallid sturgeon have been implanted with radio transmitters and sampled for blood plasma sex steroid concentrations. Blood plasma sex steroid analysis shows that five of the 28 WHR pallid sturgeon have been identified as mature males. In 2014, a wild born reproductively active female was intensively followed in an attempt to locate a spawning site. The female moved a minimum of 552 km between peak discharge and when water temperature reached 24°C (5 June to 5 July), but when the female was recaptured she still contained her eggs. Histological analysis of eggs and sex steroid analysis did not reveal that the fish had undergone atresia prior to 4 July. The female will be recaptured in spring 2015 to determine if she underwent follicular atresia or spawned after the tracking period. Additional fish will be implanted with radio transmitters and tracked along with previously tagged fish in the spring and early summer of 2015 and 2016.
Density of pallid sturgeon and food web dynamics in the Missouri River: Inferences regarding carrying capacity and density-dependent response of pallid sturgeon to the contemporary stocking protocol
Pallid sturgeon have been stocked annually in the Missouri River below Ft. Peck Reservoir and the Yellowstone River since 1998. Survival estimates for hatchery-reared pallid sturgeon are relatively high. Thus, growing concern exists among biologist that they have stocked too many pallid sturgeon, thereby negatively influencing growth and survival of conspecifics and allospecifics. The effects of hatchery-reared pallid sturgeon on food-web dynamics is unknown. The objectives of this study are to 1) estimate density and standing stock of the pallid sturgeon population, 2) estimate survival rate of the hatchery-reared pallid sturgeon, 3) compare density estimates to estimates of hatchery-reared pallid sturgeon at-large from survival estimates and stocking history, 4) estimate production of the prey base (i.e., macroinvertebrates and small-bodied fishes), 5) assess the potential of food limitation for hatchery-reared pallid sturgeon, 6) use population and production models to estimate carrying capacity, and 7) compare estimated carrying capacity to estimated historical abundance. Capture-recapture models will be used to estimate abundance of pallid sturgeon. Habitat specific estimates of macroinvertebrate and small fish production will be combined with large-scale habitat quantification to estimate the amount of macroinvertebrate and small fish production that is potentially available for fish consumers over large reaches. Additionally, quantitative diets of the most abundant fish species in each trophic guild will be used to estimate the energetic demand of the fish assemblage. Combining supply and demand approaches will allow us to create quantitative food-webs to assess the amount of energy available to the current pallid sturgeon population after accounting for major consumers in the ecosystem. These results will be used to better manage pallid sturgeon by informing future stocking recommendations.
Evaluation of management actions in the Big Hole River basin on Arctic grayling relative abundance
In North America, Arctic Grayling distribution has been documented throughout Alaska and northern Canada west of the Hudson Bay, and in association with two disjunct populations in Michigan and the upper Missouri River Basin. Populations in Alaska and Canada are considered robust; however, Arctic Grayling were extirpated from Michigan in the 1930s and, in the Missouri River Basin, are limited to five isolated indigenous populations in Montana that occupy less than 5% of the historic range. Arctic Grayling in Montana are currently designated as sensitive species and have been considered for protection under the Endangered Species Act since 1982. Conservation efforts in Montana have largely focused on the Big Hole River population since abundance and distribution began to decline in the early 1980s. In 2006, a Candidate Conservation Agreement with Assurances program was established to facilitate conservation actions on non-federal properties in the upper watershed, and the program currently includes 30 participating landowners and about 61,000 hectares. Conservation actions have been implemented to remove or mitigate the effects of environmental conditions that are hypothesized to negatively influence Arctic Grayling abundance, occurrence, or both. Specific environmental conditions include interactions with non-native salmonids, degraded and fragmented habitat, reduced stream discharge, and increased stream temperature. However, little quantitative information exists that evaluates the influence of the environmental conditions on Arctic Grayling to support the current hypotheses or inform conservation actions. Arctic Grayling and environmental conditions have been measured throughout the upper Big Hole Watershed in conjunction with conservation efforts (1982 through 2014). Arctic Grayling abundance, non-native salmonid abundance, riparian habitat condition, stream discharge, and stream temperature data have been collected at varying spatial and temporal patterns and are available for exploratory analysis. Thus, the objective of this study is to use historical data to evaluate the relationships among Arctic Grayling abundance and occurrence, non-native salmonid abundance, riparian-habitat condition, stream discharge, and stream temperature. Evaluating relationships among these data is expected to provide a better understanding of Arctic Grayling ecology, evaluate hypotheses that currently guide management, inform managers of the conservation actions that probably provide the most benefit, and establish a foundation of knowledge to guide future research.
Evaluation of juvenile bull trout outmigration in Thompson Falls Reservoir
Habitat fragmentation caused by dams adversely affects the distribution and connectivity of fish populations. In 2008, the U.S. Fish and Wildlife Service concluded that the Thompson Falls Hydroelectric Project was adversely affecting bull trout. Understanding the effects of Thompson Falls Reservoir on the out-migration behavior and survival of juvenile bull trout may lead to the development of new procedures for the operation of Thompson Falls Dam to maximize the survival of out-migrating bull trout. Data collected in 2014 revealed that low numbers (n = 5) of acoustically-tagged bull trout out-migrated from West Fork Thompson River into the mainstem Thompson River and no bull trout out-migrated into Thompson Falls Reservoir. This resulted in an adjustment of the previously stated objectives. The current objectives of this study are 1) characterize the spatial and temporal aspects of out-migrating sub-adult bull trout within the Thompson River drainage, 2) describe travel time and rate, and 3) to estimate survival rate of out-migrants. To accomplish these objectives, bull trout will be sampled in summer 2015 in Fishtrap Creek and West Fork Thompson River drainages and implanted with PIT tags. Timing of out-migration from Fishtrap Creek and West Fork Thompson River will be independently assessed with a PIT tag antenna installed at each tributary confluence with the mainstem Thompson River. Actively out-migrating juvenile bull trout will be sampled during the autumn of 2015 using directional weir traps placed in Fish Trap Creek and the West Fork Thompson River at the confluence with the mainstem Thompson River. Twenty-nine actively out-migrating juvenile bull trout (≥ 38 g) will be surgically implanted with Lotek MAP coded acoustic transmitters and monitored throughout the autumn and early winter using a combination of stationary and mobile hydrophone receivers. Because acoustic telemetry is ineffective in shallow and turbulent river systems, additional juvenile bull trout (n = 15) will be implanted with radio transmitters to increase the likelihood of obtaining movement data within the Thompson River.
Recently Completed Projects
Spawning characteristics and early life history of mountain whitefish in the Madison River, Montana
Mountain whitefish were historically common throughout much of the Intermountain West. However, within the last decade mountain whitefish have exhibited population-level declines in some rivers. In the Madison River, Montana, anecdotal evidence indicates mountain whitefish abundance has declined and the population is skewed toward larger individuals, which is typically symptomatic of recruitment problems. Spawning success and early-life history influence numbers of juveniles recruited into a population; thus, our objectives were to describe the spatial and temporal extent of spawning, determine fecundity, spawning periodicity, and age-at-maturity, identify effective sampling methods for age-0 fish, and describe the spatial distribution of age-0 fish. We implanted radio tags in mature mountain whitefish (n = 138) and relocated tagged fish in autumn 2012 – 2014. Timing of spawning was determined from spawning status of captured females (n = 85) and from density of eggs collected on egg mats. Gonad samples and otoliths were collected from fish sampled in October 2012 (n = 147) to examine age at maturity and spawning periodicity, and whole ovaries were collected and used to estimate fecundity for a subsample of females (n = 28). Four sampling gears were tested in May 2013 to compare their effectiveness at sampling age-0 mountain whitefish. In May 2014, seining was used to sample age-0 mountain whitefish at backwater and channel sites throughout the entire study site. Females in this population spawned annually, were 90% mature at age 3.7 (2.2 – 5.6, 95% CI), and fecundity was estimated to be 18,221 eggs per kg body weight. In 2013 and 2014, spawning occurred between the third week of October and first week of November. During spawning, 28% of the tagged fish were observed in an area accounting for 5% of study site length, near Varney Bridge. Seines were the most efficient sampling gear tested for capturing age-0 mountain whitefish. In 2014, the reach downstream of Varney Bridge had the highest catch per unit effort (C/f) of age-0 mountain whitefish, and the percentage of spawning adults in the 25 km upstream of a sampling site was positively associated with juvenile C/f (Z = 2.537, df =15, P < 0.001). Within this reach, age-0 mountain whitefish were associated with silt-laden backwater and eddy habitats. Future investigations on mechanisms influencing recruitment should focus on the reach where we observed high numbers of spawning adults and age-0 fish.
Exploitation, abundance, and large-scale movements of burbot in the upper Wind River Drainage
In the Wind River drainage, burbot are a popular sport fish and an important cultural resource for the Eastern Shoshone and Northern Arapahoe tribes. However, overexploitation may be limiting these populations. To address this issue, we estimated exploitation by tagging 1,041 burbot in Bull Lake and 476 burbot in the Torrey Creek drainage with Carlin-type tags from 2011 through 2013. We also estimated tag loss (20% for the 2011 cohort and 4% for the 2012 cohort) and tag reporting (16%) to minimize bias in our exploitation estimates. In Bull Lake, annual exploitation was 11% (95% CI: 4–19%); in the Torrey Creek drainage, exploitation was 2% (95% CI: 0–16%). Mean exploitation estimates were low; however, the upper ends of both confidence intervals approach values that merit concern. Using population size-structure and parameter estimates, including exploitation, natural mortality, abundance, and growth rate, we constructed a stage-structured model to investigate the effects of varying exploitation rates on the Bull Lake and Torrey Creek drainage burbot populations. Model results indicated that the burbot populations were sustainable at the observed exploitation rates in both drainages. Thus, more restrictive harvest regulations are currently not warranted in Bull Lake or the Torrey Creek drainage.
Suppression of lake trout in Quartz Lake, Glacier National Park
Prior to the recent invasion of non-native lake trout Salvelinus namaycush, Glacier National Park (GNP) supported about one-third of the remaining natural lake habitat supporting threatened bull trout Salvelinus confluentus. However, bull trout populations have recently declined and are at high risk of extirpation in several lakes in western GNP because of the establishment of lake trout. In 2009, the U.S. Geological Survey and the National Park Service began suppressing lake trout in Quartz Lake (352 ha) to reduce effects on native bull trout. The objectives of this study were to 1) describe the demography of the lake trout population during the suppression program (2009-2013), 2) identify the timing and location of lake trout spawning, 3) determine the most efficient combination of gill net mesh color and twine diameter to capture juvenile lake trout (age 2 to age 4), 4) assess the effects of suppression on the growth rate of the lake trout population and use this information to model harvest scenarios, and 5) determine whether suppression negatively affected bull trout. Lake trout exhibited slower growth, lower condition, and lower fecundity relative to other populations. Spawning locations were identified on cobble and boulder substrates (depths 2-20 m) near the base of two avalanche chutes where adults began aggregating between 1 and 9 October prior to thermal destratification (11-12 C°). Catch rates of spawning (ripe) adults were highest from 12 October through 25 October when temperatures declined to below 10 C°. Gill nets with 0.1 mm twine thickness and green color increased catchability of juvenile lake trout. Although density dependent parameters were not included, population simulation models indicated the population was growing exponentially and would probably reach carrying capacity within ten years without suppression. Suppression resulted in declining population growth rates (λ) from 1.23 prior to suppression to 0.61-0.79 during suppression. Bull trout redd abundances remained stable throughout the suppression period. Targeted suppression successfully reduced lake trout abundance and continued suppression at or above observed exploitation levels is needed to ensure continued population declines and to avoid effects on the bull trout population.
Estimate density of lake trout vulnerable to capture in trap nets using mark-recapture methods appropriate to sampling design
From June 3, 2013 through October 17, 2013, lake trout ≥ 210 mm were captured in Yellowstone Lake in gill nets and trap nets. Data from these fish were used in a mark-recapture analysis to estimate abundance using closed mark-recapture models. Abundance estimation was conducted for fish in four length classes: (1) 210-450 mm, (2) 451-540 mm, (3) 541-610 mm, and (4) > 610 mm. Abundance was estimated by using capture-recapture and removal data to estimate capture probability of fish in each length class by day. A set of competing models of capture probability was evaluated, and the model(s) that were best supported by the data were used to provide estimates of abundance for the population. Model-selection results provided strong evidence that daily capture probability varied by length class of fish, effort expended by gear type, and the previous capture history of a fish. The best-supported model included interactions between effort by each gear type and length class, which allowed effort for each of the different gear types to have different relationships with capture probability for different length classes. Evaluation of goodness-of-fit by examination of residual errors between the observed number of daily captures in each length class and the expected number based on the best-supported model did not indicate any serious problems with lack of fit. Resulting estimates of abundance and accompanying standard errors were as follows: 303,484 (SE = 22,350) fish in the 210-450 mm length class; 41,288 (SE = 4,456) fish in the 451-540 mm length class; 17,278 (SE = 4,456) fish in the 541-610 mm length class; and 5,601 (SE = 812) fish in the > 610 mm length class. Based on these estimates and the number of individual fish caught in each length class, estimated exploitation rates along with accompanying 95% confidence intervals were as follows: 0.72 (0.63-0.84) for fish in the 451-540 mm length class; 0.56 (0.46-0.71) for fish in the 451-540 mm length class; 0.48 (0.38-0.66) for fish in the 541-610 mm length class, and 0.45 (0.35-0.63) for fish in the > 610 mm length class. The estimate for fish 210-451 mm in length may not fully represent all fish in the length class. The extent to which the estimates may underestimate abundance and overestimate exploitation rate depends on gear selectivity for the smallest length class.
Use of mobile electrofishing to induce mortality in lake trout embryos in Swan Lake
An apparent rapid increase in the abundance of nonnative lake trout has occurred in Swan Lake, which is of concern to state, federal, tribal, and private entities because Swan Lake contains one of the most stable bull trout populations in Montana. Consequently, an experimental lake trout suppression program has been initiated in Swan Lake that targets juvenile and adult lake trout. Targeting lake trout embryos may be a complementary and effective method for suppressing lake trout. Exposure of fish embryos to voltage gradients in the upper range of those produced by electrofishing equipment has been shown to result in mortality. However equipment does not exist to increase mortality of embryos. We tested a grid of electrodes electrified for 60 s with 15 amps of direct current at 1000 V. Embryos were pre-positioned in spawning areas, electrodes were lowered from a pontoon boat and electrified using standard electrofishing equipment. Embryo mortality was 100.00% (± 0.00%) at the surface of the substrate, 99% (± 2%) in embryos buried 5 cm deep, 99% (± 1%) in embryos buried 10 cm deep, and 98% (±3 %) in embryos buried 20 cm deep. Average mortality in the controls (embryos placed in spawning areas but not exposed to electricity) was 8% (± 5%). Mortality in treatment groups differed significantly from the control (Kruskal–Wallis statistic, H = 63.6, df = 4, P ≤ 0.001). The portable grid of electrodes was effective in causing high mortality of lake trout embryos. The equipment should be used to supplement ongoing gillnetting operations for overall population suppression, and could be used thereafter for continued population suppression. Modifications to the electrode array, or to the electric waveforms used, could make the array effective in causing high mortality of larval and juvenile life stages.
Spawning of pallid sturgeon and shovelnose sturgeon in an artificial stream
Understanding the spawning behavior and spawning habitat requirements of shovelnose sturgeon affected by regulated rivers is necessary to inform fishery management actions directed at maintaining and recovering shovelnose sturgeon populations. Shovelnose sturgeon spawning was studied in an artificial river at the Bozeman Fish Technology Center from 2011–2014. Spawning trials performed in 2011 focused on developing methodology and describing the spawning behaviors of shovelnose sturgeon. Spawning duration varied from 3 to 18 h (defined as the shortest and longest periods from first oviposit to final oviposit for an individual female). Spawning bouts or coupling lasted 2-3 seconds. About 50 individual spawning bouts or couplings occurred for each female. Hundreds to thousands of eggs were released during each spawning bout. Courtship and mating behaviors of shovelnose sturgeon included polyandrous and polygynous mating and a single couple per spawning bout. Additionally, shovelnose sturgeon were observed to spawn over gravel (2-64 mm) and cobble substrate (65-256 mm), spawned in close proximity to the substrate (0-18 cm), and the majority of eggs released by a female attached to the substrate a few meters downstream of the spawning site. Spawning trials performed in 2012-2014 examined microhabitat characteristics (water velocity and substrate) connected to spawning site selection. Preliminary analysis showed shovelnose sturgeon spawning site selection was influenced by water velocity. Water velocities available in the living stream were characterized for each individual trial. Manly selection ratios and chi-squared log likelihood selection ratios indicated that shovelnose sturgeon did not select velocities in proportion to availability. Substrate and velocity influenced site selection. Understanding the behavioral ecology of sturgeon spawning will help river managers determine how controlled flow regimes might be best used (e.g., timing and magnitude) to promote spawning and recruitment. The information collected on shovelnose sturgeon may have management applications that can also be used to aid in the recovery of pallid sturgeon.