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Lake trout (Salvelinus namaycush) were unintentionally introduced to Yellowstone Lake, Yellowstone National Park, Wyoming, and drastically reduced the native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) population. Gillnetting suppresses adult lake trout since 1995; however, Yellowstone National Park is developing methods to suppress embryos, including adding lake trout carcasses and analog pellets to spawning sites. Decomposing carcasses and analog pellets cause lake trout embryo mortality due to low dissolved oxygen concentrations, but the effects of these methods on lower trophic levels are unknown. We estimated the degree to which adding carcasses or analog pellets to spawning sites altered nutrient limitation, nutrient concentrations, algal biomass, and ammonium uptake. We deployed nutrient diffusing substrates at three sites (control, carcass, and analog pellets) before and after carcasses or analog pellets were added to measure algal biomass in six treatments where nothing (control), nitrogen, phosphorus, nitrogen and phosphorus, carcasses or pellets were added to agar. We measured nutrient concentrations, algal biomass (chlorophyll a concentrations) and ammonium uptake at spawning sites where no carcasses were added (control), site where carcasses were added before lake trout spawned (early season sites), and sites where carcasses were added after lake trout spawned (late season sites) in 2018 and 2019 to investigate the degree to which carcasses caused bottom-up effects in periphyton and phytoplankton. Nutrient diffusing substrates indicated that nitrogen and phosphorus co-limited periphyton before treatments; however, nutrients were not limiting after carcasses or analog pellets were added to spawning sites. Analog pellets appeared to suppress algal biomass and carcasses increased algal biomass ≥2.4x after their addition. Adding carcasses to shallow spawning sites did not alter the concentration of ammonium, algal biomass or uptake compared to the control site. Periphyton had higher biomass and phytoplankton uptake was much higher. Adding carcasses to the littoral zone likely alters small areas but overall had a small effect on algal biomass and nutrient cycling. Estimating how lake trout suppression methods may alter basal resources in the littoral zone of Yellowstone Lake will help managers develop the best plan to control these invasive predators at early life stages.
Introduction of lake trout Salvelinus namaycush into a system can add a trophic level, potentially affecting organisms at lower trophic levels. Similar to many lakes and reservoirs in the western United States, lake trout were introduced into Yellowstone Lake, Wyoming. Previous studies showed that lake trout reduced the population and altered the size structure of native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri in Yellowstone Lake, but we sought to determine the degree to which lake trout predation changed lower trophic levels. We predicted that the structure of lower trophic levels would change in conformance with trophic cascade theory because Yellowstone cutthroat trout consume zooplankton. We compared zooplankton and phytoplankton assemblages between the period when Yellowstone cutthroat trout were abundant and the period after they declined. As predicted by trophic cascade theory, zooplankton biomass shifted from being dominated by copepods before lake trout introduction to being dominated by cladocerans after lake trout introduction, with zooplankton body lengths 17% longer after introduction. Vertical water clarity increased by 1.6 m because of a twofold decrease in chlorophyll a and a three- to sevenfold decrease in phytoplankton biovolume. Thus, the introduction of lake trout and subsequent decline of Yellowstone cutthroat trout likely altered lower trophic levels in Yellowstone Lake. Trophic cascades may be common in western U.S. lakes and reservoirs where native salmonids are present and where lake trout have been introduced.
Recent invasion (late 1980s) of piscivorous lake trout into Yellowstone Lake, WY, caused large changes in the aquatic community. For example, lake trout added a fourth trophic level to the food web thereby causing large declines in the population size of Yellowstone cutthroat trout, the primary food source of adult lake trout. Daphnia pulicaria is the second most abundant species of zooplankton in the lake, and the main prey item for native Yellowstone cutthroat trout. I compared D. pulicaria body size, clutch size, egg size, and probability of reproducing from archived Yellowstone National Park samples collected prior to lake trout invasion (1977-1981) to zooplankton samples collected post-invasion (2004 and 2008). Before lake trout were introduced, D. pulicaria matured at a smaller body size and carried larger clutches of smaller eggs. With these data, I cannot distinguish among several possible mechanisms for changes in life history traits, including rapid evolution due to reduced size-selective predation by cutthroat trout and adaptive or non-adaptive phenotypic plasticity. However, our results suggest that the introduction of lake trout and the subsequent changes that they caused in Yellowstone Lake have altered the life-history traits of D. pulicaria. While direct effects of invasions are often studied, indirect effects on lower trophic levels may have significant consequences for community composition and ecosystem function, but are rarely studied.
Invasive predators can induce trophic cascades in the open water of lakes; however, much less is known about their effect on benthic invertebrates, which inhabit the lake bottom, or benthic zone. Lake trout (Salvelinus namaycush) were introduced to Yellowstone Lake, Wyoming, and reduced the Yellowstone cutthroat trout (Oncorhynchus clakrii bouvieri ) population. We predicted that lake trout indirectly reduced predation of benthic invertebrates through cutthroat trout. To estimate how the benthic invertebrate assemblages differed under cutthroat trout? versus lake trout?dominated food webs, we collected benthic invertebrate samples from two areas of Yellowstone Lake in 2004 using a Ponar sampler and compared them with stomach contents from cutthroat trout. Cutthroat trout selectively ate benthic invertebrates with the largest body sizes. The amphipod genus, Gammarus, had the highest biomass of all benthic invertebrates. Gammarus biomass was higher in West Thumb (6,000 mg/m2 [0.02 oz/ft2]) where lake trout dominated and lower in South Arm (3,160 mg/m2 [0.01 oz/ ft2]) where cutthroat trout dominated (p = 0.01). Additionally, individual body mass of Gammarus was greater in West Thumb (1.6 mg/individual [0.000056 oz/individual]) than in South Arm (1.1 mg/individual [0.000039 oz/individual; p = 0.01). Our results suggest that lake trout predation on cutthroat trout indirectly reduced predation on Gammarus in West Thumb, leading to a relative increase in the local Gammarus biomass and body mass. Monitoring the benthos of Yellowstone Lake may allow managers to understand the food web dynamics at higher trophic level.
The introduction of lake trout Salvelinus namaycush into Yellowstone Lake preceded the collapse of the native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri population. As a system with a simple fish assemblage and several long-term data sets, Yellowstone Lake provided a unique opportunity to evaluate the ecology of a native salmonid in the presence of a non-native salmonid population undergoing suppression in a large natural lake. Diet data for Yellowstone cutthroat trout and lake trout were evaluated at varying densities to determine the effects of density on diet composition. Temporal diet shifts from 1996-1999 to 2011-2013 were likely caused by limitation of prey fish for lake trout. Diets, stable isotopes, and depth-related patterns in CPUE indicated lake trout> 300 mm consumed primarily amphipods, making them trophically similar to Yellowstone cutthroat trout from during 2011-2013. A lake trout removal program was initiated during 1995 to reduce predation on Yellowstone cutthroat trout. Abundance and fishing mortality were estimated for lake trout from 1998 through 2013 and Yellowstone cutthroat trout from 1986 through 2013. Density-dependence was evaluated by examining individual growth, weight, maturity, and pre-recruit survival as a function of abundance. In addition, a simulation model was developed for the lake trout- Yellowstone cutthroat trout system to determine the probability of Yellowstone cutthroat trout abundance persisting at performance metrics given potential reductions in lake trout abundance. Estimates of Yellowstone cutthroat trout abundance varied 5-fold and lake trout abundance varied 6-fold. Yellowstone cutthroat trout weight and pre-recruit survival decreased with increasing Yellowstone cutthroat trout abundance; however, individual growth and maturity were not related to abundance. Lake trout population metrics did not vary with lake trout abundance. Simulation model results were variable because of uncertainty in lake trout pre-recruit survival. Conservative estimates for required lake trout reductions were> 97% of 2013 abundance for a> 70% probability of Yellowstone cutthroat trout persistence at the performance metrics outlined in the Native Fish Conservation Plan. Lake trout removal will likely reduce lake trout abundance and result in Yellowstone cutthroat trout recovery if the amount of fishing effort exerted in 2013 is maintained for at least 15 years.
In a study of the Yellowstone Lake cutthroat trout, Salmo clarki lewisi, by the U.S. Fish and Wildlife Service, effects of environment on mortality of eggs, immature fish, spawners, and postspawners were measured for various components of the population in Yellowstone Lake (Wyoming). Five methods for estimating mortality of adults on spawning runs are described, with counting and tagging as the principal procedures. Of the total number of eggs deposited in the gravel, 60 to 75 percent died before hatching, and 99.6 percent had died by the time the fingerlings enetered Yellowstone Lake. In Arnica Creek runs, 48.6 percent died in the stream, 40.2 died later in the lake of natural causes, 7.6 were taken by fishermen, and 3.6 percent were alive 2 years later. The white pelican is a serious predator on cutthroat trout in Yellowstone Lake. From 1949 to 1953 fishermen caught 11.6 percent of the catchable trout available to them. Migrations of adult fish in Yellowstone Lake were traced through tagging.
Following the confirmation of the presence of nonnative lake trout (Salvelinus namaycush) in Yellowstone Lake during the summer of 1994, the National Park Service (NPS) launched a major suppression program to curtail potential negative consequences to the native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) and the Yellowstone Lake ecosystem. In August 2008, the NPS convened a scientific review panel to evaluate the suppression program and provide direction for future suppression and recovery activities. The review panel met August 25?29, 2008 at Chico Hot Springs, Montana. This is a report of the findings and recommendations of the panel.
Predatory fish introduction can cause cascading changes within recipient freshwater ecosystems. Linkages to avian and terrestrial food webs may occur, but effects are thought to attenuate across ecosystem boundaries. Using data spanning more than four decades (1972?2017), we demonstrate that lake trout invasion of Yellowstone Lake added a novel, piscivorous trophic level resulting in a precipitous decline of prey fish, including Yellowstone cutthroat trout. Plankton assemblages within the lake were altered, and nutrient transport to tributary streams was reduced. Effects across the aquatic-terrestrial ecosystem boundary remained strong (log response ratio ? 1.07) as grizzly bears and black bears necessarily sought alternative foods. Nest density and success of ospreys greatly declined. Bald eagles shifted their diet to compensate for the cutthroat trout loss. These interactions across multiple trophic levels both within and outside of the invaded lake highlight the potential substantial influence of an introduced predatory fish on otherwise pristine ecosystems.
This richly illustrated and thoroughly researched reference covers all the species of fish and every aspect of their existence in one of the most famous sport fisheries in the world. This edition includes new material on the impact of forest fires and the introduction of non-native species; an expanded chapter on angling; and an assessment of recent management policies. Full color plates and historic b&w photos.
Nonnative species threaten ecosystems throughout the world ? including protected reserves. In Yellowstone National Park, river otters Lontra canadensis depend on native cutthroat trout as prey. However, nonnative lake trout and whirling disease have significantly reduced the abundance of these native fish in the park's largest body of water, Yellowstone Lake. We studied the demographic and behavioral responses of otters to declining cutthroat trout on Yellowstone Lake and its tributaries. From 2002-2008, we monitored otter activity at latrine (scent-marking) sites, collected scat for prey identification, and used individual genotypes from scat and hair samples to evaluate survival and abundance with capture?recapture methods. Otter activity at latrines decreased with declines in cutthroat trout, and the prevalence of these fish in otter scat declined from 73% to 53%. Cutthroat trout numbers were the best predictor of temporal variation in apparent survival, and mean annual survival for otters was low (0.72). The density of otters in our study area (1 otter per 13.4 km of shoreline) was also low, and evidence of a recent genetic bottleneck suggests that otter abundance might have declined prior to our study. River otters in and around Yellowstone Lake appear to be responding to reductions in cutthroat trout via changes in distribution, diet, and possibly survival and abundance. Our results provide a baseline estimate for monitoring the broader outcome of management efforts to conserve native cutthroat trout and emphasize the indirect ecosystem consequences of invasive species.