FAQ on Invasive Lake Trout in Yellowstone Lake

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These Frequently Asked Questions and responses were developed by USGS and their partners with the Lake Trout Suppression Scientific Review Panel *.  The purpose of these FAQs is to provide answers to several of the more common questions concerning the lake trout suppression program in Yellowstone Lake, within Yellowstone National Park, and provide information about the current status of the program.  For more information, contact Robert Gresswell at bgresswell@usgs.gov.

How have the fish communities in Yellowstone National Park changed through time? 

Almost half of the streams and lakes in Yellowstone National Park did not support trout populations prior to the coming of European Americans (Jordon 1891).  Stocking programs initiated by the U.S. Fish Commission in the late 1800s and early 1900s established populations of rainbow, brown, brook, and lake trout, as well as Yellowstone cutthroat trout, in most of the fishless waters (Varley and Schullery 1983).  In some cases, nonnative fishes were introduced into waters where native cutthroat trout flourished.  As a consequence, native cutthroat trout were often replaced by the introduced nonnatives, and hybridization between cutthroat trout and rainbow trout was common (Varley and Schullery 1983, 1998).

The goal of these introductions was to provide angling opportunities for visitors to the Yellowstone area.  In this vein, these stockings were very successful.  There was little concern for whether these interactions with native trout were harmful; the main goal was to provide fish for anglers.  As nonnative trout expanded in Yellowstone National Park, the range of the native cutthroat trout contracted (Varley and Schullery 1998).  Despite these introductions, Yellowstone cutthroat trout flourished in many parts of their native range within the Park, and until the 1990s, Yellowstone Lake supported the largest genetically pure population of Yellowstone cutthroat trout on earth (Gresswell and Liss 1995).

 

Where did the lake trout in Yellowstone Lake come from? 

Lake trout were first documented in Yellowstone Lake in 1994 (Kaeding et al. 1996).  Evidence from chemical patterns in the ear bones of lake trout captured during the late 1990s indicated that these fish were illegally introduced from a nearby lake during the 1980s (Munro et al 2005).  An early report by historian Hiram Chittenden asserted that lake trout were stocked in the Yellowstone River above the falls in 1890 (Chittenden 1914).  However, stocking records published by the U.S. Fish Commission, which did the stocking at that time, do not support Chittenden’s claim (McDonald 1893).  In fact, U.S. Fish Commission records indicate that all of the lake trout plantings during that period occurred in Lewis and Shoshone lakes (McDonald 1893).  Apparently, Chittenden misinterpreted the stocking summary in the U.S. Fish Commission report that indicated 10,000 whitefish were stocked in the Yellowstone River above the falls.  Reference to lake trout stocking did not appear in later editions of Chittenden’s book (beginning Chittenden 1915). 

Even if the Chittenden report were true, it would mean that lake trout lived in Yellowstone Lake or the downstream Yellowstone River for about a century without being observed by anglers, rangers, or fisheries biologists.  Although this is possible, it is highly unlikely.  During the 1950s-60s, 250,000 to 300,000 cutthroat trout were being captured by anglers each year (Gresswell and Varley 1988), rigorous creel surveys were conducted (Moore, Cope, and Beckwith 1952; Cope 1955), and experienced guides were on the water with their clients almost every day, but there are no records of lake trout being caught or observed (Kendall 1915; Simon 1962; Varley and Schullery 1983).  Further, fisheries biologists did not observe any lake trout when sampling fish with nets and traps (Foster 1932, 1942; Cope 1957; Ball and Cope 1961; Benson 1961).  The evidence indicates that there was not a population of lake trout in Yellowstone Lake at that time, and the current population of lake trout is the result of a much more recent introduction.

 

How have lake trout affected the native cutthroat trout in Yellowstone Lake? 

In relation to current management efforts, it does not matter when lake trout were introduced into Yellowstone Lake or from where they originated.  What is important is the effect that lake trout have had on cutthroat trout in the lake since the late 1990s.  Scientific evidence in the form of creel surveys conducted by the U.S. Fish and Wildlife Service and the National Park Service reveal that for the 15 years prior to the discovery of lake trout, the average landing rate by anglers on Yellowstone Lake was over one and a half cutthroat trout per hour of fishing (Gresswell et al. 1994).  Subsequently, the landing rate for anglers steadily declined to less than one half of a fish per hour of fishing in 2006 (Koel et al. 2012).  Despite the very substantial lake trout suppression program in recent years, the landing rate of cutthroat trout has not exceeded one fish per hour since 2001 (Koel et al. 2012) .   

Declines in the abundance of Yellowstone cutthroat trout that move upstream into tributaries to spawn provide further evidence of the negative effects of lake trout.  In Clear Creek, where annual studies have been conducted since the 1940s, the number of cutthroat trout counted during the annual spawning migration dropped from 43,600 per year in the 1970s and 80s to 489 in 2006, the lowest in the 60-year period of record (Koel et al. 2008).  This represents a decrease of almost 99%.  Numbers of cutthroat trout spawners in other smaller tributaries have also declined, some to less than 10 fish (Koel et al. 2008).  These data suggest that in the lake cutthroat trout declined from millions prior to lake trout to only tens of thousands by the end of the first decade in the twenty-first century.

Furthermore, observations by fisheries scientists of the stomach contents of almost 600 lake trout captured in the late 1990s underscore the predatory nature of lake trout.  Scientists found that 95% of the diet of older (9 years and older), larger (greater than 23 inches) lake trout in Yellowstone Lake was composed of fish, and most of the fish were cutthroat trout (Ruzycki et al. 2003).  Furthermore, fish represented about 60% of the diet among lake trout 4 to 8 years old (Ruzycki et al. 2003).  This provides strong evidence that in the late 1990s, cutthroat trout were a major item in the diet of lake trout and that predation by lake trout was an important factor causing the decline of cutthroat in Yellowstone Lake.  It is possible the cutthroat trout are less common in the stomach contents of lake trout caught since the cutthroat trout population severely declined, and researchers from Montana State University are currently analyzing samples collected in 2013 to evaluate current diet structure (Syslo, unpublished data). 

 

Why are cutthroat trout uncommon in lake trout stomachs in Jackson Lake, Wyoming? 

Jackson Lake supports populations of native cutthroat trout and introduced lake trout, and scientists have considered how it is possible for a cutthroat trout fishery to persist in the presence of lake trout.  Stomach samples from lake trout sampled from Jackson Lake yield few cutthroat trout (Harper et al. 2007); however, this is likely because cutthroat trout in Jackson Lake and other lakes in the upper Snake River Basin evolved with a much greater number of fish species than did the cutthroat trout in Yellowstone Lake.  Consequently, the effects of a new predator in the lakes of the upper Snake River would be moderated by the greater variety of prey available.  For example, in Heart Lake there were seven fish species in the lake prior to the introduction of lake trout (Varley and Gresswell 1988).  Lake trout were abundant in Jackson Lake by 1906 (Kiefling 1981, cited by Harper et al. 2007), and by the 1930s, the number of cutthroat trout caught by anglers had declined to the point that lake trout became the primary target of the fishery (Erickson 1976, cited by Harper et al. 2007).

 

How has the presence of lake trout affected birds and mammals in the Yellowstone Lake ecosystem? 

Prior to the introduction of lake trout, the cutthroat trout in Yellowstone Lake supported a complex food web that included grizzly bears (Reinhart and Mattson 1990; Mattson and Reinhart 1995), otters (Crait and Ben-David 2006; Wengeler et al. 2010), bald eagles (Swenson et al. 1986), white pelicans (Ward 1922; Schaller 1962; Kaeding 2002), and osprey (Swenson 1978; Baril et al. 2013).  In fact, recent research has documented a sharp decline in the number of osprey breeding pairs in the Yellowstone Lake ecosystem and concomitant decrease in osprey productivity and nesting success (Baril et al. 2013).  Moreover, scientific evidence suggests that grizzly bears that previously fed on cutthroat trout during the spawning migration into small streams around the lake are now feeding on elk calves (Haroldson et al. 2005; Fortin et al. 2013; Middleton et al. 2013).  Lake trout do not substitute for the cutthroat trout that evolved in the lake because they spawn in the lake and spend most of their time in cold deep areas of the lake where they are not vulnerable to predation by birds and mammals (Varley and Schullery 1995a; Koel et al. 2005). 

 

How much does the lake trout program cost?  

Currently, about $2 million are being spent annually on lake trout suppression in Yellowstone Lake, and much of the funding comes from private donors that want to restore the cutthroat trout population in the lake (T. M. Koel, U.S. National Park Service, personal communication).  However, this amount is relatively small compared to the money that has been lost to businesses in and around Yellowstone National Park as the cutthroat trout population in the lake collapsed following the introduction of lake trout.  Estimates from the early 1990s indicate that the economic value of the Yellowstone Lake cutthroat trout fishery likely exceeded $36 million dollars annually (Varley and Schullery 1995b).  Furthermore, the social value of the fishery undoubtedly extends far beyond that.  In the 1970s, 80s, and early 90s, about 142,000 angler days occurred annually on the lake (National Park Service, unpublished data).  Fishing on the lake was often the key reason for families to visit the park from across the nation and around the world.  Even non-anglers benefited from the cutthroat trout in the lake, and almost 350,000 visitors (10% of the park visitation in 1990 and 1991) visited Fishing Bridge and LeHardy Rapids to watch the thousands of cutthroat trout that moved through those areas on their annual spawning migration (Gresswell and Liss 1995).  More recently, few visitors are found on Fishing Bridge because there are so few fish to watch.

 

Are lake trout being suppressed everywhere?  

Lake trout are not inherently bad fish.  Tens of millions of dollars are being spent to restore, conserve, and protect lake trout in the upper Midwest and other portions of their native range in North America.  Furthermore, there are many people who thoroughly enjoy lake trout and seek out places to fish for them.  However, the National Park Service in Yellowstone National Park is not alone in efforts to suppress lake trout where they have been introduced outside of their native range and native fish populations were subsequently threatened (Martinez 2009).  For example, there are suppression programs in Idaho (Hansen et al. 2008; Lake Pend Oreille) and Montana (Syslo et al. 2013, Swan Lake; Dux et al. 2011, Lake McDonald).  The bottom line is that lake trout are a good sport fish, but introduced populations of this potentially large predator have had a negative influence on native fishes in many waters of the western U.S. (Martinez 2009).

 

Are there other factors contributing to the decline of cutthroat trout in Yellowstone Lake? 

Although the evidence clearly points to lake trout predation as the primary cause of the decline of cutthroat trout, it is not the only factor affecting cutthroat trout in Yellowstone Lake.  Natural disturbances, such as drought and fire, and the invasive parasite that causes whirling disease may also influence the cutthroat trout population (Koel et al. 2005). 

Drought conditions can reduce the ability of cutthroat trout to migrate upstream into tributaries to spawn and affect the numbers of fry that are hatched (Bulkley and Benson 1962; Farnes an Bulkley 1964).  This mechanism may have had an effect on cutthroat trout reproduction during the drought of the late 1990s and early 2000s (Koel et al 2005), but there is no evidence of declines in reproductive success and subsequent abundance of fish in the lake of the magnitude observed over the last decade in Yellowstone Lake. 

Some are concerned about the potential effects of fire; however, research conducted on five major fishing streams in Yellowstone National Park following the 1988 fires failed to find any long-term negative effects on trout populations (Jones et al. 1990).  In fact, evidence from across the West suggests that even when fish populations are wiped out of small streams following catastrophic fires, they rebound when fish migrate from surrounding streams, and populations usually reach pre-fire levels within 2 years (Gresswell 1999; Rieman et al. 2012).  After the 1988 fires burned a quarter of the Yellowstone Lake watershed, there were no changes in cutthroat trout growth (Gresswell 2004), and spawning streams around the lake did not become choked with ash and fine sediments.

Whirling disease is another threat to cutthroat trout.  Studies in the mid-2000s indicated that up to 20% of the juvenile and adult Yellowstone cutthroat trout in Yellowstone Lake were infected, but infection was not uniform throughout the watershed (Koel et al. 2006).  The organism causing the disease was found in two tributaries and the Yellowstone River downstream from the lake, but the Yellowstone River upstream of the lake and 13 other spawning tributaries tested negative for the parasite.  Risk of infection was highest in the Yellowstone River and Pelican Creek (Koel et al. 2006; Murcia et al. 2006).  Although few cutthroat trout fry were observed in the lower portions of the Pelican Creek in the mid-2000s (Koel et al. 2005; Koel et al. 2006), recent angler surveys in the second largest tributary to Yellowstone Lake suggests juvenile cutthroat trout are abundant.  In fact, it appears that the prevalence of whirling disease in Yellowstone Lake was lower in 2012 than in the 1999-2001 period (Gresswell et al. 2013).  Whirling disease does occur in the cutthroat population in the Yellowstone Lake drainage, but lake trout predation appears to be the major factor influencing declines in cutthroat trout in the lake.  

 

What is the long-term outlook for cutthroat trout in Yellowstone Lake? 

Based on the most recent evidence concerning cutthroat trout restoration efforts, scientists on the Lake Trout Suppression Scientific Review Panel are optimistic that the lake trout suppression program is gaining ground, reducing the density of lake trout, and leading to an increase in cutthroat trout abundance (Gresswell et al. 2013).  Lake trout may never be completely removed from Yellowstone Lake, but the Panel believes that the National Park Service goal of restoring the Yellowstone cutthroat trout population to levels approaching those observed in the late 1980s and early 1990s is achievable.  Suppression activities may always be a part of the management of Yellowstone Lake, but as lake trout numbers begin to decline, it appears that less expensive suppression techniques that target vulnerable portions of the lake trout life cycle may be substituted for current netting strategies that are necessary when lake trout abundance is high (Gresswell et al. 2013).  It appears that in the long term Yellowstone cutthroat trout can be restored in Yellowstone Lake and returned to their important ecological roles within the greater Yellowstone ecosystem.  That is an important achievement for the National Park Service and all who visit Yellowstone Park.

 

 

Members of the Lake Trout Suppression Scientific Review Panel 

Dr. Robert E. Gresswell, Research Biologist, USGS Northern Rocky Mountain Science Center; Affiliate Assistant Professor, Department of Ecology, Montana State University

Dr. Jack E. Williams, Senior Scientist, Trout Unlimited

John Varley, (Ph.D., Hon), Chief (retired), Yellowstone Center for Resources, Yellowstone National Park; Director (retired), Big Sky Institute, Montana State University

Dr. Christopher S. Guy, Assistant Unit Leader, USGS, Montana Cooperative Fishery Research Unit; Affiliate Associate Professor, Department of Ecology, Montana State University

Dr. Michael J. Hansen, Professor, University of Wisconsin – Stevens Point; Chief, Hammond Bay Biological Station, USGS Great Lakes Science Center; Commissioner, U.S. Section, Great Lakes Fishery Commission.

Dr. Michael L. Jones, Professor and Chair, Department of Fisheries and Wildlife; Co-Director, Quantitative Fisheries Center, Michigan State University

Dr. J. Ellen Marsden, Professor and Director of Wildlife and Fisheries Biology Program, Rubenstein School of Environment and Natural Resources, University of Vermont

Patrick J. Martinez, Colorado Division of Wildlife (retired); US Fish and Wildlife Service (retired)

Dr. Wayne A. Hubert, USGS Wyoming Cooperative Fish and Wildlife Research Unit; Professor Emeritus, University of Wyoming; Past President, American Fisheries Society.

 

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