Montana Fishery Project
Using Carcass and Carcass-Analog Material to Increase Lake T...
September 2019 - December 2020
- USGS Cooperative Research Units
Invasive species introductions cause negative economic effects and disrupt natural ecosystem interactions, with the potential of inducing ecological impairment or ecosystem collapse (Elton 1958; Vitousek 1996; Sala et al. 2000). Invasive species introduction and expansion is the second greatest threat to global biodiversity decline next to habitat degradation (Wilcove 1998; WWF 2016). Additionally, such introductions are recognized as a major threat to aquatic ecosystems (Ricciardi 1999; Pimentel et al. 2005). Invasive species disrupt ecosystems by displacing and reducing native species populations (Vitousek et al. 1996), usually through competition, predation, or both (Cucherousset and Olden 2011). Invasive species have the ability to initiate trophic cascades through the alteration of ecosystem food-web dynamics (Eby et al. 2006). Lake trout have been intentionally or illegally introduced to over 170 locations in the Intermountain West (Martinez et al. 2009), often leading to invasive populations that disrupt food-webs in native ecosystems (e.g., Yellowstone Lake, Lake Pend Oreille, Flathead Lake, Lake Tahoe). Lake trout in their invasive range can predate on native fishes (Ruzycki et al. 2003), compete with native fishes (Donald and Alger 1993; USFWS 1998; Fredenberg 2002), and can affect terrestrial vertebrates dependent on native fish species (Spencer et al. 1991; Schullery and Varley 1995; Tronstad et al. 2010). After being introduced from nearby Lewis Lake in the mid-to-late 1980s (Munro et al. 2005), lake trout were discovered in Yellowstone Lake in 1994 (Kaeding et al. 1995). Yellowstone Lake contains the largest population of genetically pure Yellowstone cutthroat trout in the world (Gresswell and Varley 1988). The effect of lake trout on Yellowstone cutthroat trout populations through predation and competition for limited resources in lake ecosystems (Syslo 2010) is of particular concern for conservation of Yellowstone cutthroat trout. The persistence of Yellowstone cutthroat trout in Yellowstone Lake has been threatened by the introduction of invasive lake trout, which have greatly reduced the abundance of Yellowstone cutthroat trout (Koel et al. 2005; Tronstad et al. 2010) and disrupted aquatic and terrestrial food webs (Tronstad et al. 2010). The number of spawning individuals in one spawning tributary was reduced from 55,000 Yellowstone cutthroat trout in 1987 to 500 in 2007 (Koel et al. 2012). Additionally, the decline in Yellowstone cutthroat trout abundance triggered a trophic cascade (Tronstad et al. 2010) and disrupted linkages among non-piscene predators throughout the Yellowstone Lake basin (Crait and Ben-David 2006; Baril et al. 2013; Tiesberg et al. 2014). The Yellowstone cutthroat trout population was expected to decrease by as much as 60% within 100 years if the level of predation exerted by lake trout was not suppressed (Stapp and Hayward 2002). Due to the dramatic influence of lake trout in Yellowstone Lake and the Yellowstone Lake basin, the National Park Service initiated a lake trout suppression program in 1995. The original gillnetting program objectives were to remove lake trout from Yellowstone Lake and assess the geographic distribution and population characteristics (Kaeding and Boltz 1997; Ruzycki 2004). Subsequently, contract fishing crews are gillnetting in addition to the National Park Service to increase effort throughout Yellowstone Lake. The National Park Service additionally uses acoustic telemetry to locate lake trout spawning areas and concentrate gillnetting effort during the spawning season (Koel et al. 2015). Gillnetting effort has increased from 1997 through 2014 along with the cost of the program, which is now nearly two million dollars annually. The increase in gillnetting effort is showing some benefit; recently the catch of lake trout has declined and Yellowstone cutthroat trout numbers have slightly increased. Research studies were initiated in 2015 to explore alternative lake trout suppression methods to increase the success of meeting suppression program goals, improve suppression efficiency, and reduce program cost. Currently, the most efficient and cost-effective alternative suppression method has been to suppress lake trout embryos using whole and ground lake trout carcass material and carcass-analog pellets on spawning substrates causing hypoxic conditions unsuitable for developing lake trout embryos (Thomas 2017). Carcass analog pellets are created from plant-based organic materials (gluten/soy) and were created to mimic lake trout carcass material. Addition of lake trout carcass material and carcass-analog pellets have increased lake trout embryo mortality in controlled experiments to near 100% (Thomas 2017; Figure 1; YELL unpublished data 2018; Figure 2) and could be applicable to suppression of lake trout embryos lake-wide, potentially increasing the long-term cost efficiency of the lake trout suppression program. However, with a concentrated addition of lake trout carcass material or carcass-analog pellets at lake trout spawning sites, novel concentrations of nutrients become available for aquatic organisms at all trophic levels that were not historically available. The potential influence of concentrated lake trout carcass or carcass-analog pellet nutrients on the food-web dynamics in Yellowstone Lake is unknown. We predict that unintended consequences from lake trout carcass or carcass-analog pellet deposition will not exist, but this must be objectively evaluated using rigorous scientific methodology. Understanding the potential consequences of lake trout carcass material or carcass-analog pellets on nutrient cycling and food-web dynamics in Yellowstone Lake would be beneficial for National Park Service managers when deciding whether to implement embryo suppression lake-wide using lake trout carcass or carcass-analog pellet deposition—implementing the carcass program would result in considerable cost savings to the lake trout suppression program. The aim of this study is to provide information on the potential trophic cascading effects of suppressing embryos with carcass material or carcass-analog pellets in concentrated littoral and benthic zones of the lake. Specifically, we will determine how this novel suppression method influences nutrient cycling and food-web dynamics in Yellowstone Lake.