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Understanding factors mediating hybridization between native and invasive species is crucial for conservation. We assessed the spatial distribution of hybridization between invasive rainbow trout (Oncorhynchus mykiss) and native Yellowstone cutthroat trout (Oncorhynchus clarkii bouveri) in the Lamar River of Yellowstone National Park using a paired telemetry and genetic dataset. Spawning populations containing hybrids (15/30) occupied the full spectrum of abiotic conditions in the watershed (stream temperature, stream size, runoff timing), including an intermittent stream that dried completely in late June, and mainstem spawning locations. Hybrids and rainbow trout occupied an entire high-elevation (~2500-1900 m) tributary where rainbow trout ancestry was highest in headwaters and decreased downstream. Fluvial distance to this ostensible source population was the only covariate included in top hybridization models; effects of abiotic covariates and stocking intensity were relatively weak. In this watershed, abiotic conditions are unlikely to mediate continued hybridization. We conclude that management intervention is important for the persistence of nonhybridized Yellowstone cutthroat trout and highlight the value of pairing telemetry with genetic analysis to identify and characterize populations for hybridization assessments. Il est d'importance cruciale pour la conservation de comprendre les facteurs qui modulent l'hybridation d'especes indigenes et envahissantes. Nous avons evalue la repartition dans l'espace de l'hybridation entre des truites arc-en-ciel (Oncorhynchus mykiss), une espece envahissante, et des truites fardees de Yellowstone (Oncorhynchus clarkii bouveri), une espece indigene, dans la riviere Lamar du Parc national de Yellowstone, en utilisant un ensemble jumele de donnees genetiques et de telemetrie. Les populations reproductrices contenant des hybrides (15/30) occupent toute la fourchette des conditions abiotiques (temperature dans le cours d'eau, taille du cours d'eau, moment de l'ecoulement) observee dans le bassin versant, incluant un cours d'eau intermittent qui etait completement asseche a la fin de juin et des lieux de frai dans le cours principal. Des hybrides et des truites arc-en-ciel occupaient l'entierete d'un affluent de haute altitude (~2500-1900 m) dans lequel une ascendance de truites arc-en-ciel etait maximum dans les eaux d'amont, diminuant dans les eaux plus en aval. La distance fluviale par rapport a cette population source apparente est la seule covariable incluse dans les meilleurs modeles d'hybridation; les effets des covariables abiotiques et de l'intensite de l'ensemencement sont relativement faibles. Dans ce bassin versant, il est peu probable que les conditions abiotiques modulent l'hybridation a l'avenir. Nous concluons que des interventions de gestion sont importantes pour assurer la persistance de truites fardees de Yellowstone non hybrides et insistons sur l'utilite de jumeler la telemetrie et l'analyse genetique pour cerner et caracteriser les populations pour des evaluations de l'hybridation., Introduction Extinction by hybridization is a prominent conservation threat, especially as introductions of non-native species increase globally (Rhymer and Simberloff 1996; Todesco et al. 2016). This is a common issue [...]

Yellowstone Lake in Yellowstone National Park, USA, has the longest ongoing suppression program for non-native lake trout (Salvelinus namaycush) in the western USA. Harvest data from the suppression program, along with data from an assessment program initiated in 2011, was used to estimate lake trout abundance and mortality rates. Abundance and biomass estimates were used to estimate stock-recruitment dynamics, which were inputs to a simulation model forecasting responses to continued suppression. Abundance increased during 1998-2012 when total annual mortality exceeded 0.59 and declined thereafter. The fishing mortality rate required to reduce abundance was 67% greater than predicted by models that used prerecruit survival estimates from the lake trout's native range. Prerecruit survival in Yellowstone Lake was estimated at four to six times greater than native range survival rates. Simulated abundance continued to decline if recent suppression efforts were maintained. High prerecruit survival in Yellowstone Lake likely illustrates ecological release for an invasive species in an ecosystem containing few predators or competitors and demonstrates the potential pitfalls of assuming equal demographic rates for native and non-native populations. Le lac Yellowstone, dans le Parc national Yellowstone (Etats-Unis), est le lieu du programme de suppression continu de truites de lac (Salvelinus namaycush) non indigenes le plus ancien dans l'Ouest des Etats-Unis. Des donnees sur les prises issues du programme de suppression, combinees a des donnees d'un programme devaluation initie en 2011, ont ete utilisees pour estimer l'abondance et les taux de mortalite des truites de lac. Des estimations de l'abondance et de la biomasse ont ete utilisees pour estimer la dynamique du recrutement au stock, dont les resultats ont servi d'intrants dans un modele de simulation qui predit les reactions a la suppression continue. L'abondance a augmente de 1998 a 2012, alors que le taux de mortalite annuelle totale a depasse 0,59, pour diminuer par la suite. Le taux de mortalite par peche requis pour reduire l'abondance est de 67 % superieur au taux predit par des modeles qui utilisent des estimations de la survie prerecrutement obtenues pour l'aire de repartition naturelle des truites de lac. Le taux de survie prerecrutement estime pour le lac Yellowstone est de quatre a six fois plus grand que les taux de survie dans l'aire de repartition naturelle. L'abondance simulee continue de diminuer si les efforts de suppression recents sont maintenus. Le taux eleve de survie prerecrutement dans le lac Yellowstone reflete probablement la liberation ecologique d'une espece envahissante dans un ecosysteme comptant peu de predateurs ou concurrents et demontre les ecueils possibles de l'hypothese de taux demographiques egaux pour des populations indigenes et non indigenes. [Traduit par la Redaction], Introduction Non-native fishes have been implicated in the decline of native fish populations worldwide (Dudgeon et al. 2006; Jelks et al. 2008), causing eradication or suppression projects to increase in [...]

Expansion of an invasive Lake Trout Salvelinus namaycush population in Swan Lake, Montana, threatens a core area population of Bull Trout S. confluentus. Given the recent development of novel suppression methods, such as use of carcass analog pellets to cause high mortality of embryos, there was a need to quantify spawning season aggregation sites, site use, and spawning habitat for Lake Trout in Swan Lake. Acoustic tags were implanted in 85 Lake Trout during the summer in 2018 and 2019. Nightly tracking efforts during autumn in both years resulted in 1,744 relocations for 49 individual Lake Trout. Kernel density analysis was used to evaluate Lake Trout aggregation sites, identifying 10 distinct sites. All spawning sites were located in the littoral zone along areas of steep bathymetric relief, and these sites composed 48% of total relocations during both spawning seasons. In 2019, side‐scan sonar imaging was used to classify and quantify the total area of spawning substrate, which constituted 12.8% of the total surface area estimated for spawning sites 1, 6, and 9 and 11.4% of the total surface area for aggregation sites 2–5, 7, 8, and 10. Simultaneous treatment of all spawning sites would require 205,709 ± 86 kg of carcass analog pellet material, resulting in 370.4 ± 0.2 kg of phosphorus inputs and 7,487.9 ± 3.1 kg of nitrogen inputs to Swan Lake. Thus, pellet treatment would increase the Carlson's trophic state index (TSI) values from 20.8 to 27.7 for total phosphorus and from 22.1 to 26.2 for total nitrogen. Based on a TSI threshold of less than 40 for an oligotrophic lake, the use of carcass analog pellets could be feasible for supplementing the gill‐netting suppression of Lake Trout in Swan Lake. [ABSTRACT FROM AUTHOR]

Introduced lake trout (Salvelinus namaycush) threaten to extirpate native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) in the 34 000 ha Yellowstone Lake in Yellowstone National Park, USA. Suppression (and eventual eradication) of the lake trout population is deemed necessary for the conservation of Yellowstone cutthroat trout. A US National Park Service gill-netting program removed nearly 450 000 lake trout from Yellowstone Lake from 1995 through 2009. We examined temporal variation in individual growth, body condition, length and age at maturity, fecundity, mortality, and population models to assess the efficacy of the lake trout suppression program. Population metrics did not indicate overharvest despite more than a decade of fish removal. The current rate of population growth is positive; however, it is lower than it would be in the absence of lake trout suppression. Fishing effort needs to increase above observed levels to reduce population growth rate below replacement. Additionally, high sensitivity of population growth rate to reproductive vital rates indicates that increasing fishing mortality for sexually mature lake trout may increase the effectiveness of suppression. Lake trout suppression in Yellowstone Lake illustrates the complexities of trying to remove an apex predator to restore a relatively large remote lentic ecosystem with a simple fish assemblage. Les touladis (Salvelinus namaycush) introduits dans le lac Yellowstone de 34 000 ha dans le parc national de Yellowstone, E.-U., menacent d'en extirper la truite fardee de Yellowstone (Oncorhynchus clarkii bouvieri) indigene. On croit que la reduction (et eventuellement l'eradication) de la population de touladis est necessaire a la conservation des truites fardees de Yellowstone. Un programme de peche au filet maillant mene par le service des parcs nationaux des E.-U. a retire presque 450 000 touladis du lac Yellowstone de 1995 jusqu'a la fin de 2009. Nous examinons la variation temporelle de la croissance individuelle, de la condition corporelle, de la longueur et de l'age a la maturite, de la fecondite, de la mortalite et des modeles demographiques afin d' evaluer l' efficacite du programme d' elimination des touladis. Les metriques de la population n'indiquent pas de recolte excessive malgre le retrait de poissons pendant plus d'une decennie. Le taux actuel de croissance de la population est positif; cependant, il est plus bas qu'il ne le serait en absence du programme d'elimination des touladis. L'effort de peche devra etre accru au-dela des niveaux observes afin de reduire le taux de croissance de la population sous le seuil de remplacement. De plus, la forte sensibilite du taux de croissance de la population aux taux vitaux reproductifs indique que l' augmentation de la mortalite due a la peche chez les touladis a maturite sexuelle pourrait ameliorer l' efficacite de l' elimination. L' elimination des touladis dans le lac de Yellowstone illustre les difficultes reliees aux efforts pour retirer un predateur de sommet de reseau trophique afin de restaurer un ecosysteme lenitique relativement grand avec un peuplement simple de poissons. [Traduit par la Redaction], Introduction Non-native species have long been recognized as a driver of the global decline in biodiversity (Vitousek et al. 1997; Simberloff 2001) and are often ranked second to habitat loss [...]

Invasive Lake Trout Salvelinus namaycush have altered the once‐pristine Yellowstone Lake ecosystem through top‐down effects by consuming Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. To conserve Yellowstone Cutthroat Trout and restore the ecosystem, a Lake Trout gillnetting program was implemented to suppress the invasive population. We evaluated the spatial structure of Lake Trout in Yellowstone Lake with the intent of increasing suppression efficiency. Specifically, we addressed questions related to adult Lake Trout aggregation and movement during summer and autumn (spawning) periods and how Lake Trout used locations in the context of suppression efforts. We tracked 373 Lake Trout (>500 mm TL) during the summer and autumn of 2016 and 2017. Based on kernel density estimates, Lake Trout were highly aggregated at 9 locations during summer and 22 locations during the spawning period. Using a novel metric, individual days (product of mean individuals per survey and mean length of stay), five summer locations and five spawning locations had at least 30 individual days. These locations are suggested as priority areas for targeting Lake Trout suppression. Lake Trout were less aggregated and moved less during the summer, making them less vulnerable to a passive gear in the summer than during the autumn spawning period. Lake Trout exhibited low spawning site fidelity compared to populations elsewhere, possibly due to decades of intensive gill netting at spawning locations. Given the aggregation and movement patterns observed in Yellowstone Lake, continuing to target adult Lake Trout during the spawning period is the most cost‐effective approach to Lake Trout suppression. [ABSTRACT FROM AUTHOR]

Yellowstone Lake is the site of actions to suppress invasive Lake Trout Salvelinus namaycush and restore native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri and natural ecosystem function. Although gill netting is effective (Lake Trout λ ≤ 0.6 from 2012 through 2018), the effort costs more than US$2 million annually and only targets Lake Trout age 2 and older. To increase suppression efficiency, we developed an alternative method using organic (soy and wheat) pellets to increase mortality of Lake Trout embryos on spawning sites. Decomposition of pellets during two in situ experiments caused dissolved oxygen (DO) concentrations to temporarily decline to lethal levels (<3.4 mg/L) within days of application. Embryo mortalities during the first exposure period (days 1–6 following application) were high at all treatment levels (1.75–28.0 kg/m2) at the substrate surface and within interstices 20 cm below the surface, varying from 97 ± 1.8% (mean ± SE) to 100 ± 0.0%, but may have been enhanced by a handling effect (exposure to sunlight). Embryo mortalities during the second exposure period (days 11–22) were highest 20 cm below the surface, varying from 78 ± 9.7% to 100 ± 0.0%. Almost all (98 ± 3.1%) Lake Trout embryos died after exposure to DO < 3.4 mg/L for >200 h during the second period. Pellets caused lethal DO for several weeks below the substrate surface, despite largely dissolving and dissipating from the surface of treated areas by day 39. Broad‐scale application of pellets at 1.75 kg/m2 following the spawning period in autumn may reduce Lake Trout recruitment and enhance population suppression because the area of 14 verified spawning sites is only 11.4 ha (0.03% of lake surface area). Pellet application may be useful in other similar systems as part of an integrated pest management approach targeting multiple life stages of invasive freshwater fish. [ABSTRACT FROM AUTHOR]