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Invasive predatory lake trout Salvelinus namaycush were discovered in Yellowstone Lake in 1994 and caused a precipitous decrease in abundance of native Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri. Suppression efforts (primarily gillnetting) initiated in 1995 did not curtail lake trout population growth or lakewide expansion. An adaptive management strategy was developed in 2010 that specified desired conditions indicative of ecosystem recovery. Population modeling was used to estimate effects of suppression efforts on the lake trout and establish effort benchmarks to achieve negative population growth (λ < 1). Partnerships enhanced funding support, and a scientific review panel provided guidance to increase suppression gillnetting effort to >46,800 100-m net nights; this effort level was achieved in 2012 and led to a reduction in lake trout biomass. Total lake trout biomass declined from 432,017 kg in 2012 to 196,675 kg in 2019, primarily because of a 79% reduction in adults. Total abundance declined from 925,208 in 2012 to 673,983 in 2019 but was highly variable because of recruitment of age-2 fish. Overall, 3.35 million lake trout were killed by suppression efforts from 1995 to 2019. Cutthroat trout abundance remained below target levels, but relative condition increased, large individuals (> 400 mm) became more abundant, and individual weights doubled, probably because of reduced density. Continued actions to suppress lake trout will facilitate further recovery of the cutthroat trout population and integrity of the Yellowstone Lake ecosystem.

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.Résumé : Les touladis (Salvelinus namaycush) introduits dans le lac Yellowstone de 34 000 ha dans le parc national de Yellowstone, É.-U., menacent d'en extirper la truite fardée de Yellowstone (Oncorhynchus clarkii bouvieri) indigène. On croit que la réduction (et éventuellement l'éradication) de la population de touladis est nécessaire à la conservation des truites fardées de Yellowstone. Un programme de pêche au filet maillant mené par le service des parcs nationaux des É.-U. a retiré presque 450 000 touladis du lac Yellowstone de 1995 jusqu'à la fin de 2009. Nous examinons la variation temporelle de la croissance individuelle, de la condition corporelle, de la longueur et de l'âge à la maturité, de la fécondité, de la mortalité et des modèles démographiques afin d'évaluer l'efficacité du programme d'élimination des touladis. Les métriques de la population n'indiquent pas de récolte excessive malgré le retrait de poissons pendant plus d'une décennie. 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'élimina-tion des touladis. L'effort de pêche devra être accru au-delà des niveaux observés afin de réduire le taux de croissance de la population sous le seuil de remplacement. De plus, la forte sensibilité du taux de croissance de la population aux taux vitaux reproductifs indique que l'augmentation de la mortalité due à la pêche chez les touladis à maturité sexuelle pourrait améliorer l'efficacité de l'élimination. L'élimination des touladis dans le lac de Yellowstone illustre les difficultés reliées aux efforts pour retirer un prédateur de sommet de réseau...

The hydrologic regime of the Illinois River has been altered over the past 100 years. Locks and dams regulate water surface elevations and flow, enabling commercial navigation to continue year round. This study relates changes in water surface elevation to fish abundance in the river, and establishes target criteria for operating locks and dams. Using longterm records of daily river stage, we identified ecologically meaningful hydrological parameters for eight gage locations along the Illinois River. Inter-annual variability of a long-term fisheries dataset beginning in 1957 was related to variability in stage, flood and recession duration, frequency, timing, and rate of change of water levels. Reversals in water surface elevation, maximum stage levels, and length of the spring flood were the most important parameters influencing abundance of age-zero fishes in annual collections. Smallmouth buffalo (Ictiobus bubalus), black crappie (Pomoxis nigromaculatus), freshwater drum (Aplodinotus grunneins), and white bass (Morone chrysops) were most abundant in samples during years that approximated the natural water level regime. Of the 33 hydrologic parameters evaluated for the entire water year from an Illinois River gage site on La Grange Reach, all except average stage in January and Julian date (JD) of maximum stage had moderate or high hydrologic alteration based on the historical range of variation (RVA). The highest degree of hydrologic alteration was for minimum stage levels (1-day, 3-day, and 7-day), rate-of-rise, and rate-of-fall. Other parameters that have been severely altered were 30-day minimum stage, 90-day maximum stage, and the annual number of water level reversals. Operations of the La Grange and Peoria locks and dams could be modified so water level variability would approximate that of the late 1800s, when fish and wildlife resources were abundant. The water regime could be regulated to maintain navigation and improve conditions for native plants and animals without increasing flood damages.