Aerial Application of Organic Pellets Eliminates Lake Trout Recruitment from a Primary Spawning Reef in Yellowstone Lake

Abstract

Invasive Lake Trout Salvelinus namaycush in the Yellowstone Lake ecosystem have been gillnetted since 1995 to suppress the population and allow for recovery of native Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. Although gillnetting is effective (Lake Trout population growth rate λ ≤ 0.6 during 2012-2022), the effort only targets free-swimming, age-2 and older Lake Trout. We developed a complementary suppression method using organic (soy and wheat) pellets to cause Lake Trout embryo mortality and reduce recruitment from spawning areas. The entire Carrington Island spawning reef (0.5 ha) was aerially treated with 3.56 and 3.00 kg/m 2 of pellets in 2019 and 2020, respectively. Pellet decomposition caused dissolved oxygen concentrations to decline to lethal levels at 20 cm depth in the substrate, and pellets mostly dissipated from the reef within 12 d. Lake Trout fry trap CPUE was reduced to zero

Fisheries Volume 49 Number 2 February 2024

“…Carcass analog pellets have been found to increase mortality in Lake Trout embryos at spawning reefs (Koel et al 2020b(Koel et al , 2023a and are believed to be a promising complimentary suppression method for Yellowstone Lake in addition to gill nets. Pellet application is unlikely to have long-term effects on nontarget organisms (Briggs et al 2020 ;Lujan et al 2022 ), did not deter future spawning by adults, or result in other unintended consequences (Koel et al 2023a ). We did not have specifi c information regarding the production of Lake Trout at different spawning sites and making assumptions about the effect of pellets at the different sites without those data could be misleading.…”

Yellowstone Cutthroat Trout Recovery in Yellowstone Lake: Complex Interactions Among Invasive Species Suppression, Disease, and Climate Change

“…Carcass analog pellets have been found to increase mortality in Lake Trout embryos at spawning reefs (Koel et al 2020b(Koel et al , 2023a and are believed to be a promising complimentary suppression method for Yellowstone Lake in addition to gill nets. Pellet application is unlikely to have long-term effects on nontarget organisms (Briggs et al 2020;Lujan et al 2022), did not deter future spawning by adults, or result in other unintended consequences (Koel et al 2023a). We did not have specific information regarding the production of Lake Trout at different spawning sites and making assumptions about the effect of pellets at the different sites without those data could be misleading.…”

“…Therefore, we assumed carcass analog pellets had equal application and equal mortality at all spawning sites to represent an ideal pellet application scenario. Some spawning sites are concentrated in distant locations or at depths that are too great for uniform pellet application from the lake surface (Koel et al 2020b(Koel et al , 2023a. We acknowledge that application of carcass analog pellets to all spawning sites is currently infeasible due to the logistics that would be involved to make, transport, and apply the carcass analog pellets.…”

“…This approach has been useful to estimate the amount of gill netting effort needed to suppress Lake Trout (Syslo et al 2020), with a suppression program cost of US$2.85 million in 2019 (Koel et al 2020a). Ways to reduce suppression cost, including reducing gill netting effort and using alternative suppression methods, such as embryo suppression via carcass analog pellets (Koel et al 2020b(Koel et al , 2023a, have been suggested. Carcass analog pellets deposited on Lake Trout spawning sites have proven to increase mortality in Lake Trout embryos at spawning locations (Koel et al 2020b(Koel et al , 2023a.…”

“…Ways to reduce suppression cost, including reducing gill netting effort and using alternative suppression methods, such as embryo suppression via carcass analog pellets (Koel et al 2020b(Koel et al , 2023a, have been suggested. Carcass analog pellets deposited on Lake Trout spawning sites have proven to increase mortality in Lake Trout embryos at spawning locations (Koel et al 2020b(Koel et al , 2023a. Pellet application is unlikely to have long-term effects on nontarget organisms, such as benthic invertebrates and periphyton (Briggs et al 2020;Lujan et al 2022), making pellets a promising alternative suppression method in Yellowstone Lake.…”