Study design: The focus of our study was Hidden Lake, Banff National Park, Canada (Table 1), a high elevation mountain lake in the Canadian Rockies that received rotenone treatment during the summers of 2018 and 2019 to eradicate non-native brook trout (Salvelinus fontinalis) that were introduced and established in the 1970s, leading to the extirpation of a population of Westslope cutthroat trout (Oncorhynchus clarkii lewisi). The rotenone treatment was done accordingly to Montana State (USA) rotenone policy in the absence of a Canadian equivalent. This policy recommends two rotenone treatment for brook trout eradication because their spawning is not perfectly synchronous and because brook trout eggs in the gravel are not susceptible to rotenone (MFWP 2017). Moreover, several fish and traces of environmental DNA from brook trout were detected between the two rotenone treatments in summer 2018 and 2019 (Derry A, unpubl. data). The rotenone formulation applied to Hidden Lake (21-22 Aug 2018 and 13 Aug 2019) was Nusyn-Noxfish® and contained 2.5 % rotenone active ingredient. The theoretical rotenone concentration of Hidden Lake once it penetrated the thermocline by pumping was 30 ppb and 25 ppb in 2018 and 2019 respectively (Parks Canada 2020).
For whole macroinvertebrate communities, Hidden Lake was sampled in mid-July 2018 (5 weeks before the first rotenone application), early September 2018 (3 weeks after the first rotenone application) and late July 2019 (11 months after the first rotenone application). To partition seasonal variance and to determine if the range of responses observed in Hidden Lake to rotenone treatment were different from the range of community variation observed between lakes in the region, samples from Hidden Lake were compared with 11 Rocky Mountain lakes that did not receive rotenone treatment (nine lakes with brook trout populations and two fishless lakes) in Banff, Kootenay, and Yoho National Parks, Canada. The 11 reference lakes were sampled for macroinvertebrate communities at the same seasonal sampling periods of Hidden Lake, but only in 2018 (mid-July and early September 2018).
Field and laboratory: Nearshore littoral macroinvertebrate communities were collected semi-quantitatively with a D-frame kick net at eight sampling stations located around the lake. Sampling was performed by the Traveling-Kick-and-Sweep method with a 500 μm “D-net” as recommended by Jones et al. (2007) on a surface of approximatively 2 m2. Samples were concentrated with a 500-µm sieve and preserved in 95% ethanol. Identification was done to the family level for insect larvae, at the sub-class level for Collembola, and at the phylum level for Platyhelminthes and Mollusca with the exception of the Bivalvia class that was counted separately. Identification was done using a 6.3-63x dissecting microscope and Merritt et al. (2008) and Moisan (2010). Sub-samples were taken and counted until the 100th individual was reached. A ratio was calculated between the sample volume and the subsample volume to estimate the density of macroinvertebrates.
References:
Jones C, Somers K, Craig B, Reynoldson T. 2007. Ontario benthos biomonitoring network: protocol manual. Dorset (ON): Ontario Ministry of Environment. Environmental Monitoring and Reporting Branch.
Merritt RW, Cummins KW, Berg MB. 2008. An introduction to the aquatic insects of North America. 4th ed. Dubuque (IA): Kendall Hunt publishing company.
[MFWP] Montana Fish, Wildlife and Parks: Fisheries Division. 2017. Montana Rotenone Policy. Approved by: Eileen Ryce, Fisheries Division Administrator. Issued April 18, 1996. Revised April 5, 2017
Moisan J. 2010. Guide d’identification des principaux macroinvertébrés benthiques d’eau douce du Québec – Surveillance volontaire des cours d’eau peu profonds. Bibliothèque et archives nationales du Québec (CA) : Direction du suivi de l’état de l’environnement, ministère du Développement durable, de l’Environnement et des Parcs. Québec (CA). ISBN : 978-2-550-58397-4.
Parks Canada. 2020. Summary report for the chemical removal of brook trout from Hidden Lake, Upper Corral Creek and Tributaries. Radium Hot-Springs (BC): Internal report.
