Research Document 2020/074
Lake Superior Pygmy Whitefish (Prosopium coulterii) population trends, habitat characteristics, and abundance
By van der Lee, A.S., and Koops, M.A.
Abstract
Pygmy Whitefish (PWF, Prosopium coulterii) in Lake Superior have been assessed as Threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). In support of a recovery potential assessment, long-term bottom trawl survey data are used to assess trends in density, identify important habitat characteristics, and estimate population size. This analysis makes use of a statistical approach, R-INLA, that can accommodate complex covariance structures in spatial-temporal data. Trends from the spatial-temporal model indicate that lake-wide PWF biomass (kg/ha) has been in decline since 2013 (~ 1 generation) and may be at the lowest mean biomass since the nearshore bottom trawl survey was expanded to include Canadian locations. Prior to 2013, PWF biomass appeared to follow periodic fluctuations suggesting the population could rebound following successful recruitment. Depth was the only important habitat characteristic, fitted as a quadratic term, in predicting occurrence or biomass. PWF were more likely to inhabit depths between ~ 50 and 110 m, preferring 75 to 90 m, with maximum biomass occurring between 80 and 95 m. Based on lake-wide spatial projections 2018 PWF biomass is estimated to be 68,707 kg (CI: 2,465 – 1,357,612) with 9,774 km2 (CI: 712 – 26,014) of suitable habitat.
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