Abundance Estimates and Genetic Analysis of the Large-bodied Population of Lake Utopia Rainbow Smelt (Osmerus mordax) in Mill Lake Stream, 2023

Abstract

Lake Utopia Rainbow Smelt (LURS) comprise two sympatric populations, referred to as large-bodied (LbP) and small-bodied (SbP), that are differentiated by their morphology, choice of spawning streams and timing of spawning runs. The two populations are also genetically distinct, although there is evidence of gene flow between them. The updated recovery abundance objective for the LbP of a minimum of 5,000 fish in Mill Lake Stream observed on at least one night of spawning was met on both nights of mark-recapture sampling in 2023. Nightly estimates calculated using the adjusted Petersen method, were 25,014 and 29,093 for April 12 and 13, respectively. Nightly estimates scaled using the 170 mm, 142 mm and 130 mm fork length (FL) criteria to account for potential SbP and hybrid smelt present in Mill Lake Stream during nights of sampling range from 10,506 to 24,514 for April 12, 2023, and from 12,219 to 28,511 for April 13, 2023. Large and small-bodied smelt remain genetically and morphometrically distinguishable. Available genetic data do not support the use of one FL cutoff over another due to the range of sizes and genetic variability observed in fish spawning in Mill Lake Stream. While the LURS LbP has met the updated recovery abundance objective, the total number of spawners occupying Mill Lake Stream during annual spawning runs cannot be estimated given that sampling only covers a small portion of the spawning run and the proportion of the whole population spawning on any given night is unknown. Abundance estimates cannot be compared between years due to high annual variability in the estimates and associated error between sampling nights and the limited sampling period. Similarly, the changes in the degree of hybridization over time cannot currently be assessed due to limited sampling and changes in the genetic marker panel post-2016. It is recommended that the updated recovery abundance target of 5,000 spawners be applied to nightly estimates of early (late March to mid-April) spawning fish in Mill Lake Stream until improved genetic and morphological metrics are developed to differentiate the LbP and SbP. Lack of certainty in the proportion of LbP smelt that make up the early spawning run of Mill Lake Stream can result in over-estimated abundance.