Reproductive dynamics of a native brook trout population following removal of non-native brown trout from a stream in Minnesota, north-central USA

Abstract

Manual removal of non-native brown trout (Salmo trutta) led to increased abundance and improved size structure of a native brook trout (Salvelinus fontinalis) population. Reproductive dynamics of brook trout in response to release from this competitor were quantified using 14 microsatellite DNA loci to estimate effective population sizes, family sizes, and parentage across five cohorts. We hypothesized that brown trout removal allowed more brook trout to reproduce and distribute reproductive success more evenly among individuals, thereby increasing generational effective population size (Ne) and cohort effective number of breeders (Nb). However, $${\hat{N}}_{\text{e}}$$ varied little (27–32), but was estimated for only one generation before and after removals began. Similarly, $${\hat{N}}_{\text{b}}$$ differed little between pre-removal (21–25) and post-removal (19–23) cohorts, because increased numbers of reproductively successful adults were offset by highly skewed family sizes. Variance in family size increased following brown trout removal, but was uncorrelated with brook trout abundance across all years. Although most individuals matured at a small size, reproductive success increased with length. Increased abundance of adult brook trout has not increased $${\hat{N}}_{\text{b}}$$ and $${\hat{N}}_{\text{e}}$$ remains low. Managers should consider moving adults from nearby populations to increase genetic diversity of this isolated population.