The effects of riverine impoundment on genetic structure and gene flow in two stream fishes in the Mobile River basin

Abstract

Summary Riverine impoundments (reservoirs) are thought to impede natural migration in small‐stream‐inhabiting fishes, resulting in spatially and genetically fragmented populations. However, this hypothesis remains poorly tested, and the genetic consequences of riverine impoundment for stream fishes with differing dispersal capabilities are not well understood. This study utilised a combination of microsatellite DNA loci from 479 individuals and the mitochondrial (mt) DNA cytochrome b gene (810–1140 bp) from 83 individuals from eight streams to compare genetic structure and diversity between reservoir‐fragmented and non‐fragmented groups of two species of stream fishes with differing dispersal capabilities in the Mobile River basin of the south‐eastern United States. For both species, analysis of microsatellite loci revealed genetic discontinuities between neighbouring tributaries that have been fragmented by reservoir construction. This finding was in stark contrast to the high degree of continuity for both species between reference tributaries in a natural river setting. Results from mtDNA revealed no significant genetic structure within reservoir‐fragmented or non‐fragmented groups, indicating a lack of historical genetic structure (i.e. prior to reservoir construction). Microsatellite‐based estimates of genetic diversity and migration were differentially affected in the two species, indicating that stream fishes with relatively high dispersal abilities may be equally or more susceptible to reservoir fragmentation when compared to species with relatively low dispersal abilities. Collectively, our data revealed that riverine impoundment and reservoir‐induced habitat fragmentation adversely affect genetic characteristics in small‐stream‐inhabiting fishes. This is of particular interest in biodiversity hotspots such as the south‐eastern United States where hydroelectric and recreational reservoirs restrict connectivity in aquatic systems.

Crayfish populations genetically fragmented in streams impounded for 36–104 years

“…For both F. validus and F. erichsonianus , gene flow from down‐ to upstream populations was lower in impounded streams compared to unimpounded streams, which is consistent with the expected response to stream habitat fragmentation (Fluker et al, ; Hudman & Gido, ; Meffe & Vrijenhoek, ). In contrast, crayfish populations in up‐ and downstream sections of unimpounded streams displayed high genetic connectivity and bidirectional dispersal and gene flow.…”

“…Detecting genetic signatures of recent fragmentation using molecular data has been difficult (Richmond, Reid, Ashton, & Zamudio, ; Sumner, Jessop, Paetkau, & Moritz, ), particularly when using markers that do not mutate at exceptionally fast rates. For example, genetic differences between fragmented populations of Alabama stream fishes were detected using hypervariable nuclear microsatellite loci but not using mitochondrial DNA sequences (Fluker et al, ). However, in the present study using mitochondrial COI sequences, we detected differences between crayfishes in up‐ and downstream sections of streams that were impounded for only 36 years (Cedar Creek), 40 years (Little Bear), and 104 years (Little Cahaba River).…”

Riverscape genetic variation, migration patterns, and morphological variation of the threatened Round Rocksnail, Leptoxis ampla

“…Given this, populations of pleurocerid species separated by artificial impoundments (e.g. Leptoxis coosaensis ; Whelan et al, ) are probably not as affected as organisms with greater dispersal abilities such as fishes (Fluker et al, ). Impoundments are often considered a primary cause of species decline in the Mobile River Basin (Johnson et al, ; Neves et al, ; Williams, Bogan, & Garner, ), but they may not be the biggest risk to remaining species that have survived initial impoundment.…”

“…Pleurocerid species affected by impoundment probably had similarly low population connectivity prior to anthropogenic habitat modification. Thus, artificial dispersal barriers, particularly on the timescale they have existed, may not affect unnaturally disjunct pleurocerid populations as much as they have been shown to affect animals with greater dispersal abilities like fish (Bessert & Ortí, ; Fluker et al, ).…”

“…Indeed, much of our understanding of gene flow and population genetic patterns in fluvial systems comes from studies of fish and aquatic insects (e.g. Faulks, Gilligan, & Beheregaray, ; Finn & Adler, ; Finn, Blouin, & Lytle, ; Fluker, Kuhajda, & Harris, ; Fluker, Kuhajda, Lang, & Harris, ; Kang, Ma, & He, ; Lecaudey, Schliewn, Osinov, Taylor, Bernatchez, Schliewn, & Osinov, ; Robinson, Simmons, Williams, & Moyer, ; Stobie, Oosthuizen, Cunningham, & Bloomer, ). Moreover, hypotheses that predict diversity patterns across riverine landscapes have been mostly developed with data on freshwater insects and fish (reviewed by Davis et al, ; Hughes et al, ).…”