Impoundments facilitate upstream invasion and introgression: Case studies of fluvial black basses (Micropterus spp.) in the southeastern USA

Abstract

Impoundment construction has resulted in the alternation and loss of fluvial habitats, threatening the persistence of many native fishes. Compounding this threat, non-native species stocked into impoundments often invade interconnected fluvial habitats, where they may negatively affect native species. Black basses (genus Micropterus) are popular sportfishes with divergent ecologies: some taxa are tolerant of impoundments and widely stocked to create fishing opportunities, whereas others are endemic fluvial specialists that are threatened by introgression with non-native congeneric taxa. We investigated whether impoundments facilitate non-native invasion and introgression in two case study systems: Lake Lanier, Georgia, and Lake Tenkiller, Oklahoma. In both case studies, native fluvial taxa inhabited upstream tributaries and a non-native was established within the downstream impoundment. Results from longitudinal surveys of upstream tributaries provided clear evidence that non-natives invaded upstream from impoundments, and in some cases, extensive introgression with native taxa also occurred. Variation in spatial trends of invasion and directionalities of introgression across case studies provided insights into eco-evolutionary drivers. Within the riverscapes studied, proximity to impoundment appeared to influence invasion and introgression dynamics, and in one case, stream size was also influential. Introgression rates also varied markedly across the species pairs studied-from very little introgression to the onset of hybrid swarming-illustrating the importance of underlying eco-evolutionary mechanisms such as habitat alteration, propagule pressure, and reproductive isolation. Our results underscore the need to consider the upstream influences of impoundments, and the non-natives that invade from them, to create more holistic riverscape conservation plans for fluvial fishes, including native black basses.

Fig 1

Study area and longitudinal sampling sites for A) Case Study I in Lake Lanier and B) Case Study II in Lake Tenkiller.

Fig 2. Individual STRUCTURE proportional assignments (q) for Case Study I (Lake Lanier, Georgia), ordered by sampling site.

Cluster colors correspond to the following black bass taxa: non-native Alabama Bass (ALB), native Chattahoochee Bass (CHB), native Florida Bass or Largemouth Bass (FLB/LMB), previously undocumented Redeye Bass (REB), and native Shoal Bass (SHB).

Fig 3

Case Study I (Lake Lanier, Georgia) site-level summaries of A) genomic proportions from STRUCTURE and B) hybrid classifications from NEWHYBRIDS. Cluster colors correspond to the following black bass taxa: non-native Alabama Bass (ALB), native Chattahoochee Bass (CHB), native Florida Bass or Largemouth Bass (FLB/LMB), previously undocumented Redeye Bass (REB), native Shoal Bass (SHB). Hybrid classes include backcrosses (BC), first filial hybrids (F₁), and second filial hybrids (F₂).

Fig 4. Individual NEWHYBRIDS results for Case Study I (Lake Lanier, Georgia) displaying estimated posterior probabilities of belonging to a given hybrid class (P of Z).

Species pairings examined were: A) native Shoal Bass and non-native Alabama Bass; B) native Chattahoochee Bass and non-native Alabama Bass; and C) native Shoal Bass and previously undocumented Redeye Bass. Hybrid classes include backcrosses (BC), first filial hybrids (F₁), and second filial hybrids (F₂).

Fig 5. Triangle plots for Case Study I (Lake Lanier, Georgia) generated in ‘introgress’ that also display corresponding hybrid class assignments from NEWHYBRIDS.

Species pairings examined were: A) native Shoal Bass and non-native Alabama Bass; B) native Chattahoochee Bass and non-native Alabama Bass; and C) native Shoal Bass and previously undocumented Redeye Bass. Shapes were used to contrast hybrid classifications, wherein circles represent non-hybrids, squares represent backcrosses (BC) and triangles represent both first filial (F₁) and second filial (F₂) hybrids.

Fig 6. Individual assignments for Case Study II (Lake Tenkiller, Oklahoma), ordered by sampling site.

Panels include: A) STRUCTURE proportional assignments wherein cluster colors correspond to native Neosho Bass (NEO), non-native Smallmouth Bass (SMB), and native Spotted Bass (SPB); and B) NEWHYBRIDS results displaying estimated posterior probabilities of belonging to a given hybrid class (P of Z) for mixtures of Neosho Bass and Smallmouth Bass. Note that discretizing hybrid classes at a critical posterior probability threshold of ≥0.50 resulted in some signal loss, such as backcrosses towards Smallmouth Bass (BCSMB) in Fig 7B. Hybrid classes include backcrosses (BC), first filial hybrids (F₁), second filial hybrids (F₂).

Fig 7

Case Study II (Lake Tenkiller, Oklahoma) site-level summaries of A) genomic proportions from STRUCTURE and B) hybrid classifications from NEWHYBRIDS. Black bass taxa and hybrid classes include: native Neosho Bass (NEO), non-native Smallmouth Bass (SMB), native Spotted Bass (SPB), backcrosses to parental species (BC), first filial hybrids (F₁), and second filial hybrids (F₂).

Fig 8. Triangle plots for Case Study II (Lake Tenkiller, Oklahoma) generated in ‘introgress’ that display patterns of introgression between native Neosho Bass (NEO) and non-native Smallmouth Bass (SMB).

Panel A) displays corresponding hybrid class assignments from NEWHYBRIDS. Shapes were used to contrast hybrid classifications, wherein circles represent non-hybrids, squares represent backcrosses (BC) and triangles represent both first filial (F₁) and second filial (F₂) hybrids. Panel B) displays corresponding STRUCTURE proportional assignments (q) to non-native Smallmouth Bass (SMB).

Fig 9. Our simple “TLT” (pronounced tilt) framework for contextualizing the eco-evolutionary drivers of invasion and introgression among black basses.

Panel A) illustrates two sliding weights along a beam that toggle habitat alteration and propagule pressure levels, which can become elevated when near impoundments that often harbor non-native black basses. The fulcrum can also move slightly in either direction to: set the playing field” of interactions based on divergence time and ecological niche differentiation of the species in question. Bottom panels show B) an approximation of Case Study I’s (Lake Lanier, Georgia) pairing of native Shoal Bass and non-native Alabama Bass, and C) an approximation of Case Study II’s (Lake Tenkiller, Oklahoma) pairing of native Neosho Bass and non-native Smallmouth Bass.