Genetic changes caused by restocking and hydroelectric dams in demographically bottlenecked brown trout in a transnational subarctic riverine system

Abstract

Habitat discontinuity, anthropogenic disturbance, and overharvesting have led to population fragmentation and decline worldwide. Preservation of remaining natural genetic diversity is crucial to avoid continued genetic erosion. Brown trout (Salmo trutta L.) is an ideal model species for studying anthropogenic influences on genetic integrity, as it has experienced significant genetic alterations throughout its natural distribution range due to habitat fragmentation, overexploitation, translocations, and stocking. The Pasvik River is a subarctic riverine system shared between Norway, Russia, and Finland, subdivided by seven hydroelectric power dams that destroyed about 70% of natural spawning and nursing areas. Stocking is applied in certain river parts to support the natural brown trout population. Adjacent river segments with different management strategies (stocked vs. not stocked) facilitated the simultaneous assessment of genetic impacts of dams and stocking based on analyses of 16 short tandem repeat loci. Dams were expected to increase genetic differentiation between and reduce genetic diversity within river sections. Contrastingly, stocking was predicted to promote genetic homogenization and diversity, but also potentially lead to loss of private alleles and to genetic erosion. Our results showed comparatively low heterozygosity and clear genetic differentiation between adjacent sections in nonstocked river parts, indicating that dams prevent migration and contribute to genetic isolation and loss of genetic diversity. Furthermore, genetic differentiation was low and heterozygosity relatively high across stocked sections. However, in stocked river sections, we found signatures of recent bottlenecks and reductions in private alleles, indicating that only a subset of individuals contributes to reproduction, potentially leading to divergence away from the natural genetic state. Taken together, these results indicate that stocking counteracts the negative fragmentation effects of dams, but also that stocking practices should be planned carefully in order to ensure long‐term preservation of natural genetic diversity and integrity in brown trout and other species in regulated river systems.

Effects of run‐of‐river hydropower plants on fish communities in montane stream ecosystems in Serbia

“…Heggenes and Røed (2006) stated that dams increased overall genetic diversity of brown trout in the river Mana in Norway, assigning genetic drift to be the strongest evolutionary mechanism in small, recently fragmented populations. Junge, Museth, Hindar, Kraabøl, and Vøllestad (2014) reported brown trout to be the fish species most affected by damming the river Otta's system in mid-Norway and Klütsch et al (2019) reported genetic isolation between brown trout in sections between dams and loss of genetic diversity in the subarctic river Paswik system subdivided by several dams at the border between Norway, Finland, and Russia.…”

“…Kohout, Jaškov a, Papoušek, Šediva, and Šlehta (2012) reported that stocking practice in Czech and Slovakian streams resulted in massive hybridization between the Atlantic and Danubian brown trout strains and loss of population heterogeneity. Klütsch et al (2019) recorded the occurrence of bottlenecks and the homogenization of brown trout stocks in stocked sections of the river Paswik, which implies loss of the original genetic state. They regarded the stocking as a mechanism that cannot counteract the negative fragmentation effect of dams, while additionally aggravating effects of fragmentation, resulting in irreparable harm to native brown trout stocks, which has already been witnessed in streams of Serbia (Simonovi c et al, 2015).…”

Quantifying the individual impact of artificial barriers in freshwaters: A standardized and absolute genetic index of fragmentation

“…Although restrictive, this adjacent sampling design has the advantage of making the F INDEX valid for almost any freshwater species, regardless of their life-history traits: The effective population size, a key parameter that may obviously differ across species, is indeed directly taken into consideration in the F INDEX computation, while differences in dispersal abilities can be considered as null at very short distances. It yet implies the exclusion of migratory fish species, though at the heart of great conservation issues (Junge et al, 2014;Klütsch et al, 2019):…”

“…Although restrictive, this adjacent sampling design has the advantage of making the F INDEX valid for almost any freshwater species, regardless of their life‐history traits: The effective population size, a key parameter that may obviously differ across species, is indeed directly taken into consideration in the F INDEX computation, while differences in dispersal abilities can be considered as null at very short distances. It yet implies the exclusion of migratory fish species, though at the heart of great conservation issues (Junge et al., 2014; Klütsch et al., 2019): complex life cycles such as anadromy (“river‐sea‐river” migrations), catadromy (“sea‐river‐sea” migrations), or potamodromy (“river‐lake‐river” migrations) indeed preclude the delineation of upstream and downstream populations and do not allow proper estimates for the F INDEX . In nonmigratory fish species, this assumption also prevents the use of the F INDEX in large‐scale studies, in which the distance between the upstream and the downstream sampling sites lies beyond the dispersal capacities of the studied species.…”

Temporal analysis shows relaxed genetic erosion following improved stocking practices in a subarctic transnational brown trout population

“…Hence, standing genetic variation is the basis for responses to anthropogenic pressures and environmental change, and it ensures the long-term survival of species 1 – 3 . Spatial genetic analyses have greatly improved our understanding of intraspecific genetic variation, including phenomena like gene flow and connectivity and/or isolation of populations 1 , 3 , 4 . In comparison, temporal changes in genetic variation are less studied, despite their importance for predicting impacts of anthropogenic activities and environmental disturbances on natural populations 1 , 3 , 5 , 6 .…”