Faculty of Life and Environmental Sciences, University of Iceland, Reykjavík, Iceland.
Marine and Freshwater Research Institute, Selfoss, Iceland.
PeerJ. 2023 Sep 5;11:e15985. doi: 10.7717/peerj.15985. eCollection 2023.
Lake Þingvallavatn in Iceland, a part of the river Ölfusá drainage basin, was presumably populated by brown trout soon after it formed at the end of the last Ice Age. The genetic relatedness of the brown trout in Þingvallavatn to other populations in the Ölfusá drainage basin is unknown. After the building of a dam at the outlet of the lake in 1959 brown trout catches declined, though numbers have now increased. The aim of this study was to assess effects of geographic isolation and potential downstream gene flow on the genetic structure and diversity in brown trout sampled in several locations in the western side of the watershed of River Ölfusá. We hypothesized that brown trout in Lake Þingvallavatn constituted several local spawning populations connected by occasional gene flow before the damming of the lake. We also estimated the effective population size (N) of some of these populations and tested for signs of a recent population bottleneck in Lake Þingvallavatn.
We sampled brown trout inhabiting four lakes and 12 rivers within and near the watershed of River Ölfusá by means of electro- and net- fishing. After stringent data filtering, 2,597 polymorphic loci obtained from ddRADseq data from 317 individuals were ascertained as putative neutral markers.
Overall, the genetic relatedness of brown trout in the Ölfusá watershed reflected the connectivity and topography of the waterways. Ancestry proportion analyses and a phylogenetic tree revealed seven distinct clusters, some of which corresponded to small populations with reduced genetic diversity. There was no evidence of downstream gene flow from Lake Þingvallavatn, although gene flow was observed from much smaller mountain populations. Most locations showed low N values (., ~14.6 on average) while the putative anadromous trout from River Sog and the spawning population from River Öxará, that flows into Lake Þingvallavatn, showed notably higher N values (., 71.2 and 56.5, respectively). No signals of recent population bottlenecks were detected in the brown trout of Lake Þingvallavatn.
This is the first time that the genetic structure and diversity of brown trout in the watershed of River Ölfusá have been assessed. Our results point towards the presence of a metapopulation in the watershed of Lake Þingvallavatn, which has been influenced by restoration efforts and is now dominated by a genetic component originated in River Öxará. Many of the locations studied represent different populations. Those that are isolated in headwater streams and lakes are genetically distinct presenting low genetic diversity, yet they can be important in increasing the genetic variation in downstream populations. These populations should be considered for conservation and direct management.
冰岛的廷瓦拉湖是上一个冰河时代末期形成的，据推测，在形成之后不久，这里就出现了褐鳟。廷瓦拉湖中的褐鳟与 Ölfusá 流域的其他种群的遗传关系尚不清楚。1959 年在湖出口修建大坝后，褐鳟的捕捞量下降，但现在数量已经增加。本研究的目的是评估地理隔离和潜在的下游基因流对 Ölfusá 河流域西部几个地点采集的褐鳟遗传结构和多样性的影响。我们假设在筑坝之前，廷瓦拉湖中的褐鳟由几个通过偶然基因流连接的局部产卵种群组成。我们还估计了其中一些种群的有效种群大小（N），并检测了廷瓦拉湖是否存在近期种群瓶颈的迹象。
我们通过电捕鱼和网捕鱼的方式在 Ölfusá 流域内和附近的四个湖泊和 12 条河流中采集褐鳟。经过严格的数据筛选，从 317 个个体的 ddRADseq 数据中确定了 2597 个多态性位点作为假定的中性标记。
总的来说，Ölfusá 流域的褐鳟遗传关系反映了水道的连通性和地形。亲缘关系比例分析和系统发育树显示出七个不同的聚类，其中一些与遗传多样性降低的小种群相对应。虽然从小型山区种群观察到基因流，但没有来自廷瓦拉湖的下游基因流的证据。大多数地点的 N 值较低（平均约为 14.6），而来自 Sog 河的假定溯河洄游鳟和流入廷瓦拉湖的 Öxará 河的产卵种群的 N 值则明显较高（分别为 71.2 和 56.5）。在廷瓦拉湖的褐鳟中未检测到近期种群瓶颈的信号。
这是首次评估 Ölfusá 河流域褐鳟的遗传结构和多样性。我们的结果表明，廷瓦拉湖流域存在一个复合种群，该种群受到了恢复工作的影响，目前主要由源自 Öxará 河的遗传成分主导。所研究的许多地点代表不同的种群。那些孤立在源头溪流和湖泊中的种群具有独特的遗传特征，遗传多样性较低，但它们在增加下游种群的遗传变异方面可能很重要。这些种群应该考虑进行保护和直接管理。
