Revealing patterns of genetic diversity in the brown trout of Lake Þingvallavatn and the surrounding area | Institute of Life and Environmental Sciences

21. September 2023

In the first study of its kind, scientists at the University of Iceland and Marine and the Freshwater Research Institute have investigated the genetic structure and diversity of the brown trout populations in Lake Þingvallavatn and other parts of the Ölfusá watershed. They have discovered that genetic relatedness between trout populations reflects the connectivity of the waterways and formation of impassable waterfalls. Their findings were published this September in the journal PeerJ. The scientists aimed to map the genetic diversity of brown trout in the western part of the Ölfusá watershed and use the genetic data to answer questions about relatedness between fish in different locations and effective population sizes.

The Ölfusá watershed is large. It begins in the glaciers of Langjökull and Hofsjökull and extends over a large part of South and West Iceland, including the volcano Hengill and the river Varmá, Lake Þingvallavatn and the river Sog, the last of which merges with the river Hvítá to become the river Ölfusá. The Ölfusá watershed contains a multitude of rivers and lakes, which are home to all Iceland's main species of freshwater fish, i.e. salmon, brown trout, Arctic char, sticklebacks and European eels. Some of the brown trout and Arctic char populations are non-migratory, spending their entire life cycles in freshwater. Populations are always non-migratory where they have become isolated over time by the formation of impassable waterfalls, but sometimes even fish that are able to access the ocean do not migrate, especially populations that mature in lakes.

The research focused on Lake Þingvallavatn, which has long been famed for its particularly large brown trout, as well as nearby rivers and lakes, including streams on the slopes of Hengill. Numbers of brown trout in Þingvallavatn declined after an outlet of the lake was dammed in 1959, but several attempts have been made to revive the spawning population in the southern part of the lake and catches have increased significantly over the last two decades.

Pattern of genetic diversity determined by topography

The scientists sampled brown trout from four lakes, including Þingvallavatn, Úlfljótsvatn and Hestvatn, and 12 rivers, including Öxará, Efra-Sog, Sog, Hvítá, the streams flowing into Hengladalsá and Ölfusvatnsá. DNA from just over 300 fish was analysed, revealing clear signs of genetic differences related to the way the landscape of the Ölfusá watershed has evolved since the end of the last Ice Age.

The trout that were caught in lakes and rivers above waterfalls within the same watershed, such as Úlfjótsvatn, Þingvallavatn and their tributaries Öxará and Efra-Sog, were genetically very similar and distinct from both headwater populations in other watersheds and lowland populations in the Ölfusá watershed. This suggests that Þingvallavatn and Úlfljótsvatn were populated by trout which then became isolated shortly after the end of the Ice Age. Although fish do occasionally find their way down the waterfalls in the river Sog, this study found no evidence that they breed with the lowland populations to any great extent. However, there does seem to have been downstream gene flow from smaller populations on Hengill to the rivers Varmá and Ölfusá.

The effective population size proved very small in most of the locations, except in the rivers Öxará and Sog (where it was 2-3 times larger, respectively). The scientists found no signs that the brown trout population in Þingvallavatn suffered such a dramatic decline after the lake was dammed that it caused a population bottleneck.

The brown trout is a key player in the ecosystem of the Ölfusá watershed and the identification of unique populations and genetic relations among populations is an important tool for proper management and conservation in this area. This study is an important contribution to our knowledge of brown trout in this region. Diversity within a species is an important aspect of biodiversity, particularly in Arctic regions such as Iceland. The genetic structure and diversity of brown trout populations in a specific region reveals the way they have evolved since the settlement of Iceland, and also has significance for the future of the species.

The research was funded by the National Power Company and the University of Iceland, but these parties had no influence over the design of the study or the interpretation of results.