The brown trout of Thingvallavatn and Ölfusá

Marcos Lagunas and his collaborators from the University of Iceland and the Marine and Freshwater Institute teamed up for a project on the genetics of brown trout. Last year, they published an article where they looked into the genetic structure of different trout populations in Thingvallavatn and Ölfusá.

The decline of trout in Thingvallavatn

The number of brown trout in Thingvallavatn heavily declined between 1960 to 1990. Why the trout declined has never been fully known but the building of a dam in 1959 at the South of the lake might have contributed. This dam dried up a small river leading out of the lake and destroyed an important feeding ground for juvenile (young) trout. 

To help the trout recover, fish from the breeding grounds in the river Óxará were used to rear eggs in the 1990s and early 2000s. This river is connected to Thingvallavatn on the northern shore of the lake and is an important breeding ground for trout. Juvenile trout and eggs were then released into the lake to increase the number of trout. Currently, the number of brown trout is high again, so these efforts seem to have been successful.

In 2016 and 2017 Marcos and his team caught brown trout from Thingvallavatn and surrounding rivers using gill nets and electrofishing. To look into the genetics of these trout he took a small piece of the caudal fin (the fin on the tail) that was used to extract the DNA of each trout. 

The effect of restoration efforts is shown in trout DNA

Marcos and his team looked into the genetic structure of the trout in Thingvallavatn. They could see that many trout from the lake had a genetic component that originated from trout in the river Öxará. This was not too much of a surprise as this may have been a result of the restoration efforts from the 1990s and 2000s. 

Also, they found that different trout populations in the area have small genetic diversity compared to trout that have access to the ocean. This could hinder their ability to adapt to changing environments which is crucial in these times of rapid climate change.

But why is it so important that we look into the genetics of fish?

We hear more and more about fish species being threatened, especially due to climate change and overfishing. The first step in ensuring conservation is to identify fish that are unique or have reduced genetic diversity.

Going back to the study, the authors found a specific trout population in the Hengill volcanic area that provides genetic variability to trout in Thingvallavatn. Some of the streams where these Hengill trout live have an inflow of hot water due to hot springs in the area. Because of this, the trout living there might have genetic variability linked to their tolerance to higher water temperatures. This might be essential to the survival of brown trout in Thingvallavatn and Ölfusá with climate change on the rise!

Read Marcos’ paper here if you want to learn more about the brown trout in Thingvallavatn and the Ölfusá watershed.