The brown trout
The brown trout in Lake Þingvallavatn, whose origin can be traced to Britain, has long been one of the most talked-about freshwater fish in Iceland - and abroad. The brown trout's fame is based primarily on its large size and great numbers. After it became isolated in Þingvallavatn in the wake of the last ice age, living conditions proved beneficial, and it was found in many populations all over the lake.
The best-known population was connected with Efra-Sog, which was the natural output of Þingvallavatn to the south, while another well-known population still has its spawning site in the river Öxará. The main reasons why the lake's largest brown trout population stayed in Efra-Sog were the strong current and river gravel that created good conditions for spawning and for the development of black flies, a good food source.
The brown trout can reach an incredible size, that once attracted anglers from all over the world. Records show that it wasn't uncommon to catch a 20-30 pound brown trout. When the Steingrímsstöð hydroelectric plant was built in 1959 at the southern end of the lake, the largest spawning site of the brown trout was destroyed, and the largest brown trout population in the lake has yet to recover.
Recently, the life habits of the brown trout have been researched in detail in order to learn more about this lake giant. During this research, brown trout up to 20 pounds in weight have been observed in the river Öxará.