Oldest brown trout in Lake Thingvallavatn 19 years old 2019

Johannes Sturlaugsson

thingvallaurridi-brown_trout-salmo_trutta-copyright_johannes_sturlaugsson_laxfiskarPicture shows a male brown trout in river Oxara

During my annual monitoring of the brown trout in Lake Thingvallavatn for roughly 20 years, I have experienced many significant changes. For example, the brown trout stocks has expanded enormously during that period. In 1999 the number of spawners participating in spawning in the rivers were within 100, but last autumn they were around 3000. Also, the numbers of inactive spawners taking a break from spawning 2019, were estimated around 1000. So the total numbers of spawners in 2019 were around 4000, when considering the number of both the active and the inactive spawners. In addition to the spawners, there are all the thousands of immature brown trout in the lake. These piscivorous brown trout stocks of Lake Thingvallavatn are characterized by longevity, late maturation and hence a large body size. An immature fish can e.g. exceed 10 kg in body weight. The largest brown trout in my studies according to weight was a female that weighed 12,7 kg, but the longest one was 111 cm long old and skinny male. The brown trout of stocks of Lake Thingvallavatn and some other brown trout stocks from other lakes also located in higher areas inland are often called Iceage brown trout in Iceland (ice. √≠saldarurri√įi), for they were landlocked after the land rose following the last iceage. The features of the brown trout stocks in Lake Thingvallavatn are the same as of the ferox trout stocks in Scotland, Ireland and also Britain.

Studies show drastic increase in size of stocks in Lake Thingvallavatn              

 As my studies on the brown trout in Lake Thingvallavatn show, there has been a drastic increase in the size of the stocks, especially during the last decade. Parallel to this is the increased number of larger fish and older fish observed. In the autumn 2019 I  captured many old-timers in river Oxara during my research fishing at the spawning grounds. Whereof the oldest being 19 years old, which is the highest known age of a brown trout in Lake Thingvallavatn. Among these oldest fish at the spawning grounds was one that had participated for 8 years in the spawning, but had also taken a year break from spawning in between, so his total period of spawning participation did cover 12 years. Due to these characteristics of the brown trout in Lake Thingvallavatn, some fish have been captured many times. In some cases the fish was captured, both in the lake itself as well as at the spawning grounds in the rivers. One example of this is a brown trout I managed to recapture for 8 years in a row, when the fish entered the spawning grounds of the river.

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