Lake Langtjern - fish studies in the Langtjern area 1966-2000

Abstract

Langtjern is located 120 km northwest of Oslo east of Gulsvik in the county of Buskerud. The area is underlain by felsic gneisses and granites. The catchment area is 4.69 km², the lake surface area is 0.227 km² with an average runoff of 750 mm/yr. Langtjern and most of the lakes and ponds in the area are acidified humic lakes. The research activities in the Langtjern area started in 1965. Langtjern lost its population of brown trout in the early 1960's due to effects of acid rain. Recapture of stocked fingerlings (0+) of brook trout and a hybrid between brook trout and Arctic char (baptised brar) in 12 small humic lakes located in the Langtjern area during the years 1966-1975 averaged 28%. For stocked brown trout the recapture was significantly lower. Sporadic stockings of brook trout and brown trout took place from 1976-2000. Brook trout had a high survival with a high percent of recaptures (20-60%), a short life span, and obtained a good quality with red flesh and good condition. After a period of 1-3 years, however, most of the fish had disappeared. The brown trout stockings have given more variable results than the brook trout. The mean annual yield of brook trout and brown trout in Langtjern in the years 1973-2000 was 0.77 kg/ha, 0.42 kg as brown trout and 0,35 as brook trout. The highest yield occurred in 1978 with about 2.1 kg/ha. The improved recapture of the stockings of brown trout may have been caused by the general improvement in water quality during the last 10 years. During the years 1991-1995 11 small ponds in the Langtjern area were stocked with brook trout and brown trout in the spring for growth through the summer season. Fish weighing 20-60 g when stocked grew to weights of 200-300 g during about 110 days. On the average, 10 fish per hectare was stocked and the average yield was 1.5 kg/hectare (range: 1-5 kg). Brook trout gave much better recapture and growth than brown trout.