River Imsa Research Articles

Effects of Carlin tagging and fin clipping on survival of Atlantic salmon ( Salmo salar L.) released as smolts

During the period 1976–1979, in total 2567 wild migrating smolts of Atlantic salmon were Carlin-tagged, 2528 were adipose fin-clipped and 2309 individuals were released unmarked into the River Imsa, S.W. Norway. The smolts were caught in a permanent Wolf trap. The adult return rate to the river was highest for unmarked and lowest for Carlin-tagged fish. The main reason for this is probably a mortality of smolts due to handling, anaesthesia and tagging.

Growth, migration and survival of lake reared juvenile anadromous Atlantic salmon Salmo salar L.

A release of 16.741 underyearlings of Atlantic salmon Salmo salar in Lake Storevatnet in the River Imsa system resulted in a downstream migration of 1.580 smolts. Most of the smolts were 2 years of age, but there was also a significant proportion of one year old smolts. Growth of the lake reared smolts was excellent, but they migrated downstream over a much longer period than naturally produced smolts which descended mainly in May. In total 0,7% of Carlin-tagged lake reared smolts were recaptured as adults compared with 15,4% of the naturally produced smolts. It is suggested that the low survival of lake reared smolts is mainly due to their irregular migration time.

Migration of farmed adult Atlantic salmon with and without olfactory sense, released on the Norwegian coast

The dispersal and migration of farmed Atlantic salmon, Salmo salar, allowed to escape during the summer was studied. Three groups of 4–year–old fish of the River Imsa stock were released in coastal waters off south‐western Norway: one group, with functional olfactory organs, was released at a fish farm 4 km away from the R. Imsa; two other groups, one with transected olfactory nerves and the other with functional olfactory organs, were released in the sea 90 km from the R. Imsa. To compare them with the migration pattern of reared, large smolts of the Imsa stock, a group of 3 + smolts was released in the R. Imsa.Adults of salmon released as 3–year–old smolts homed with high precision to the R. Imsa. Four– year–olds released in the sea were recaptured in the fjord and in the coastal current, the majority north of the places of release. Immatures migrated to feeding areas in the North Atlantic. Matures seemed to enter rivers at random when ready to spawn. There was no difference in migration pattern between anosmics and controls. The olfactory sense was not mandatory for entering fresh water. The results indicate that the homing behaviour of Atlantic salmon is not a direct consequence of a single imprinting of the smolts, and that there is not a direct genetic link for return to a particular river. The present results support the sequential imprinting hypothesis proposed by Harden Jones (1968).

Introduction of Atlantic salmon Salmo salar L. to the river Akerselv, Oslo

In 1981 Carlintagged Atlantic salmon smolts from the river Imsa and river Sandvikselv stocks, and in 1984 smolts from the river Loneelv stock were released in the river Akerselv. The smolts left the river and the inner Oslofjord few weeks after release. Most adult recaptures were reported from the sea fisheries. Adults from the Imsa and Sandvikselv stocks were caught in more distant waters than fish from the Loneelv stock. The main reason for this is probably due to size selective fishery. Straying to neighbouring rivers in Oslojord was very common, especially for the smolts released in 1981. The Loneelv smolts released in 1984 gave the best adult return to the river Akerselv. It is concluded that smolt releases in the river Akerselv particularly of a grilse stock like that of the river Loneelv may be a useful way to develop a recreational fishery in the Oslofjord and the river Akerselv.

Environmental Factors Regulating the Seaward Migration of European Silver Eels (Anguilla anguilla)

We examined the connection between descent of European silver eels (Anguilla anguilla) and water discharge, water temperature, turbidity, photoperiod, light intensity, and moon phase based on 10 yr of field data and field experiments. The eel migration takes place between August and December, and mean water temperature for July–August and mean water discharge for August–October explained 91% of the total variation in the start of the yearly silver eel run. Low mean water temperatures during July–August and high mean water discharge during August–October resulted in an early start of the yearly silver eel run, whereas high mean water temperature and low water discharge gave the opposite effect. There was no significant correlation between the duration of the entire yearly silver eel run and the environmental variables investigated. But the duration of a part of the run, e.g. number of days from 5 to 25% cumulative eel descent, was significantly correlated with mean water temperature and increase in water discharge for the period in question (R2 = 0.84). Maximum silver eel descent was at a water temperature of 9 °C. Few silver eels descended at temperatures below 4 °C or above 18 °C. The migration speed of transplanted silver eels in the River Imsa was correlated with water discharge and water temperature (R2 = 0.88). Water discharge alone explained 85% of the total variation in migration speeds. The recapture rate of silver eels transplanted within the River Imsa was highest at 9 °C and decreased at higher and lower temperatures (R2 = 0.95). Illumination of 20 lx upon the river reduced the descent of silver eels.

Downstream migration of hatchery-reared smolts of Atlantic salmon ( Salmo salar L.) in the River Imsa, Norway

On 16 May 1983 1750 tagged 1+ and 1280 tagged 2+ Atlantic salmon smolts of the River Imsa strain were released into the River Imsa in the evening. In 1984 1409 tagged 2+ smolts were divided into three groups. The first was released at 09.00 h on 21 May, the second group at 21.00 h the same evening, and the third group was released at 21.00 h the next evening. The smolts were recaptured in a Wolf trap, 1000 m downstream. To test effects on smolt migration, the light at the trap (4 lux) was switched on and off successively every 15 min during the first night after the 1983 release. In 1984 the whole width of a fast flowing part of the river was constantly illuminated (88 lux) during the first night after the last release. Most smolts descended during the first 12 h after release, and the smolts released in the morning descended in considerable numbers during daytime. 2+ smolts descended faster than did 1+ smolts. Small 1+ smolts descended faster than larger 1+ smolts whereas large 2+ smolts descended faster than smaller ones. Smolts released in the evening descended faster than smolts released in the morning. Illumination of the river during the night reduced the speed of descent, the effect being most pronounced in the pool just above the trap. Males that were mature in autumn 1983 smoltified and descended when released in 1984, but they were smaller than immature males and females. It is suggested that wild and hatchery smolts staying in the river may be attracted to shoals of released hatchery smolts and join them when migrating downstream.

Migration of wild and hatchery reared smolts of Atlantic salmon, Salmo salar L., through lakes

In 1982 and 1983 descending wild Atlantic salmon smolts, Salmo salar L., were caught in a fish‐trap at the mouth of the River Imsa. Together with hatchery‐reared smolts of the River Imsa strain they were tagged and released at three different sites of the Imsa‐Lutsi watercourse: in the Imsa River 1 km above the trap, and in two lakes, 3 and 11 km upstream of the trap. The recapture‐rate in the fish‐trap decreased with increased migration distance. The hatchery‐reared smolts migrated downstream faster than wild smolts. Lake‐released smolts were considerably delayed in their downstream migration compared to the river released smolts. The results are discussed in relation to environmental and physiological parameters.