Sammendrag
Atlantic salmon (Salmo salar) has spawning and nursery grounds in the lower parts of the river Glomma, downstream from Sarpsfossen. It is assumed that the best areas for spawning and nursery are in the upper 3 km, between Sandesundbrua (E6) and Sarpsfossen. It is further assumed that the so-called “grusørene” (gravel bars) are especially important for salmon spawning and nursery. At low river discharge it is possible to do biological surveys at the gravel bars, and the highest densities of salmon juveniles have been found there.
Borregaard AS in Sarpsborg discharges its effluent to Glomma in the middle and the upper parts of the stretch between Sandesundbrua and Sarpsfossen.
Biological indicators (benthic macroinvertebrates and heterotrophic growth) have indicated considerable organic pollution and poor ecological status downstream of Borregaard’s points of discharge. Heterotrophic growth also occurs in the assumed important salmon spawning and nursery areas and may therefore have a negative effect on the recruitment to the salmon population in the lower part of the river Glomma.
Electro fishing was conducted in the river Glomma at Borregaard 26th of September 2019, mainly to investigate densities of Atlantic salmon in the assumed spawning and nursery areas and to monitor recruitment of salmon. Electro fishing was conducted at six stations from the gravel bars near Melløs to areas near Glomma Papp.
The number of juvenile salmon (0+) captured pr. 100 m2 at the gravel bars (st. 5A+B) in 2019 was lower than the previous years. The number was considerably lower than in 2015, but nearly the same level as in 2018. The number of juvenile salmon varies between the years 2013–2019, without any clear trend during this period.
Nine of 29 investigated individuals were colour marked stocked fish (ca. 31 %), hence a large share of the investigated fish was of natural origin. This small number of investigated individuals, however, provides only a slight indication on juvenile composition in the lower Glomma. Even so, it is apparent that both natural reproduction and stocked fish contributes to the juvenile population.
In addition to Atlantic salmon, we also observed alpine bullhead (Cottus poecilopus), European eel (Anguilla anguilla) and common bleak (Alburnus alburnus). Alpine bullhead and European eel were caught at nearly all stations, with the highest numbers at the gravel bars, where the conditions seem to be favorable for both species.
During the ordinary rod fishing season in Glomma in 2019, it was caught 83 fin-clipped spawners (stocked fish) and 32 non-fin-clipped spawners. This indicated that stocked fish are a major contribution the spawning population in Glomma.
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