As part of building of a new highway (E6) between Trondheim and Værnes in central Norway, plans are in place to fill part of Hellstranda beach, a part of the river Stjørdalselva estuary, with boulders. An extension of the road intersection Værneskrysset is also included in the plans for the new highway, which may ultimately have an impact on the old riverbed east of Langøra sør. The purpose of our study was to use electronic tagging of individual fish with acoustic transmitters (acoustic telemetry) during August 2020 to September 2021 to better understand the behavior and space use of individual anadromous brown trout (sea trout, Salmo trutta) and Atlantic salmon (Salmo salar) at Hellstranda beach and in the old riverbed at Langøra sør. Information from this study was used to evaluate possible negative impacts of the road building on these fish species and develop and evaluate mitigation measures that may reduce these negative impacts.
The results showed that the planned intervention area at Hellstranda is an important feeding area for sea trout. Sea trout were observed in this area year-round, but with spring and summer as the most important periods. Most sea trout veterans (sea trout that have been at sea for at least one season before being tagged) stayed in the area from late April to late September, while most trout smolts (first-time migrants) were registered in May. The old riverbed east of Langøra sør is also an important feeding area for sea trout, and in this area, they were also commonly detected during the winter.
Monitoring of sea trout smolt with a fyke net in river Gråelva, a tributary to river Stjørdalselva, showed that smolts from this river migrated downstream during the whole period when the trap was in operation (15 April – 28 May). In the rotary screw trap at Sona bridge in the main river (upstream of river Gråelva), the first trout smolts were caught in mid-April, but most smolts were caught in the last half of May. Most sea trout smolts arrived in the estuary during the night (77% of tagged smolts).
During the tracking period, the sea trout at Hellstranda and in the old riverbed at Langøra sør remained in the brackish water in the upper water layers of the water column (mean swimming depth 1.5-2.0 m) and not in the more saline water layer closest to the bottom.
The Atlantic salmon spent only a short time at Hellstranda and in the old riverbed at Langøra sør when they migrated from the river to their feeding grounds at sea. In general, both salmon kelts and smolts stayed less than a day in the river estuary. Although the residence time in the river mouth for salmon was short, the transition phase can still represent an important part of the seaward migration, particularly for salmon smolts. In comparison to adult stages of the salmon’s life cycle, it is more demanding for smolts to regulate salt balance when move from freshwater to sea water. This makes them more vulnerable also to other negative impacts, and this is a critical phase of their life cycle.
The monitoring of the salmon smolt migration in Stjørdalselva watercourse showed that salmon smolts from the tributary Gråelva migrated out during the first half of May, while salmon smolts from the main river and tributaries upstream of river Gråelva mainly migrated out during the last half of May. The seaward migration most likely continued into June, but the monitoring was terminated on 28 May due to the start of the angling season in the river. Most salmon smolts (69%) arrived the estuary at night.
Three salmon were tagged in the river mouth in August 2020 when they returned from the marine feeding migration. These fish used the entire survey area until October, after which they migrated upstream in the river - probably to spawn. Excluding one salmon tagged in December, the remaining salmon were all tagged in river Stjørdalselva prior to the seaward migration in May 2021. Consequently, there are only few data available on spatio-temporal habitat use of returning salmon kelts at Hellstranda and in the old riverbed at Langøra sør.
The significant increase in sea trout activity levels in May to July at both Hellstranda, the old riverbed at Langøra sør and in the main river channel running through the estuary showed that all three areas are important feeding areas for sea trout in the summer. Loss of feeding areas in the estuary, due to the planned development of new E6, will likely increase competition for food in the remaining areas around Hellstranda and in the old riverbed. As favorable estuarine feeding areas in river Stjørdalselva already have been heavily modified, mainly due to the airport development in the 1950s, further interventions will worsen the situation for sea trout. Furthermore, both the current and a previous study of area use for the sea trout at Langøra sør, indicated that the habitat in the old riverbed is especially important for the youngest sea trout veterans at low water temperatures in winter and early spring. It is therefore strongly recommended to reduce the degradation of the estuary to a minimum and then, as far as possible, to compensate for degraded areas corresponding to what is lost.
For Atlantic salmon, the habitat alterations in parts of the estuary will have less consequences than for sea trout since the salmon spent a significantly shorter time in this area than the sea trout. Nevertheless, the transition zone with brackish water is important, especially for salmon smolts, and mitigating and compen-satory measures for the benefit of sea trout will thus also be favorable for salmon.
As the mapping of the area use for sea trout and salmon has shown that sea trout in particular use the area actively, the following mitigating measures are recommended during the construction phase:
1) Monitor, on a day-to-day basis, behavior and stress level of adult sea trout in the river estuary to be able to adjust specific activities related to the road construction since these may have an extra-large impact on sea trout in the area.
2) Avoid blasting and backfilling in or nearby the estuary between 21:00 - 06:00 during the main migration period (15 April - 1 June) for brown trout and Atlantic salmon smolts.
Three different measures are recommended to mitigate the negative effects of the completed development:
1) Reduce the degradation of the estuary to a minimum strictly necessary for the construction of a new E6.
2) Adapt the new beach zone along Hellstranda so it consists of shallow sand and silt areas that are exposed at low tide towards.
3) Increase the opening between the old riverbed at Langøra sør and the main river so as to ensure that fresh water from the river and marine water from the fjord can still flow in and out.
Further, the current survey supports previously proposed compensation measures. As the survey showed that the shallow sand and silt areas, which are exposed at low tide, are important feeding areas for sea trout in particular, compensatory measures must correspond to such a habitat. Two suggestions include construct-ion of new tidal flats influenced by brackish water and do hence satisfy this requirement.
1) Construction of a new shallow sea area with tidal flats between Sjetéen and the airport runway.
2) Restructuring of the river outlet by constructing a new pier from Billedholmen.