DOI: 10.21335/nmdc-1940051508
title: Ascending Atlantic salmon (Salmo salar) and sea trout (S. trutta) in the river Etneelva, 2013-2017.

description: Ever since monitoring commenced in the late 1980s, the number of escaped farmed salmon that have entered the river Etneelva has been a matter of serious concern to Norway’s environmental and fisheries management authorities. The association of property owners along the river Etneelva has repeatedly requested that measures be taken against the escapees. As a consequence, the board of directors of the national pilot project “Immediate Priority Measures for Securing the Viability of Anadromeous Salmonid Stocks in the Hardangerfjord Region Pending Longer-Term Management Measures” decided to carry out a three-year test of the Resistance Board Weir, a type of salmon trap developed in North America.

The objective of the project is to test this technology’s efficacy in preventing upstream migration of escaped salmon, and to provide accurate data on the escapees’ number, size, sexual maturation, and time of migration up the river Etneelva. Additional aims are to obtain data on migratory wild salmon and sea trout, including their time of migration, their age, and the smolt year class to which they belong.

The trap was in operation in 2013 and 2014. Classified by external characteristics, a total of 1154 migratory wild salmon were recorded in 2013, compared to 417 in 2014; in other words, the 2014 total was 36 % of what was registered in 2013. The decline was dramatic for all age groups examined, i.e. salmon from the 2013, 2012 and 2011 smolt year classes. These observations deviate from the situation for that of one-sea-winter (1SW) salmon along large stretches of the Norwegian coast. By contrast, the river Suldalslågen reported catches totalling 977 salmon in 2013, and 1153 in 2014. The development in Etneelva is thus quite different from that of stocks in rivers outside the Hardangerfjord region. For sea trout in Etneelva, 360 individuals were registered in 2014, a reduction to 39 % compared to the 922 sea trout registered in 2013.

The date which 50 % of the wild salmon were registered was about four weeks later in 2014 than in 2013. Another 2014 observation was that escaped farmed salmon migrated upstream later than the wild salmon, and also that the average escapee arrived roughly four weeks later than in 2013.

The capture efficacy for wild salmon in 2013 and 2014 was 80–85 %, and slightly higher for escaped fish. In 2014, the percentage of escaped farmed salmon caught in the weir decreased from approximately 28 % to 4 %.

The Resistance Board Weir represents a unique research platform. In a national context, the weir is a valuable monitoring station for collecting a wide range of data on escaped farmed salmon, including their number, size, migration date, migration and distribution relative to the escape point, as well as sexual maturation, fish health and infection rates, and genetic identification of different groups and breeding lines. The weir is a relatively effective means for removing escaped farmed fish from certain rivers. For wild salmon, sea trout and escaped farmed salmon, the weir is a key part of a national network for monitoring migration rates for each smolt year class, the growth of the various year classes, migration dates and fish health. The weir is also useful for investigating salmon louse infections and their impact on various stocks.

The Resistance Board Weir is robust and has been able to withstand the extreme water flow of Etneelva, although it does require significant human resources to carry out the manual sampling and daily cleaning. If this trapping method is to be continued in Etneelva or introduced to other rivers, it would be advantageous to develop the weir technology to achieve a greater degree of automation.