Salmon migrating through a maze in a changing world: building a management regime for a multi-stock system affected by mixed-stocked fishing

The river Tana support one of the largest stocks of wild Atlantic salmon in the world, with more than 1200 km accessible for ascending salmon, including nearly 30 tributaries. For many years the river catches in Tana amounted to 50 % of the Norwegian river catches and more than 20 % of the river catches of Atlantic salmon in whole Europe. Catches in Tana have fluctuated strongly among years, from 250 tons in 1975 and 2001, to less than 60 tons in 2009. The average catches from 1973-2009 were 135 tons, while the average catches the last 10 years have been scarcely 75 tons. The size of Tana salmon varies from 1-3 kg in one-sea-winter fish to more than 20 kg for fish feeding in sea for 4-5 years. The average size the last 25 years have been around 3.5-4 kg. Researches from NINA (project leader Martin-A. Svenning) have in cooperation with Norwegian, Finish and Russian colleges, tried to explore the reason for the strong decline in catches by examining historical catch data. Further, biological and genetic parameters have been collected from more than 16 000 wild salmon captured along the North-Norwegian coast, together with 20 000 wild salmon captured in the Tana river. Based on a genetic analysis of ?Tana-salmon-scales? sampled from 36 localities in the Tana river system, we produced an extensive genetic baseline (including 33 microsatellite markers). We identified 28 hierarchically structured and genetically distinct population segments in Tana River, and the strong genetic structuring allowed for accurate stock identification of individuals and enabled assessment of stock compositions contributing to the mixed-stock fisheries in the river. In order words, all Tana salmon captured could be assigned to their natal river/tributary. Thus, the biological knowledge gained from this study is essential for explaining the impact of the selective fishery on each of the sup-populations of Tana salmon. We have used a similar genetic approach, when we generated a comprehensive Atlantic salmon genetic baseline consisting of 185 populations along the Norwegian, Barents and White Sea coasts, genotyped for a high number (33 microsatellites) of genetic markers, which has a dense geographical coverage and includes all notable stocks in this region. The hierarchical genetic structure observed was the basis for defining 7 regional (RGR) and 26 local (RGL) reporting groups (RGs) enabling practical implementation of genetic stock identification (GSI). Although the GSI accuracy varied across populations and the study area, depending on the level of genetic structuring, a high level of accuracy was achieved when individuals were classified to regional (97%) or to local (90%) RGs. This baseline made it possible to confirm the home region (natal river) of more than 16 000 salmon captured in the North-Norwegian coastal fisheries, and to develop a stock-specific coastal migration model for the largest Barents Sea Atlantic salmon populations. Results from this study will also assist in providing a more precise, and informed regulatory regime for the management of Barents Sea Atlantic salmon stocks. Our results show that exploitation varies strongly among stocks, both spatially and temporally. Most Atlantic salmon stocks reached the North-Norwegian coast in June-July, with the multi- sea-winter salmon arriving almost a month earlier than the one- sea-winter salmon. The estimated proportions of the pre-fishery abundance in the coastal fisheries for Tana salmon were for instance less than 15 %, while the exploitation rates for Alta salmon were close to 50 %. The migration patterns also differed strongly among populations. The Målselv stock, being the largest salmon stock in Troms County, approached the coast more or less directly westwards from the open sea, i.e. showing an accurate homing from the open ocean. This was true both for multi-sea-winter and one-sea-winter salmon. The Alta stock showed a similar coastal migration patterns as the Målselv salmon, although it entered the coastal waters in a more widespread manner. The Tana salmon, however, as opposed to both Målselv and Alta salmon, reach the coastal areas both from southwest, west, north and east, i.e. approaching the coast from all over the Barents Sea. We also found the abundance of Tana salmon, as opposed to the other northern river populations, has declined the last 10 years. This negative development in the Tana stock is worrying and most probably connected to a very high exploitation rate the last decade. A new agreement on joint fishing regulations between Norway and Finland to protect the Tana salmon become operative in 2017. Although the agreement is controversial, especially among the Sami people, the agreement ensure 1) that the future exploitation rate will decrease and 2) that the management of the Tana salmon will be population specific (treat each of the 28 populations/rivers specifically). Hopefully, this will ensure a rebuilding of the Tana stock.

Salmon rivers in northernmost Norway, Finland, and the Kola peninsula (Russia), support important fisheries, both in coastal areas and in the rivers themselves, and contribute ca 50 % of the world freshwater catch of wild Atlantic salmon (Salmo salar). The River Tana, a large complex system with 30 tributaries (stocks), that forms the border between northernmost Norway and Finland, is among the largest wild Atlantic salmon producing stocks in the world. During the last 6-8 years, however, and unlike other rivers in northernmost Norway and Russia, the salmon catches in Tana have declined strongly, especially the important 3- and 4SW fish. The only slight demographic increase has been in previous spawners. In addition to the mixed stock coastal fisheries along the North-Norwegian coast, the Tana salmon stocks are exploited heavily in a mixed stock fishery in the river by use of various fishing methods, including weirs, gill nets, seines, drift nets and hook and line. High fishing effort in sea and/or in river may be contributing (the reason for?) to the decline in recent River Tana catches. Consequently, the objective of this proposal is to examine the historical fisheries to explain the historical (temporal) variation and recent decline in the abundance of Atlantic salmon from the Tana river. The study will apply molecular genetic methods to identify the contribution of individual stock components of the mixed-stock Tana salmon complex to the mixed-stock fishery, both in North-Norwegian coastal areas, in the Tana estuary and within the River Tana itself. The same genetic methods will also be used to quantify the total coastal run of Tana salmon, the life history-specific migration pattern of Tana salmon along the coast, as well as the total stock- and life history-specific spawning run sizes to each of the Tana tributaries. Biological knowledge gained from this project will be essential for explaining the impact of the selective fishery on the Tana salmon.