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Atlantic salmon Salmo salar supports highly valuable commercial and recreational fisheries in Europe, but its stocks are currently overexploited and threatened by climate change. Its southernmost populations (in northern Spain) play a key role in conserving the species' original genetic diversity, which is endangered due to decades-long (1970s to 1990s) massive stocking with non-native stocks. Their decline is well documented, but the effect of stock transfer and conservation efforts is unclear. Nine microsatellite loci were amplified from archival samples (scales from 1958–1959) from eight Spanish rivers to analyse the species' natural genetic dynamics before its decline started. Allelic richness was high in the historical populations (the 1950s) and above most contemporary estimates. Private alleles were found in most rivers, indicating high local uniqueness and relative isolation among river basins. Some alleles are regional markers since they are rare or absent from contemporary northern European populations. Effective population size suggested good conservation status, with higher values than those estimated for contemporary populations. Strong population structure and genetic differentiation between rivers were found, with limited gene flow, restricted to geographically close populations. Our estimates of historical genetic diversity and structure from southernmost salmon populations are a powerful benchmark to guide conservation programs. [ABSTRACT FROM AUTHOR]

Historical data on the oceanic distribution and migration routes of southernmost Atlantic salmon Salmo salar populations from Europe are almost non-existent, as no rigorous tagging initiatives have been conducted. Here, we used stable isotope data (δ13C and δ15N) of historic scale collections to identify the potential marine feeding areas of the largest salmon population in the Iberian Peninsula. Data were compared with published datasets from Northern Ireland, Wales, south England, and northeast UK coast, which correspond to series between 15- and 33-year long within the time period from 1958 to 2009. Temporal covariation in sea surface temperature, primary productivity, and δ13C values suggests that feeding areas of Iberian salmon are located around Greenland, both in the Labrador and the Irminger seas. Furthermore, δ13C values of Atlantic salmon from Canadian rivers reported in the literature are similar to those found in individuals from Spanish rivers. Our results suggest that Iberian salmon follow a westerly migration route towards Greenland instead of following the easterly branch of the North Atlantic current into the Norwegian Sea. Characterization of feeding patterns and migration routes might help to understand the causes of ongoing population decline and establish targeted conservation programmes for threatened Iberian salmon. [ABSTRACT FROM AUTHOR]

The consistency of the global declining trend of Atlantic salmon (Salmo salar) populations suggests that climate-driven reduced survival and growth at sea are the main driving factors. The southernmost populations have experienced the greatest declines, consistent with harsher conditions in natal fresh waters. We analyzed temporal trends in Spanish Atlantic salmon, important food organisms at sea, and climatic variables in the breeding (fresh water) and feeding (marine) salmon areas from 1950 onwards to elucidate drivers of declining patterns. Salmon abundance dropped abruptly in 1970–1971, plausibly linked to widespread overfishing coincident with incipient changes in the marine food webs and freshwater hydrology. A major regime shift in biophysical conditions throughout the North Atlantic salmon feeding grounds occurred in 1986–1987, driven by the concurrence of an abrupt acceleration in the anthropogenic warming trend and the warm phase of the Atlantic Multidecadal Oscillation. This regime shift may be the proximate cause of the collapse of Spanish salmon observed in 1988–1989, which kept declining in parallel to trends of ever-increasing ocean and freshwater temperatures, decreasing river flows, and poorer marine trophic conditions. [ABSTRACT FROM AUTHOR]

The abundance of Atlantic salmon is declining throughout its geographical area. Fisheries and global warming were assumed as main drivers of the decline, and recent studies suggest that habitat changes in freshwater is a third contributor. Southern populations experience the greatest decline, and face the highest risk of extinction as global warming moves its thermal niche northwards. We analysed long-term catch data (1949–2013) from a salmon fishery in northern Spain, and examined its relationship with local and global indicators of temperature and hydrological change. CPUE data, analysed by ARIMA time-series models, exhibited a significant negative trend and a marked decrease since 1973–1974, possibly triggered by overfishing at sea and a sudden outbreak of disease. Temperature increased in the same period, particularly so since 1986–1988, being negatively correlated with CPUE. A significant change in magnitude and duration of extreme water conditions occurred from 1970s onwards. Indicators of hydrological shift were also significantly correlated with CPUE of returning salmon. The best ARIMAX models indicated however, that the decrease in salmon CPUE was mainly driven by temperature trends. This indicates that both local (temperature and flow in the river) and global (ocean temperature) factors have contributed to the decrease in salmon numbers, and that temperature has played the major role. Despite a strong reduction in fishing pressure after the 1970s widespread collapse, our study population did not recover to previous abundance levels. This suggests the operation of additional factors, being climate warming and changes in food webs of the North Atlantic the most likely reasons. [ABSTRACT FROM AUTHOR]