Spatial and temporal variation of genetic diversity and estimation of effective population sizes in Atlantic salmon (Salmo salar, L.) populations from Asturias (Northern Spain) using microsatellites

Rivers in Asturias (northern Spain) constitute the southern limit of the distribution of Atlantic salmon (Salmo salar L.) in Europe, a biological resource facing one of the more serious challenges for conservation today. In this work, eight microsatellite loci have been used to analyse samples collected in 1993 and 1999 from four Asturian rivers (Esva, Narcea, Sella, and Cares), obtaining information about the temporal and the spatial genetic variation in these populations and, in addition, estimations of their effective population sizes. The temporal analysis revealed a general decrease in all the estimated genetic variability parameters when samples from 1993 (mean A(1993) = 6.47, mean HO(1993) = 0.472, mean HE(1993) = 0.530) were compared with those obtained in 1999 (mean A(1999) = 6.16, mean HO(1999) = 0.460, mean HE(1999) = 0.490). This reduction was particularly notable for the case of the Esva river. Our results pointed to a pattern of spatial genetic differentiation inside the Asturian region (FST (1993) = 0.016 P < 0.01; FST (1999) = 0.023 P < 0.01). Using the standard Temporal Method we found estimates of Ne^(Esva) = 75.1 (33.2–267.2); Ne^(Cares) = 96.6 (40.0–507.5), Ne^(Sella) = 106.5 (39.1–9396.4) and Ne^(Narcea) = 113.9 (42.0–3693.3). The use of likelihood-based methods for the Ne^ estimations improved the results (smaller CIs) for the Esva and Cares rivers (Ne^(Esva) = 63.9 (32.3–165.3); Ne^(Cares) = 76.4 (38.8–202.0) using a Maximum likelihood approach) and suggested the presence of larger populations for the Sella and Narcea rivers (Ne^≈200). These results showed that the Asturian Atlantic salmon populations (in particular Esva and Cares river populations) could be close to the conservation genetic borderline for avoiding inbreeding depression although we discuss some implications of the analysis of temporal genetic change in populations with overlapping generations.