Isolation by distance within a river system: genetic population structuring of Atlantic salmon, Salmo salar, in tributaries of the Varzuga River in northwest Russia.

An important issue for designing any conservation programme aimed at preserving genetic diversity is estimation of the scale at which genetic structuring occurs. Additional relevant factors include distinguishing whether or not population structuring is expected to be stable as predicted by the member-vagrant hypothesis, or alternatively, whether populations are more prone to local extinction-recolonization processes, as predicted by the metapopulation evolutionary model. In this study, the population genetic structure of Atlantic salmon from 11 locations within or nearby the Varzuga River tributary system was assessed using 17 microsatellites. Mantel tests and spatial autocorrelation analyses revealed a significant isolation-by-distance signal within the tributary system as well as a negative association between the level of genetic diversity and waterway distance from the river mouth, indicating that dispersal is less likely to occur to populations deep in the tributary system. Individual-level spatial autocorrelation analyses indicated that the majority of migration occurred between populations situated within 20 km of each other. The relatively high level of genetic structuring and significant isolation-by-distance signal observed in the Varzuga tributaries are concordant with the predictions of the member-vagrant evolutionary model. However, one subpopulation in particular revealed signs of instability which may be due to its location in the tidal zone, or due to the fact that it is more affected by human impacts. The results suggest that preservation of a number of spawning sites spaced throughout the tributary system is recommendable for ensuring sustainable fishing tourism in the river.

[Microsporidians of the genus Myxidium in rattail fish from the North Atlantic and the Pacific Ocean]. / Mikrosporidii roda Myxidium u nekotorykh makrurid iz Severnoi Atlantiki i Tikhogo okeana.

In contrast to macrurids from the Pacific, for which 5 species of myxosporidia of the genus Myxidium are known, only one species, M. coryphaenoidium, is recorded for this group of fishes from northern Atlantic. In 9 of 13 species of macrurids there were found M. melanostigmum, M. melanocetum, M. noblei sp. n., Myxidium sp. M. coryphaenoidium, the diagnosis of which is rather wider and includes the parameters of other species, was not found. The spores found have a large size, rough polar filament, flattened ends of spores and unequal polar capsules. This favours apparently the sinking of spores to a considerable depth and so is an adaptation for parasitism in deepwater fishes.