Across rather than between river genetic structure in Atlantic salmon Salmo salar in north-east Scotland, UK: potential causes and management implications.

The aim of this study was to examine the genetic structuring, based on c. 4300 single nucleotide polymorphic markers, of juvenile Atlantic salmon Salmo salar sampled from 11 rivers in north-east Scotland, which form part of a radial drainage system. Within this area, sites in the upper mountainous and lower coastal sections of the different rivers were more closely related than sites from the upper and lower sections of the same river. Differentiation between fish from upper and lower sections was related to both distance from the sea and elevation, which were themselves correlated. The occurrence of such cross-river genetic grouping appears to be the result of both similar selective pressures resulting in homologous adaptive differences within each river and also recent common ancestry between rivers. Examination of historical tagging information showed high rates (27·4%) of between-river straying in this region that may help to maintain the across rather than between-river structure. The existence of cross-river groupings highlights the complex structuring of S. salar populations and may confound genetic identification of single-river stocks. Furthermore, the results presented show that cross-river structuring should also be an important consideration in managing S. salar stocks.

An analysis of levels of infectious pancreatic necrosis virus in Atlantic salmon, Salmo salar L., broodstock in Scotland between 1990-2002.

Throughout this study period the prevalence of infectious pancreatic necrosis virus (IPNV) in Scottish farmed Atlantic salmon was high in the marine environment but relatively low in fresh water. In order to minimize the risk of vertical transmission of infection from parent to progeny, all IPNV infected broodstock populations had to undergo testing of all fish for the virus at the time of stripping and eggs from positive parents were destroyed. Between 1990 and 2002 over 68 000 Atlantic salmon broodfish were individually screened for IPNV by cell culture isolation and enzyme linked immunosorbent assay. Generalized linear mixed models were used to assess the influence of geographical region, age, sex and year on IPNV prevalence in Atlantic salmon broodstock. This analysis determined that the age and sex of the broodfish and the geographical region of the broodstock stripping site did not have a statistically significant influence on IPNV prevalence within the broodstock parental population. However, there was a statistically significant temporal increase in IPNV prevalence from 1990 to 2002.

Spatial and temporal distribution of Lepeophtheirus salmonis (Krøyer) larvae in a sea loch containing Atlantic salmon, Salmo salar L., farms on the north-west coast of Scotland.

A plankton survey investigated spatial and temporal distributions of larval sea lice in a sea loch on the north-west coast of Scotland. Plankton tows were collected approximately weekly at two depths (0 and 5 m) at five sample stations over a 2-year period. The survey began in March 2002 when Atlantic salmon, Salmo salar L., farms in the study area stocked with fish and finished in March 2004, after farms completed harvesting. Over 90% of lice recovered were Lepeophtheirus salmonis (Krøyer); therefore, the results of this paper refer largely to that species. The data were analysed using generalized additive models with explanatory variables for time, depth and station. Greatest densities of nauplii were recovered at stations adjacent to farms indicating that the local salmon farms were a likely source of larvae at times during the production cycle. There were significant temporal trends in larval densities and the stocking and harvesting of farms can possibly account for these increases and decreases, particularly at the start and at the end of the production cycle. The distribution of copepodids was more widespread than that of nauplii, indicating that the larvae can be transported several kilometres from the point of release.